.ds CP 2003-2005
.ds TC \fCtconfpy\fP
.TH TCONFPY 3 "TundraWare Inc."
.SH NAME tconfpy.py
Configuration File Support For Python Applications
.SH SYNOPSIS
It is common to provide an external "configuration file" when writing
sophisticated applications. This gives the end-user the ability to
easily change program options by editing that file.
\fCtconfpy\fP is a Python module for parsing such configuration
files. \fCtconfpy\fP understands and parses a configuration "language"
which has a rich set of string-substitution, variable name,
conditional, and validation features.
By using \fCtconfpy\fP, you relieve your program of the major
responsibility of configuration file parsing and validation, while
providing your users a rich set of configuration features.
.B NOTE:
Throughout this document we use the term "configuration file". However,
\fCtconfpy\fP can parse configurations both in files as well as in-memory
lists. Whenever you see the term "file", think "a file or a set of
configuration statements stored in a list".
If you run \fCtconfpy\fP directly, it will dump version and copyright
information, as well as the value of the current predefined
System Variables:
.ft C \" courier
.nf
python tconfpy.py
.fi
.ft \" revert
.SH DOCUMENT ORGANIZATION
This document is divided into 4 major sections:
.B PROGRAMMING USING THE \fCtconfpy\fP API
discusses how to call the configuration file parser, the options
available when doing this, and what the parser returns. This is
the "Programmer's View" of the module and provides in-depth
descriptions of the API, data structures, and options available to
the programmer.
.B CONFIGURATION LANGUAGE REFERENCE
describes the syntax and semantics of the configuration language
recognized by \fCtconfpy\fP. This is the "User's View" of the package, but
both programmers and people writing configuration files will find this
helpful.
.B ADVANCED TOPICS FOR PROGRAMMERS
describes some ways to combine the various \fCtconfpy\fP features to
do some fairly nifty things.
.B INSTALLATION
explains how to install this package on various platforms. This
information can also be found in the \fCREAD-1ST.txt\fP file distributed
with the package.
.SH PROGRAMMING USING THE \fCtconfpy\fP API
\fCtconfpy\fP is a Python module and thus available for use by any Python
program. This section discusses how to invoke the \fCtconfpy\fP parser, the
options available when doing so, and what the parser returns to
the calling program.
One small note is in order here. As a matter of coding style and
brevity, the code examples here assume the following Python import
syntax:
.ft C \" Courier
.nf
from tconfpy import *
.fi
.ft \" revert
If you prefer the more pedestrian:
.ft C \" Courier
.nf
import tconfpy
.fi
.ft \" revert
you will have to prepend all references to a \fCtconfpy\fP object with
\fCtconfpy.\fP. So \fCretval=ParseConfig(...\fP becomes
\fCretval = tconfpy.ParseConfig(...\fP and so on.
You will also find the test driver code provided in the \fCtconfpy\fP package
helpful as you read through the following sections. \fCtest-tc.py\fP is
a utility to help you learn and exercise the \fCtconfpy\fP API. Perusing the
code therein is helpful as an example of the topics discussed below.
.SS Core Objects
In order to use \fCtconfpy\fP effectively, you need to be familiar
with the objects used to communicate between your program and the
parser. This section provides a brief overview of these objects for
reference use later in the document.
.TP
.B The Symbol Table Object
A "symbol table" is a \fCtconfpy\fP object defined to hold the symbol table
proper, the results of the parse (upon return), and some other data
structures used internally by the parser.
The full Symbol Table object definition and initialization looks
like this:
.ft C \" Courier
.nf
class SymbolTable(object):
def __init__(self):
self.Symbols = {}
self.DebugMsgs = []
self.ErrMsgs = []
self.WarnMsgs = []
self.LiteralLines = []
self.Templates = Template()
self.ALLOWNEWVAR = True
self.TEMPONLY = False
self.LITERALVARS = False
self.INLITERAL = False
self.DEBUG = False
self.CondStack = [["", True],]
self.TotalLines = 0
.fi
.ft \" revert
This object is optionally passed to the API when beginning a parse,
if you wish to provide an initial symbol table. In this case,
only \fCSymbols\fP need be populated.
When the parse is complete, an object of this same type is returned.
\fCSymbols\fP, \fCDebugMsgs\fP, \fCErrMsgs\fP, \fCWarnMsgs\fP, and
\fCLiteralLines\fP will be populated with the parse results as
appropriate.
.TP
.B The Variable Descriptor Object
The actual symbol table is kept in the \fCSymbolTable.Symbols\fP
dictionary. There is one entry for each symbol (variable) encountered
in the parse. The symbol's name is used as the dictionary key and an
object of type \fCVarDescriptor\fP as its entry. This "variable
descriptor" is an object that describes the variable's current and
default values, whether it can be modified, and any validation
constraints:
.ft C \" Courier
.nf
class VarDescriptor(object):
# Default variable type is a writeable string with no constraints
def __init__(self):
self.Value = ""
self.Writeable = True
self.Type = TYPE_STRING
self.Default = ""
self.LegalVals = []
self.Min = None
self.Max = None
.fi
.ft \" revert
.TP
.B The Template Object
As described later in this document, it is possible to pre-define
a variable's type, default value, and validation contraint(s)
to be applied only if the variable is actually brought into existence
in the configuration file. This is a so-called "template". Templates
have their own object definition:
.ft C \" Courier
.nf
class Template(object):
def __init__(self):
self.Symbols = {}
.fi
.ft \" revert
In effect, this is a subset of the \fCSymbolTable\fP object.
\fCTemplate.Symbols\fP is populated just like \fCSymbolTable.Symbols\fP
using symbol names and variable descriptors.
.SS API Overview
The \fCtconfpy\fP API consists of a single call. Only the configuration file
to be processed is a required parameter, all the others are optional
and default as described below:
.ft C \" Courier
.nf
from tconfpy import *
retval = ParseConfig(cfgfile,
CallingProgram="tconfpy version-num",
InitialSymTable=SymbolTable(),
AllowNewVars=True,
Templates=Template(),
TemplatesOnly=False,
LiteralVars=True,
ReturnPredefs=True,
Debug=False
)
.fi
.ft \" revert
where:
.TP
.B cfgfile (Required Parameter - No Default)
Declares where the configuration is found. If this parameter is a
.B string,
it is treated as the name of a file to parse. If this parameter
is a
.B list,
\fCtconfpy\fP presumes this to be an in-memory configuration, and parses the
list in sequential order, treating each entry as a configuration line.
This allows you to use \fCtconfpy\fP for parsing either configuration files or
in-memory configurations. If you pass anything other than a string
or list here, \fCtconfpy\fP will produce an error.
.TP
.B CallingProgram (Default: \fC\fCtconfpy\fP + Version Number\fP)
Change the prompt introducer string used for Debug, Error, and Warning
messages.
By default, each time \fCtconfpy\fP produces an Error, Warning, or Debug
message, it prepends it with the string \fCtconfpy\fP followed by the
version number. Since \fCtconfpy\fP is designed to be called from
applications programs, you may wish to substitute your own program
name or other information in these messages. You do so by setting the
\fCCallingProgram\fP keyword to the desired text.
.TP
.B InitialSymTable (Default: \fCSymbolTable()\fP)
Used to pass a pre-initialized Symbol Table from the application to
the parser.
.TP
.B AllowNewVars (Default: \fCTrue\fP)
Allow the user to create new variables in the configuration file.
.TP
.B Templates (Default: \fCTemplate()\fP)
This option is used to pass variable templates to the parser. If
present, \fCtconfpy\fP expects this option to pass an object of type
\fCTemplate\fP. See the section below entitled,
.B Using Variable Templates
for all the details. By default, an empty template table (no templates)
is passed to the parser.
.TP
.B TemplatesOnly (Default: \fCFalse\fP)
If this option is set to \fCTrue\fP, \fCtconfpy\fP will not permit a new variable to
be created unless a variable template exists for it. By default, \fCtconfpy\fP will
use a variable template if one is present for a new variable, but it does
not require one. If a new variable is created, and no Template exists for
it, the variable is just created as a string type with no restrictions on
content or length. When this option is set to \fCTrue\fP, then a
template
.B must
exist for each newly created variable.
.TP
.B LiteralVars (Default: \fCFalse\fP)
If set to \fCTrue\fP, this option enables variable substitutions within
\fC.literal\fP blocks of a configuration file. See the section in the
language reference below on \fC.literal\fP usage for details.
.TP
.B ReturnPredefs (Default: \fCTrue\fP)
\fCtconfpy\fP "prefefines" some variables internally. By default,
these are returned in the symbol table along with the variables
actually defined in the configuration file. If you want a
"pure" symbol table - that is, a table with
.B only
your variables in it - set this option to \fCFalse\fP.
.TP
.B Debug (Default: \fCFalse\fP)
If set to \fCTrue\fP, \fCtconfpy\fP will provide detailed debugging information
about each line processed when it returns.
.TP
.B retval
An object of type \fCSymbolTable\fP used to return parsing results.
The results of the parse will be returned in various data structures:
.ft C \" Courier
.nf
SymbolTable.Symbols => All symbols and their values as a result of the parse
SymbolTable.DebugMsgs => Any Debug Messages if debug was requested
SymbolTable.ErrMsgs => Any Error Messages
SymbolTable.WarnMsgs => Any Warning Messages
Symboltable.LiteralLines => Any Literal Text found in the configuration file
.fi
.ft \" revert
You can tell whether a parse was successful by examining \fCErrMsgs\fP.
A successful parse will leave this data structure empty (though there may
well be Warning Messages present in \fCWarnMsgs\fP.)
.SS The Initial Symbol Table API Option
The simplest way to parse a configuration is just to call
the parser with the name of a file or an in-memory list
containing the configuration statements:
.ft C \" Courier
.nf
retval = ParseConfig("MyConfigFile")
- OR -
myconfig = [configline1, configline2, ....]
retval = ParseConfig(myconfig)
.fi
.ft \" revert
Assuming your configuration is valid, \fCParseConfig()\fP will
return a symbol table populated with all the variables defined in the
configuration and their associated values. This symbol table will have
.B only
the symbols defined in that configuration (plus a few built-in and predefined
symbols needed internally by \fCtconfpy\fP).
However, the API provides a way for you to pass a "primed" symbol
table to the parser that contains predefined symbols/values of
your own choosing. Why on earth would you want to do this? There
are a number of reasons:
.IP \(bu 4
You may wish to write a configuration file which somehow depends
on a predefined variable that only the calling program can know:
.ft C \" Courier
.nf
.if [APPVERSION] == 1.0
# Set configuration for original application version
.else
# Set configuration for newer releases
.endif
.fi
.ft \" revert
In this example, only the calling application can know its own
version, so it sets the variable \fCAPPVERSION\fP in a symbol table
which is passed to \fCParseConfig()\fP.
.IP \(bu 4
You may wish to "protect" certain variable names be creating them
ahead of time and marking them as "Read Only"
(\fCVarDescriptor.Writeable=False \fP). This is useful when you want
a variable to be available for use within a configuration file, but
you do not want users to be able to change its value. In this case,
the variable can be referenced in a string substitution or conditional
test, but cannot be changed.
.IP \(bu 4
You may want to place limits on what values can be assigned to a
particular variable. When a variable is newly defined in a a
configuration file, it just defaults to being a string variable
without any limits on its length or content (unless you are using
Variable Templates). But variables that are created by a program have
access to the variable's "descriptor". By setting various attribues
of the variable descriptor you can control variable type, content, and
range of values. In other words, you can have \fCtconfpy\fP "validate" what
values the user assigns to particular variables. This substantially
simplifies your application because no invalid variable value will
ever be returned from the parser.
.SS How To Create An Initial Symbol Table
A \fCtconfpy\fP "Symbol Table" is really nothing more than a Python
dictionary stored inside a container object. The key for each
dictionary entry is the variable's name and the value is a
\fCtconfpy\fP-specific object called a "variable descriptor".
Creating new variables in the symbol table involves nothing more than
this:
.ft C \" Courier
.nf
from tconfpy import *
# Create an empty symbol table
MySymTable = SymbolTable()
# Create descriptor for new variable
MyVarDes = VarDescriptor()
# Code to fiddle with descriptor contents goes here
MyVarDes.Value = "MyVal"
# Now load the variable into the symbol table
MySymTable.Symbols["MyVariableName"] = MyVarDes
# Repeat this process for all variables, then call the parser
retval = ParseConfig("MyConfigFile", InitialSymTable=MySymTable)
.fi
.ft \" revert
The heart of this whole business the \fCVarDescriptor\fP object. It
"describes" the value and properties of a variable. These
descriptor objects have the following attributes and defaults:
.ft C \" Courier
.nf
VarDescriptor.Value = ""
VarDescriptor.Writeable = True
VarDescriptor.Type = TYPE_STRING
VarDescriptor.Default = ""
VarDescriptor.LegalVals = []
VarDescriptor.Min = None
VarDescriptor.Max = None
.fi
.ft \" revert
When \fCtconfpy\fP encounters a new variable in a configuration file, it just
instantiates one of these descriptor objects with these defaults for
that variable. That is, variables newly-defined in a configuration
file are entered into the symbol table as string types, with an
initial value of "" and with no restriction on content or length.
But, when you create variables under program control to "prime" an
initial symbol table, you can modify the content of any of these
attributes for each variable. These descriptor attributes are what
\fCtconfpy\fP uses to validate subsequent attempts to change the variable's
value in the configuration file. In other words, modifying a
variable's descriptor tells \fCtconfpy\fP just what you'll accept as "legal"
values for that variable.
Each attribute has a specific role:
.TP
.B VarDescriptor.Value (Default: \fCEmpty String\fP)
Holds the current value for the variable.
.TP
.B VarDescriptor.Writeable (Default: \fCTrue\fP)
Sets whether or not the user can change the variable's value. Setting
this attribute to \fCFalse\fP makes the variable
.B Read Only.
.TP
.B VarDescriptor.Type (Default: \fCTYPE_STRING\fP)
One of \fCTYPE_BOOL,\fP \fCTYPE_COMPLEX,\fP \fCTYPE_FLOAT,\fP
\fCTYPE_INT,\fP or \fCTYPE_STRING. This\fP defines the type of the
variable. Each time \fCtconfpy\fP sees a value being assigned to a
variable in the configuration file, it checks to see if that variable
already exists in the symbol table. If it does, the parser checks the
value being assigned and makes sure it matches the type declared for
that variable. For example, suppose you did this when defining the
variable, \fCfoo\fP:
.ft C \" Courier
.nf
VarDescriptor.Type = TYPE_INT
.fi
.ft \" revert
Now suppose the user puts this in the configuration file:
.ft C \" Courier
.nf
foo = bar
.fi
.ft \" revert
This will cause a type mismatch error because \fCbar\fP cannot be coerced
into an integer type - it is a string.
As a general matter, for existing variables, \fCtconfpy\fP attempts to coerce
the right-hand-side of an assignment to the type declared for that
variable. The least fussy operation here is when the variable is
defined as TYPE_STRING because pretty much everything can be coerced
into a string. For example, here is how \fCfoo = 3+8j\fP is treated for
different type declarations:
.ft C \" Courier
.nf
VarDescriptor.Type VarDescriptor.Value
------------------ -------------------
TYPE_BOOL Type Error
TYPE_COMPLEX 3+8j (A complex number)
TYPE_FLOAT Type Error
TYPE_INT Type Error
TYPE_STRING "3+8j" (A string)
.fi
.ft \" revert
This is why the default type for newly-defined variables in
the configuration file is TYPE_STRING: they can accept
pretty much
.B any
value.
.B NOTE:
\fCtconfpy\fP always stores a variable's value in it native type in
the symbol table entry for that variable - i.e., You'll get the
variable's value back from the parser in the proper type. However,
when the variable is actually referenced within a configuration file,
it is converted to a
.B string
for purposes of configuration processing.
For instance, when doing conditional comparisons, the parser coerces
the variable's value into a string for purposes of the comparsion.
Say you have the floating point variable \fCmyfloat\fP whose value is
\fC6.023\fP and the configuration file contains a statement like:
.ft C \" Courier
.nf
.if myfloat == 6.023
.fi
.ft \" revert
When doing this conditional check, the parser will convert the current
value of \fCmyfloat\fP into a string and compare it to the
.B string
\fC"6.023"\fP on the Right Hand Side.
This can be tricky when dealing with Boolean variables. As described
later in this document, you can do conditional tests based on the
.B state
of a Boolean, but if you do this:
.ft C \" Courier
.nf
.if myboolean == whatever...
.fi
.ft \" revert
The parser converts \fCmyboolean\fP to a
.B string
of either \fC"True"\fP or \fC"False"\fP, so watch out for this. As a
general matter, you're more likely to need the boolean tests that
check the state of the variable, but the above construct is handy if
you want to use regular string variables to control conditional
bodies:
.ft C \" Courier
.nf
MYCONTROL = True
.if myboolean == MYCONTROL
.fi
.ft \" revert
Where, \fCMYCONTROL\fP is a regular old string variable - i.e., It has not
been defined to be a boolean by either a Template or Initial Symbol table
passed to the parser.
.TP
.B VarDescriptor.Default (Default: \fCEmpty String\fP)
This is a place to store the default value for a given variable. When
a variable is newly-defined in a configuration file, \fCtconfpy\fP places the
first value assigned to that variable into this attribute. For
variables already in the symbol table, \fCtconfpy\fP does nothing to this
attribute. This attribute is not actually used by \fCtconfpy\fP for anything.
It is provided as a convenience so that the calling program can easily
"reset" every variable to its default value if desired.
.TP
.B VarDescriptor.LegalVals (Default: \fC[]\fP)
Sometimes you want to limit a variable to a specific set of values.
That's what this attribute is for. \fCLegalVals\fP explictly lists
every legal value for the variable in question. If the list is
empty,then this validation check is skipped.
The exact semantics of LegalVals varies depending on
the type of the variable.
.ft C \" Courier
.nf
Variable Type What LegalVals Does
------------- -------------------
Boolean Nothing - Ignored
Integer, Float, Complex List of numeric values the
user can assign to this variable
Examples: [1, 2, 34]
[3.14, 2.73, 6.023e23]
[3.8-4j, 5+8j]
String List of Python regular expressions.
User must assign a value to this
variable that matches at least
one of these regular expressions.
Example: [r'a+.*', r'^AnExactString$']
.fi
.ft \" revert
The general semantic here is "If Legal Vals is not an empty list, the
user must assign a value that matches one of the items in LegalVals."
One special note applies to \fCLegalVals\fP for string variables. \fCtconfpy\fP
always assumes that this list contains Python regular expressions.
For validation, it grabs each entry in the list, attempts to compile
it as a regex, and checks to see if the value the user wants to set
matches. If you define an illegal regular expression here, \fCtconfpy\fP will
catch it and produce an appropriate error.
You may also want to specify a set of legal strings that are
.B exact matches
not open-ended regular expressions. For example, suppose you have
a variable, \fCCOLOR\fP and you only want the user to be able to only set it
to one of, \fCRed\fP, \fCWhite\fP, or \fCBlue\fP. In that case, use the Python
regular expression metacharacters that indicate "Start Of String" and
"End Of String" do do this:
.ft C \" Courier
.nf
des = VarDescriptor()
des.LegalVals = [r'^Red$', r'^White$', r'^Blue$']
...
SymTable['COLOR'] = des
.fi
.ft \" revert
.B NOTE:
If you want this test to be skipped, then set \fCLegalVals\fP to an
empty list, []. (This is the default when you first create an
instance of \fCtconfpy.VarDescriptor\fP.) Do not set it to a Python
None or anything else. \fCtconfpy\fP expects this attribute to be a list in
every case.
.TP
.B VarDescriptor.Min and VarDescriptor.Max (Default: \fCNone\fP)
These set the minimum and maxium legal values for the variables,
but the semantics vary by variable type:
.ft C \" Courier
.nf
Variable Type What Min/Max Do
------------- ---------------
Boolean, Complex Nothing - Ignored
Integer, Float Set Minimum/Maxium allowed values.
String Set Minimum/Maximum string length
.fi
.ft \" revert
In all cases, if you want either of these tests skipped, set \fCMin\fP
or \fCMax\fP to the Python None.
.P
All these various validations are logically "ANDed" together.
i.e., A new value for a variable must be allowed
AND of the appropriate type AND one of the legal values AND
within the min/max range.
\fCtconfpy\fP makes no attempt to harmonize these validation
conditions with each other. If you specify a value in
\fCLegalVals\fP that is, say, lower than allowed by
\fCMin\fP you will always get an error when the user sets
the variable to that value: It passed the \fCLegalVals\fP
validation but failed it for \fCMin\fP.
.SS The Initial Symbol Table And Lexical Namespaces
The
.B CONFIGURATION LANGUAGE REFERENCE
section below discusses lexical namespaces in some detail from the
user's point-of-view. However, it is useful for the programmer
to understand how they are implemented.
\fCtconfpy\fP is written to use a predefined variable named \fCNAMESPACE\fP as
the place where the current namespace is kept. If you do not
define this variable in the initial symbol table passed to the parser,
\fCtconfpy\fP will create it automatically with an initial value of "".
From a programmer's perspective, there are are few important things
to know about namespaces and the \fCNAMESPACE\fP variable:
.IP \(bu 4
You can manually set the initial namespace to something other than "".
You do this by creating the \fCNAMESPACE\fP variable in the initial
symbol table passed to the parser, and setting the \fCValue\fP attribute
of its descriptor to whatever you want as the initial namespace. At
startup \fCtconfpy\fP will check this initial value to make sure it conforms
to the rules for properly formed names - i.e., It it will check for
blank space, a leading \fC$\fP, the presence of square brackets, and so
on. If the initial namespace value you provide is illegal, \fCtconfpy\fP will
produce an error and reset the initial namespace to "".
.IP \(bu 4
Because lexical namespaces are implemented by treating \fCNAMESPACE\fP
as just another variable, all the type and value validations available
for string variables can be applied to \fCNAMESPACE\fP. As discussed above,
this means you can limit the length and content of what the user assigns
to \fCNAMESPACE\fP. In effect, this means you can limit the number and
name of namespaces available for use by the user. There is one slight
difference here than for other variables.
.B The root namespace is always legal,
regardless of what other limitations you may impose via
the \fCLegalVals\fP, \fCMin\fP, and \fCMax\fP attributes of the \fCNAMESPACE\fP
variable descriptor.
.IP \(bu 4
When the call to \fCParseConfig()\fP completes, the \fCValue\fP
attribute of the \fCNAMESPACE\fP variable descriptor will contain
the namespace that was in effect when the parse completed. i.e.,
It will contain the last namespace used.
.SS How The \fCtconfpy\fP Parser Validates The Initial Symbol Table
When you pass an initial symbol table to the parser, \fCtconfpy\fP does some
basic validation that the table contents properly conform to the
\fCVarDescriptor\fP format and generates error messages if it finds
problems. However, the program does
.B not
check your specifications to see if they make sense. For instance
if you define an integer with a minimum value of 100 and a maximum
value of 50, \fCtconfpy\fP cheerfully accepts these limits even though they
are impossible. You'll just be unable to do anything with that
variable - any attempt to change its value will cause an error
to be recorded. Similarly, if you put a value in \fCLegalVals\fP that
is outside the range of \fCMin\fP to \fCMax\fP, \fCtconfpy\fP will accept
it quietly.
.SS The \fCAllowNewVars\fP API Option
By default, \fCtconfpy\fP lets the user define any new variables they
wish in a configuration file, merely by placing a line in the
file in this form:
.ft C \" Courier
.nf
Varname = Value
.fi
.ft \" revert
However, you can disable this capability by calling the parser like
this:
.ft C \" Courier
.nf
retval = ParseConfig("myconfigfile", AllowNewVars=False)
.fi
.ft \" revert
This means that the configuration file can "reference"
any predefined variables, and even change their values
(if they are Writeable), but it cannot create
.B new
variables.
This feature is primarily intended for use when you pass an initial
symbol table to the parser and you do not want any other variables
defined by the user. Why? There are several possible uses for
this option:
.IP \(bu 4
You know every configuration variable name the calling program
will use ahead of time. Disabling new variable names keeps
the configuration file from getting cluttered with variables
that the calling program will ignore anyway, thereby
keeping the file more readable.
.IP \(bu 4
You want to insulate your user from silent errors caused
by misspellings. Say your program looks for a configuration
variable called \fCMyEmail\fP but the user enters something
like \fCmyemail = foo@bar.com\fP. \fCMyEmail\fP and \fCmyemail\fP
are entirely different variables and only the former is
recognized by your calling program. By turning off new
variable creation, the user's inadvertent misspelling of
the desired variable name will be flagged as an error.
Note, however, that there is one big drawback to disabling new
variable creation. \fCtconfpy\fP processes the configuration file on a
line-by-line basis. No line "continuation" is supported. For really
long variable values and ease of maintenance, it is sometimes helpful
to create "intermediate" variables that hold temporary values used to
construct a variable actually needed by the calling program. For
example:
.ft C \" Courier
.nf
inter1 = Really, really, really, really, long argument #1
inter2 = Really, really, really, really, long argument #2
realvar = command [inter1] [inter2]
.fi
.ft \" revert
If you disable new variable creation you can't do this
anymore unless all the variables \fCinter1\fP, \fCinter2\fP,
and \fCrealvar\fP are predefined in the initial symbol
table passed to the parser.
.SS Using Variable Templates
By default, any time a new variable is encountered in a configuration
file, it is created as a string type with no restrictions on its
content or length. As described above, you can predefine the variable
in the initial symbol table you pass to the parser. This allows you
to define that variable's type and to optionally place various
restrictions on the values it may take. In other words, you can "declare"
the variable ahead of time and \fCtconfpy\fP will do so-called "type
and value enforcement".
"Variable Templates" are a related kind of idea, with a bit of
a twist. They give you a way to "declare" variable type
and content restrictions for selected
.B new variables
discovered in the configuration file. In other words,
by using Variable Templates, you can make sure that
a new variable also has restrictions placed on its type
and/or values.
The obvious question here is, "Why not just do this
by predefining every variable of interest in the
initial symbol table passed to the parser?" There
are several answers to this:
.IP \(bu 4
The \fCtconfpy\fP configuration language has very powerful "existential"
conditional tests. These test to see if a variable "exists".
If you predefine every variable you will ever need, then
the kinds of existential tests you can do will be
somewhat limited (since every variable
.B does
already exist).
With Variable Templates, you can define the type and value
constraints of a variable which will be applied,
.B but only if you actually bring that variable into existence.
This allows constructs like this to work:
.ft C \" Courier
.nf
.if [.PLATFORM] == posix
posix = True
.endif
.if [.PLATFORM] == nt
nt = True
.endif
.ifall posix
...
.endif
.ifall nt
...
.endif
.ifnone posix nt
...
.endif
.fi
.ft \" revert
In this example, notice that the variables \fCposix\fP and \fCnt\fP
may- or may not be actually created, depending on the value of
\fC.PLATFORM\fP. The logic later in the example depends upon this.
If you were to predefine these two variables (to specify type
and/or value restrictions), this type of logical flow would not
be possible.
By providing Variable Templates for \fCposix\fP and \fCnt\fP,
you can define their type (likely Boolean in this case)
ahead of time
.B and this will be applied if the variable does come into existence.
.IP \(bu 4
The other reason for Variable Templates is more subtle, but
gives \fCtconfpy\fP tremendous versatility beyond just processing
configuration files. Variable Templates give you a way to use
\fCtconfpy\fP to build data validation tools.
Suppose you have a list of employee records exported in this
general format (easy to do with most databases):
.ft C \" Courier
.nf
[Employee#]
LastName = ...
FirstName = ...
Address = ...
City = ...
... and so on
.fi
.ft \" revert
By using the empoyee's ID as a lexical namespace, we end up creating
new variables for each employee. Say the employee number is
\fC1234\fP. Then we would get, \fC1234.LastName\fP,
\fC1234.FirstName\fP, and so on.
Now, here's the subtle part. Notice that the type and content
restrictions of these variables is likely to be the
.B same
for each different employee.
By defining Variable Templates for each of the variables we
intend to use over and over again in different namespace
contexts, we can
.B validate
each of them to make sure their content, type, length, and so forth
are correct. This makes it possible to use \fCtconfpy\fP as the underpinnings
of a "data validation" or "cleansing" program.
.IP \(bu 4
Another way to look at this is that Variable Templates give you
a way to define type/value restrictions on an entire "class"
of variables. Instead of having to explictly predefine
variables for every employee in our example above, you
just define templates for the variable set that is common to
all employees. This is
.B way
simpler than predefining every possible variable combination
ahead of time.
.SS The \fCTemplates\fP And \fCTemplatesOnly\fP API Options
Variable Templates are supported with two API options:
\fCTemplates\fP And \fCTemplatesOnly\fP. \fCTemplates\fP
is used to pass a symbol table (separate from the main symbol
table) containing the Variable Templates. By default, this
option is set to \fC{}\fP which means no templates are
defined.
So what exactly is a "Variable Template"? It is the
.B exact same thing
as a predefined variable you might pass in the initial symbol
table. In other words, it is a Python dictionary entry where the
key is the variable name and the entry is in \fCVarDescriptor\fP
format.
The only difference is that a templated variable does not come into
existence in the main symbol table until a variable by that
.B name
is encountered in the configuration file. Then the variable is
created using the template as its entry in the main symbol table.
For example:
.ft C \" Courier
.nf
[1234]
LastName = Jones
FirstName = William
Address = 123 Main Street
City = Anywhere
State = WI
ZIP = 00000-0000
[1235]
LastName = Jones
FirstName = Susan
Address = 123 Main Street
City = Anywhere
State = WI
ZIP = 00000-0000
.fi
.ft \" revert
Suppose you define variable templates for \fCLastName\fP,
\fCFirstName\fP, \fCAddress\fP, and so on. That is, you
define variables by these names, and define whatever type
and content restrictions you want in each of their
\fCVarDescriptor\fPs. You then pass these to the parser
via the \fCTemplates=\fP option.
As \fCtconfpy\fP parses the file and encounters the new variables
\fC1234.LastName\fP ... \fC1235.ZIP\fP, it uses the following
"rules" when creating new variables:
.IP \(bu 4
See if there is a template variable whose name is the same as
the "base" name of the new variable. (The "base" name is just
the variable name without the prepended namespace.)
If there is a template with a matching name, see if the value the
user wants to assign to that variable passes all the type/validation
rules. If so, load the variable into the symbol table and set its
value as requested,
.B using the \fCVarDescriptor\fP object from the template.
(This ensures that future attempts to change the variable's value will
also be type/validation checked.)
If the assignment fails the validation tests, issue an appropriate
error and do
.B not
create the variable in the symbol table.
.IP \(bu 4
If there is no template with a matching name, then just
create a new variable as usual - string type with no
restrictions,
.B unless
\fCTemplatesOnly\fP is set to \fCTrue\fP. Setting this
option to \fCTrue\fP tells the program that you want to
allow the creation of
.B only
those variables for which templates are defined. This is
a way to restrict just what new variables can be created
in any namespace. \fCTemplatesOnly\fP defaults to \fCFalse\fP
which means you can create new variables even when no template
for them exists.
.P
In summary, Variable Templates give you a way to place
restrictions on variable type and content
.B in the event that the variable actually comes into existence.
They also give you a way to define such restrictions for an
entire class of variables without having to explicitly name
each such variable ahead of time. Finally, Variable Templates
are an interesting way to use \fCtconfpy\fP as the basis for data
validation programs.
.SS The \fCLiteralVars\fP API Option
\fCtconfpy\fP supports the inclusion of literal text anywhere in a
configuration file via the \fC.literal\fP directive. This
directive effectively tells the \fCtconfpy\fP parser to pass
every line it encounters "literally" until it sees a
corresponding \fC.endlinteral\fP directive. By default,
\fCtconfpy\fP does
.B exactly
this. However, \fCtconfpy\fP has very powerful variable substitution
mechanisms. You may want to embed variable references in a
"literal" block and have them replaced by \fCtconfpy\fP.
Here is an example:
.ft C \" Courier
.nf
MyEmail = me@here.com # This defines variable MyEmail
.literal
printf("[MyEmail]"); /* A C Statement */
.endliteral
.fi
.ft \" revert
By default, \fCParseConfig()\fP will leave everything within
the \fC.literal\fP/\fC.endliteral\fP block unchanged. In our
example, the string:
.ft C \" Courier
.nf
printf("[MyEmail]"); /* A C Statement */
.fi
.ft \" revert
would be in the list of literals returned by \fCParseConfig()\fP.
However, we can ask \fCtconfpy\fP to do variable replacement
.B within
literal blocks by setting \fCLiteralVars=True\fP in the
\fCParseConfig()\fP call:
.ft C \" Courier
.nf
retval = ParseConfig("myconfigfile", LiteralVars=True)
.fi
.ft \" revert
In this example, \fCtconfpy\fP would return:
.ft C \" Courier
.nf
printf("me@here.com"); /* A C Statement */
.fi
.ft \" revert
At first glance this seems only mildly useful, but it is
actually very handy. As described later in this document,
\fCtconfpy\fP has a rich set of conditional operators and string
sustitution facilities. You can use these features along
with literal block processing and variable substitution
within those blocks. This effectively lets you use
\fCtconfpy\fP as a preprocessor for
.B any
other language or text.
.SS The \fCReturnPredefs\fP API Option
As described below, \fCtconfpy\fP internally "predefines" a number of
variables. These include variables that describe the current runtime
environment as well as variables that substitute for language
keywords.
These predefined variables are just stored in the symbol table like
any other variable. By default, they are returned with all the "real"
variables discovered in the configuration file. If you want
.B only
the variables actually encountered in the configuration file itself,
set \fCReturnPredefs=False\fP in the \fCParseConfig()\fP API call.
This will cause \fCtconfpy\fP to strip out all the predefined
variables before returning the final symbol table.
Note that this option also removes the \fCNAMESPACE\fP variable since
it is understood to also be outside the configuration file (even
though you may have passed an initial version of \fCNAMESPACE\fP to
the parser).
Note also that this option applies only to the variables predefined
by
.B \fCtconfpy\fP
itself. Any variables
.B you
predefine when passing an initial symbol table will be returned as usual,
regardless of the state of this option.
.SS The \fCDebug\fP API Option
\fCtconfpy\fP has a fairly rich set of debugging features built into its
parser. It can provide some detail about each line parsed as well
as overall information about the parse. Be default, debugging is
turned off. To enable debugging, merely set \fCDebug=True\fP in the
API call:
.ft C \" Courier
.nf
retval = ParseConfig("myconfigfile", Debug=True)
.fi
.ft \" revert
.SS How \fCtconfpy\fP Processes Errors
As a general matter, when \fCtconfpy\fP encounters an error in the
configuration file currently being parsed, it does two things. First,
it adds a descriptive error message into the list of errors returned
to the calling program (see the next section). Secondly, in many
cases, noteably during conditional processing, it sets the parser
state so the block in which the error occurred is logically \fCFalse\fP.
This does not happen in every case, however. If you are having
problems with errors, enable the Debugging features of the package and
look at the debug output. It provides detailed information about what
caused the error and why.
.SS \fCtconfpy\fP Return Values
When \fCtconfpy\fP is finished processing the configuration file, it returns an
object that contains the entire results of the parse. This includes
a symbol table, any relevant error or warning messages, debug information
(if you requested this via the API), and any "literal" lines encountred
in the configuration.
The return object is an instance of the class \fCtwander.SymbolTable\fP which has
been populated with the results of the parse. In the simplest
case, we can parse and extract results like this:
.ft C \" Courier
.nf
from tconfpy import *
retval = ParseConfig("myconfigfile", Debug=True)
.fi
.ft \" revert
\fCretval\fP now contains the results of the parse:
.TP
.B retval.Symbols
A Python dictionary which lists all the defined symbols and their
associated values. A "value" in this case is always an object of type
\fCtconfpy.VarDescriptor\fP (as described above).
.TP
.B retval.DebugMsgs
A Python list containing detailed debug information for each line
parsed as well as some brief summary information about the parse.
\fCretval.Debug\fP defaults to an empty list and is only populated if you
set \fCDebug=True\fP in the API call that initiated the parse (as in the
example above).
.TP
.B retval.ErrMsgs
A Python list containing error messages. If this list is empty, you
can infer that there were no parsing errors - i.e., The configuration
file was OK.
.TP
.B retval.WarnMsgs
A Python list containing warning messages. These describe minor
problems not fatal to the parse process, but that you really ought to
clean up in the configuration file.
.TP
.B retval.LiteralLines
As described below, the \fCtconfpy\fP configuration language supports a
\fC.literal\fP directive. This directive allows the user to embed
literal text anywhere in the configuration file. This effectively
makes \fCtconfpy\fP useful as a preprocessor for any other language or text.
\fCretval.LiteralLines\fP is a Python list containing all literal text
discovered during the parse. The lines appear there in the order
they were discovered in the configuration file.
.SH CONFIGURATION LANGUAGE REFERENCE
\fCtconfpy\fP recognizes a full-featured configuration language that includes
variable creation and value assignment, a full preprocessor with
conditionals, type and value enforcement, and lexical namespaces.
This section of the document describes that language and provides
examples of how each feature can be used.
.SS \fCtconfpy\fP Configuration Language Syntax
\fCtconfpy\fP supports a fairly simple and direct configuration
language syntax:
.IP \(bu 4
Each line is treated independently. There is no line
"continuation".
.IP \(bu 4
The \fC#\fP can begin a comment anywhere on a line. This is done
blindly. If you need to embed this symbol somewhere within a variable
value, use the \fC[HASH]\fP variable reference.
.IP \(bu 4
Whitespace is (mostly) insignificant. Leading and trailing
whitespace is ignored, as is whitespace around comparison
operators. However, there are some places where
whitespace matters:
- Variable names may not contain whitespace
- Directives must be followed by whitespace if they take
other arguments.
- When assigning a value to a string variable, whitespace
within the value on the right-hand-side is preserved.
Leading- and trailing whitespace around the right-hand-
side of the assignment is ignored.
- Whitespace within both the left- and right-hand-side
arguments of a conditional comparison
(\fC.if ... == / != ...\fP) is significant for purposes
of the comparison.
.IP \(bu 4
Case is always significant except when assigning a value to Booleans
(described in the section below entitled,
.B Some Notes On Boolean Variables
).
.IP \(bu 4
Regardless of a variable's type, all variable references
return
.B a string representation of the variable's value!
This is done so that the variable's value can be used for comparison
testing and string substitution/concatenation. In other words,
variables are stored in their native type in the symbol table that is
returned to the calling program. However, they are treated as strings
during the parsing of the configuration file whenever they are used in
a comparison test or in a substitution.
.IP \(bu 4
Text inside a literal block (see section below on the
\fC.literal\fP directive) is left untouched. Whitespace,
the \fC#\fP symbol, and so on are not intepreted in any
way and are passed back to the calling program as-is. The
one exception to this rule is when variable substitution
inside literal blocks is enabled. This is discussed in a later
section of this document as well.
.IP \(bu 4
Any line which does not conform to these rules and/or is not
in the proper format for one of the operations described below,
is considered an error.
.SS Creating Variables And Assigning A Value
The heart of a configuration file is a "variable". Variables are
stored in a "Symbol Table" which is returned to the calling program
once the configuration file has been processed. The calling program
can predefine any variables it wishes before processing a
configuration file. You can normally also define your own new
variables in the configuration file as desired (unless the programmer
has inhibited new variable creation).
Variables are assigned values like this:
.ft C \" Courier
.nf
MyVariable = Some string of text
.fi
.ft \" revert
If \fCMyVariable\fP is a new variable, \fCtconfpy\fP will create it on the spot.
If it already exists, \fCtconfpy\fP will first check and make sure that \fCSome
string of text\fP is a legal value for this variable. If not, it will
produce an error and refuse to change the current value of
\fCMyVariable\fP.
Anytime you create a new variable, the first value assigned to
it is also considered its "default" value. This may (or may
not) be meaningful to the application program.
Variables which are newly-defined in a configuration file are
always understood to be
.B string
variables - i.e., They hold "strings" of text. However, it is
possible for the applications programmer to predefine
variables with other types and place limitations on what values
the variable can take and/or how short or long a string variable
may be. (See the previous section,
.B PROGRAMMING USING THE \fCtconfpy\fP API
for all the gory details.)
The programmer can also arrange for the configuration file to only
have access to variables predefined by the program ahead of time. In
that case, if you try to create a new variable, \fCtconfpy\fP will produce an
appropriate error and the new variable will not be created.
.SS Variable Names
Variables can be named pretty much anything you like, with certain
restrictions:
.IP \(bu 4
Variable names may not contain whitespace.
.IP \(bu 4
Variable names may not begin with the \fC$\fP character. The one exception
to this is when you are referencing the value of an environment
variable. References to environment variables begin with \fC$\fP:
.ft C \" Courier
.nf
# A reference to an environment variable is legal
x = [$TERM]
# Attempting to create a new variable starting with $ is illegal
$MYVAR = something
.fi
.ft \" revert
.IP \(bu 4
Variable names cannot have the \fC#\fP character anywhere in them
because \fCtconfpy\fP sees that character as the beginning a comment.
.IP \(bu 4
Variable names cannot begin with \fC.\fP character. \fCtconfpy\fP understands
a leading period in a variable name to be a "namespace escape".
This is discussed in a later section on lexical namespaces.
.IP \(bu 4
Variable names cannot contain the \fC[\fP or \fC]\fP characters. These
are reserved symbols used to indicate a variable
.B reference.
.IP \(bu 4
You cannot have a variable whose name is the empty string. This is
illegal:
.ft C \" Courier
.nf
= String
.fi
.ft \" revert
.IP \(bu 4
The variable named \fCNAMESPACE\fP is not available for your own
use. \fCtconfpy\fP understands this variable to hold the current lexical
namespace as described later in this document. If you set it
to a new value, it will change the namespace, so be sure this is
what you wanted to do.
.SS Getting And Using The Value Of A Variable
You can get the value of any currently defined variable by
.B referencing
it like this:
.ft C \" Courier
.nf
.... [MyVariable] ...
.fi
.ft \" revert
The brackets surrounding any name are what indicate that you want that
variable's value.
You can also get the value of any Environment Variable on your
system by naming the variable with a leading \fC$\fP:
.ft C \" Courier
.nf
... [$USER] ... # Gets the value of the USER environment variable
.fi
.ft \" revert
However you cannot set the value of an environment variable:
.ft C \" Courier
.nf
$USER = me # This is not permitted
.fi
.ft \" revert
This ability to both set and retrieve variable content makes it easy
to combine variables through "substitution":
.ft C \" Courier
.nf
MYNAME = Mr. Tconfpy
MYAGE = 101
Greeting = Hello [MYNAME], you look great for someone [MYAGE]!
.fi
.ft \" revert
Several observations are worth noting here:
.IP \(bu 4
The substitution of variables takes place as soon as the parser
processes the line \fCGreeting = ...\fP. That is, variable substitution
happens as it is encountered in the configuration file. The only
exception to this is if an attempt is made to refer to an
undefined/non-existent variable. This generates an error.
.IP \(bu 4
The variable \fCGreeting\fP now contains the
.B string
"Hello Mr. Tconfpy, you look great for someone 101!" This is true even
if variable \fCMYAGE\fP has been defined by the calling program to be
an integer type. To repeat a previously-made point: All variable substitution
and comparison operations in a configuration file are done with
.B strings
regardless of the actual type of the variables involved.
.IP \(bu 4
Variables must be
.B defined
before they can be
.B referenced.
\fCtconfpy\fP does not support so-called "forward" references.
.IP \(bu 4
Unless a variable as been marked as "Read Only" by the application
program, you can continue to change its value as you go. Simply
adding another line at the end of our example above will change the
value of \fCGreeting\fP to something new:
.ft C \" Courier
.nf
Greeting = Generic Greeting Message
.fi
.ft \" revert
In other words, the last assignment statement for a given variable
"wins". This may seem sort of pointless, but it actually has great
utility. You can use the \fC.include\fP directive to get, say, a
"standard" configuration provided by the system administrator for a
particular application. You can then selectively override the
variables you want to change in your own configuration file.
.SS Indirect Variable Assignment
The dereferencing of a variable's value can take place on either
the right- or
.B left-hand-side
of an assignment statement. This means so-called "indirect"
variable assignments are permitted:
.ft C \" Courier
.nf
CurrentTask = HouseCleaning
[CurrentTask] = Dad
.fi
.ft \" revert
To understand what this does you need to realize that before
\fCtconfpy\fP does anything with a statement in a configuration file, it
replaces every variable reference with its associated value
(or produces an error for references to non-existent variables).
So the second statement above is first converted to:
.ft C \" Courier
.nf
HouseCleaning = Dad
.fi
.ft \" revert
i.e., The value \fCDad\fP is assigned to a (new) variable called
\fCHouseCleaning\fP. In other words, putting a variable reference
on the left-hand-side of an assignment like this allows you
to access another variable which is named "indirectly".
You have to be careful when doing this, though. Consider a similar,
but slightly different example:
.ft C \" Courier
.nf
CurrentTask = House Cleaning # This is fine
[CurrentTask] = Dad # Bad!
.fi
.ft \" revert
The reason this no longer works is that the indirect
reference causes the second line to parse to:
.ft C \" Courier
.nf
House Cleaning = Dad
.fi
.ft \" revert
This is illegal because whitespace is not permitted in variable names.
\fCtconfpy\fP will produce an error if it sees such a construct. As a general
matter, any variable you construct through this indirection method
must still conform to all the rules of variable naming: It cannot
contain whitespace, begin with \fC$\fP, contain \fC#\fP, \fC[\fP, or \fC]\fP and
so on.
Another example of how indirection can "bite" you is when the
value of the variable begins with a period. As you'll see in the
following section on Lexical Namespaces, a variable name beginning
with a period is understood to be an "absolute" variable name
reference (relative to the root namespace). This can cause unexpected
(though correct) behavior when doing indirect variable access:
.ft C \" Courier
.nf
NAMESPACE = NS1
foo = .bar # Creates variable NS1.foo with value .bar
[foo] = baz # Means [NS1.foo] = baz
.fi
.ft \" revert
The second assignment statement in this example does not do what you
might initially think. Remember, \fCtconfpy\fP always does variable
dereferencing before anything else, so the second statement becomes:
.ft C \" Courier
.nf
.bar = baz
.fi
.ft \" revert
As you'll see in the section on Lexical Namespaces below, this actually
means, "Set the variable \fCbar\fP in the root namespace to the value
\fCbaz\fP." In other words, if you do indirect variable assignment, and
the content of that variable begins with a period, you will be creating/setting
a variable in
.B the root namespace.
Be sure this is what you intended to do.
Get into the habit of reading \fC[something]\fP as, "The current value
of \fCsomething\fP". See if you understand what the following does (if
you don't, try it out with \fCtest-tc.py\fP):
.ft C \" Courier
.nf
foo = 1
bar = 2
[foo] = bar
[bar] = [foo]
.fi
.ft \" revert
You can get pretty creative with this since variable references
can occur pretty much anywhere in an assignment statement.
The only place they cannot appear is
.B within
another variable reference. That is, you cannot "nest" references:
.ft C \" Courier
.nf
# The Following Is Fine
FOO = Goodness
BAR = Me
Oh[FOO][BAR] = Goodness Gracious Me!
# But This Kind Of Nesting Attempt Causes An Error
[FOO[BAR]] = Something Or Other
.fi
.ft \" revert
.SS Introducing Lexical Namespaces
So far,the discussion of variables and references has conveniently
ignored the presence of another related \fCtconfpy\fP feature, "Lexical
Namespaces." Namespaces are a way to automatically group
related variables together. Suppose you wanted to describe
the options on your car in a configuration file. You might do
this:
.ft C \" Courier
.nf
MyCar.Brand = Ferrari
MyCar.Model = 250 GTO
MyCar.Color = Red
# And so on ...
.fi
.ft \" revert
You'll notice that every variable start with the "thing" that
each item has in common - they are features of \fCMyCar\fP.
We can simplify this considerably by introducing a lexical namespace:
.ft C \" Courier
.nf
[MyCar]
Brand = Ferrari
Model = 250 GTO
Color = Red
.fi
.ft \" revert
The first statement looks like a variable reference, but it is not.
.B A string inside square brackets by itself on a line introduces a namespace.
The first statement in this example sets the namespace to \fCMyCar\fP.
From that point forward until the namespace is changed again, every
variable assignment
.B and
reference is "relative" to the namespace. What this really means is
that \fCtconfpy\fP sticks the namspace plus a \fC.\fP in front of every
variable assigned or referenced. It does this automatically and
invisibly, so \fCBrand\fP is turned into \fCMyCar.Brand\fP and so on. You
can actually check this by loading the example above into a test
configuration file and running the \fCtest-tc.py\fP program on it. You will
see the "fully qualified" variable names that actually were loaded
into the symbol table, each beginning with \fCMyCar.\fP and ending with
the variable name you specified.
Realize that this is entirely a naming "trick". \fCtconfpy\fP has no clue
what the namespace
.B means,
it just combines the current namespace with the variable name to
create the actual variable name that will be returned in the symbol
table.
You're likely scratching your head wondering why on earth this
feature present in \fCtconfpy\fP. There are several good reasons for it:
.IP \(bu 4
It reduces typing repetetive information throughout the configuration
file. In turn, this reduces the likelyhood of a typographical or
spelling error.
.IP \(bu 4
It helps visibly organize the configuration file. A namespace makes
it clear which variables are related to each other somehow. This is no
big deal in small configurations, but \fCtconfpy\fP was written with the idea
of supporting configuration files that might contain thousands or
even tens of thousands of entries.
.IP \(bu 4
It simplifies the application programmer's job. Say I want to write a
program that extracts all the information about your car from the
configuration file, but I don't know ahead of time how many things you
will describe. All I really have to know is that you are using
\fCMyCar\fP as the namespace for this information. My program can then
just scan the symbol table after the configuration file has been parsed,
looking for variables whose name begins with \fCMyCar.\fP. So if you
want to add other details about your auto like, say, \fCAge\fP, \fCPrice\fP,
and so on, you can do so later
.B and the program does not have to be rewritten.
.IP \(bu 4
It helps enforce correct configuration files. By default, you can
introduce new namespaces into the configuration file any time you
like. However, as described in the previous section on the \fCtconfpy\fP API,
the application programmer can limit you to a predefined set of legal
namespaces (via the \fCLegalVals\fP attribute of the \fCNAMESPACE\fP
variable descriptor). By doing this, the programmer is helping you
avoid incorrect configuration file entries by limiting just which
namespaces you can enter to reference or create variables.
.SS Rules For Using Lexical Namespace
Creating and using lexical namespaces is fairly straightforward,
but there are a few restrictions and rules:
.IP \(bu 4
The default initial namespace is the empty string, "". In this one
case, \fCtconfpy\fP does nothing to variables assigned or referenced. That's
why our early examples in the previous section worked. When we
assigned a value to a variable and then referenced that variable
value, we did so while in the so-called "root" namespace, "". When
the namespace is "", nothing is done to the variable names.
Bear in mind that the programmer can change this default namespace to
something other than "" before the configuration file is ever
processed. If they do this, they would be well advised to let their
users know this fact.
.IP \(bu 4
There two ways to change to a new namespace:
.ft C \" Courier
.nf
[NewNameSpace] # May optionally have a comment
OR
NAMESPACE = NewNamespace # May optionally have a comment
.fi
.ft \" revert
If, at any point, you want to return to the root namespace, you can use
one of these two methods:
.ft C \" Courier
.nf
[]
OR
NAMESPACE =
.fi
.ft \" revert
So, why are there two ways to do the same thing? The first way is the more
common, and the more readable way to do it. It appears on a line by itself and
makes it clear that the namespace is being changed. However, because variable
references cannot be "nested", you can only use strings of text here.
Suppose you want to change the namespace in a way that depends on the
value of another variable. For instance:
.ft C \" Courier
.nf
LOCATION = Timbuktu
NAMESPACE = [LOCATION]-East
.fi
.ft \" revert
In other words, the second form of a namespace change allows you to
employ the \fCtconfpy\fP string substitution and variable referencing
features. Bear in mind that \fCtconfpy\fP is case-sensitive so this
will not work as you expect:
.ft C \" Courier
.nf
Namespace = something
.fi
.ft \" revert
This just set the value of the variable \fCNamespace\fP to
\fCsomething\fP and has nothing whatsoever to do with lexical
namespaces.
.IP \(bu 4
Whichever method you use to change it,
.B the new namespace must follow all the same rules used for naming variables.
For example, both of the following will cause an error:
.ft C \" Courier
.nf
[$FOO]
OR
x = $FOO
NAMESPACE = [x]
.fi
.ft \" revert
.IP \(bu 4
By default, all variable assignments and references are
.B relative to the currently active namespace:
.ft C \" Courier
.nf
[MyNameSpace]
foo = 123 # Creates a variable called MyNameSpace.foo
x = [bar] # Means: MyNameSpace.x = [MyNameSpace.bar]
.fi
.ft \" revert
.IP \(bu 4
If you want to set or reference a variable in a namespace different
than the current namespace, you must use a so-called "absolute"
variable name. You do this by "escaping" the variable name. To escape the
name, begin it with a \fC.\fP and then use the
.B full name (including namespace)
of that variable. (This is called the "fully qualified variable
name".) For example:
.ft C \" Courier
.nf
[NS1] # Switch to the namespace NS1
foo = 14 # Creates NS1.foo
[NS2] # Switch to the NS2 namespace
foo = [.NS1.foo] # Sets NS2.foo = 14
.fi
.ft \" revert
There is another clever way to do this without using the escape
character. \fCtconfpy\fP has no understanding whatsoever of what a
lexical namespace actually is. It does nothing more than
"glue" the current namespace to any variable names and references
in your configuration file. Internally, all variables are
named
.B relative to the root namespace.
This means that you can use the fully qualified variable name
without any escape character any time you are in the root
namespace:
.ft C \" Courier
.nf
[NS1] # Switch to the namespace NS1
foo = 14 # Creates NS1.foo
[] # Switch to the root namespace
foo = [NS1.foo] # Sets foo = 14 - no escape needed
.fi
.ft \" revert
.IP \(bu 4
Lexical namspaces are implemented by having \fCNAMESPACE\fP just be
nothing more than (yet) another variable in the symbol table. \fCtconfpy\fP
just understands that variable to be special - it treats it as the
repository of the current lexical namespace. This means you can use
the value of NAMESPACE in your own string substitutions:
.ft C \" Courier
.nf
MyVar = [NAMESPACE]-Isn't This Cool?
.fi
.ft \" revert
You can even use the current value of NAMESPACE when setting a new
namespace:
.ft C \" Courier
.nf
NAMESPACE = [NAMESPACE]-New
.fi
.ft \" revert
One final, but very important point is worth noting here. The
\fCNAMESPACE\fP variable itself is always understood to be
.B relative to the root namespace.
No matter what the current namespace actually is, \fC[NAMESPACE]\fP or
\fCNAMESPACE = ...\fP always set a variable by that name in the
root namespace. Similarly, when we use a variable reference to get
the current namespace value (as we did in the example above), \fCNAMESPACE\fP
is understood to be relative to the root namespace. That's why things
like this work:
.ft C \" Courier
.nf
[MyNewSpace]
x = 100 # MyNewSpace.x = 100
y = [NAMESPACE]-1 # MyNewSpace.y = MyNewSpace-1
NAMESPACE = NewSpace # .NAMESPACE = NewSpace
.fi
.ft \" revert
.SS Predefined Variables
\fCtconfpy\fP predefines a number of variables. The \fCNAMESPACE\fP variable we
discussed in the previous section is one of them, but there are a number
of others of which you should be aware. Note that all predefined variables
.B are relative to the root namespace.
Except for the \fCNAMESPACE\fP variable, they are all Read Only and
cannot be modified in your configuration file.
The first group of predefined variables are called "System Variables".
As the name implies, they provide information about the system on
which you're running. These are primarily useful when doing
conditional tests (described later in this document). For example,
by doing conditional tests with System Variables you can have
one configuration file that works on both Unix and Windows operating
systems. The System Variables are:
.ft C \" Courier
.nf
Variable Name Contains
------------- --------
.MACHINENAME - The name of the computer on which you are running.
May also include full domain name, depending on system.
.OSDETAILS - Detailed information about the operating system in use.
.OSNAME - The name of the operating system in use.
.OSRELEASE - The version of the operating system in use.
.OSTYPE - Generic name of the operating system in use.
.PLATFORM - Generic type of the operating system in use.
.PYTHONVERSION - The version of Python in use.
.fi
.ft \" revert
By combining these System Variables as well as the content of
selected Environment Variables, you can create complex conditional
configurations that "adapt" to the system on which a Python
application is running. For example:
.ft C \" Courier
.nf
.if [.MACHINENAME] == foo.bar.com
BKU = tar
.else
BKU = [$BACKUPPROGRAM]
.endif
.fi
.ft \" revert
The other kind of predefined variables are called "Reserved Variables".
\fCtconfpy\fP understands a number of symbols as part of its own language.
For example, the string \fC#\fP tells \fCtconfpy\fP to begin a comment until
end-of-line. There may be times, however, when
.B you
need these strings for your own use. In other words, you would like
to use one of the strings which comprise the \fCtconfpy\fP language for your
own purposes and have \fCtconfpy\fP ignore them. The Reserved Variables
give you a way to do this. The Reserved Variables are:
.ft C \" Courier
.nf
Variable Name Contains
------------- --------
DELIML [
DELIMR ]
DOLLAR $
ELSE .else
ENDIF .endif
ENDLITERAL .endliteral
EQUAL =
EQUIV ==
HASH #
IF .if
IFALL .ifall
IFANY .ifall
IFNONE .ifnone
INCLUDE .include
LITERAL .literal
NOTEQUIV !=
PERIOD .
.fi
.ft \" revert
For instance, suppose you wanted to include the \fC#\fP symbol
in the value of one of your variables. This will not work,
because \fCtconfpy\fP interprets it as the beginning of a comment,
which is not what you want:
.ft C \" Courier
.nf
MyJersey = Is #23
.fi
.ft \" revert
So, we use one of the Reserved Variables to get what we want:
.ft C \" Courier
.nf
MyJersey = Is [HASH]23
.fi
.ft \" revert
One word of warning, though. At the end of the day, you still have
to create variable names or namespace names that are legal. You
can't "sneak" illegal characters into these names using Reserved
Variables:
.ft C \" Courier
.nf
foo = [DOLLAR]MyNewNamespace # No problem
NAMESPACE = [foo] # No way - namespace cannot start with $
.fi
.ft \" revert
.SS Type And Value Enforcement
By default, any variable (or namespace) you create in a configuration file
is understood to just hold a string of characters. There are no limits
to what that string may contain, how long it is, and so on.
However, \fCtconfpy\fP gives the programmer considerable power to enforce variable
types and values, if they so choose. (See the section above entitled,
.B PROGRAMMING USING THE \fCtconfpy\fP API
for the details.) The programmer can set all kinds of limitations
about a variable's type, permissible values, and (in the case of
strings) how long or short it may be. The programmer does this by
defining these limitations for each variable of interest
.B prior to calling \fCtconfpy\fP to parse your configuration file.
In that case, when \fCtconfpy\fP actually processes the configuration file,
it "enforces" these restrictions any time you attempt to change the
value of one of these variables. If you try to assign a value that
fails one of these "validation" tests, \fCtconfpy\fP will produce an error
and leave the variable's value unchanged.
For instance, suppose the programmer has defined
variable "Foo" to be a floating point number, and that it must have
a value between -10.5 and 100.1. In that case:
.ft C \" Courier
.nf
Foo = 6.023E23 # Error - Value is out of range
Foo = MyGoodness # Error - Value must be a FP number, not a string
Foo = -2.387 # Good - Value is both FP an in range
.fi
.ft \" revert
.SS What Specific Validations Are Available?
The programmer has several different restrictions they can place on
a variable's value. You do not need to understand how they work,
merely what they are so that any error messages you see will make sense.
.IP \(bu 4
The programmer may declare any variable to be
.B Read Only.
This means you can still use references to that variable to extract
its value, but any attempt to change it value within the configuration
file will fail and produce an error.
.IP \(bu 4
The programmer may specify the variable's
.B type
as string (the default), integer, floating point, complex, or boolean.
.IP \(bu 4
The programmer may specify
.B the set of all legal values
that can be assigned to a variable. For instance, the programmer
might specify that the floating point variable \fCTranscend\fP can only
be set to either 3.14 or 2.73. Similarly, the programmer might
specify that the string variable \fCCOLOR\fP can only ever be set to
\fCRed\fP, \fCGreen\fP, or \fCBlue\fP. In fact, in the case of string
variables, the programmer can actually specify a set of patterns
(regular expressions) the value has to match. For instance, they can
demand that you only set a particular string variable to strings that
begin with \fCa\fP and end with \fCbob\fP.
.IP \(bu 4
For integer and floating point variables, the programmer can specify
a legal
.B value range
for the variable. If you change the value of such a variable, that value
must be within the defined range or you'll get an error.
.IP \(bu 4
For string variables, the programmer can specify a minimum and maxium
.B length
for the strings you assign to the variable in question.
.IP \(bu 4
The programmer can limit you to
.B only being able to use existing variables.
(.i.e. The Predefined variables and any variables the programmer
has defined ahead of time.) In that case, any attempt to create a
new variable in the configuration file will fail and produce an error.
.IP \(bu 4
The programmer can limit you to
.B only being able to use namespaces they have defined ahead of time.
In that case, if you attempt to enter a namespace not on the list
the programmer created ahead of time will fail and produce an error.
.IP \(bu 4
The programmer can enable or prevent
.B the substitution of variable references in literal blocks
(see below). If they disable this option, something like \fC[Foo]\fP
is left unchanged within the literal block. i.e., It too, is treated
"literally".
.SS Notes On Variable Type/Value Enforcement
There are a few other things you should know about how \fCtconfpy\fP
enforces restrictions on variables:
.IP \(bu 4
For purposes of processing the configuration file,
.B variable references are always converted to strings
regardless of the actual type of the variable in question. (Variables are
stored in the symbol table in their actual type.)
For instance, suppose the programmer defines variable \fCFoo\fP to be
floating point. Then:
.ft C \" Courier
.nf
Foo = 1.23
Bar = Value is [Foo] # Creates a new *string* variable with the
# value: "Value is 1.23"
.fi
.ft \" revert
In other words, variable values are "coerced" into strings for
the purposes of substitution and conditional testing within a
configuration file. This is primarily an issue with
the conditional comparisons below. For example, the following
conditional is \fCFalse\fP because the string representations of the
two numbers are different. Assume \fCf1\fP and \fCf2\fP have been
defined as floating point variables by the calling program:
.ft C \" Courier
.nf
f1 = 1.0
f2 = 1.00
.if [f1] == [f2] # False because "1.0" is not the same string as "1.00"
...
.fi
.ft \" revert
.IP \(bu 4
You cannot create anything but a string variable within a
configuration file. This variable will have no restrictions placed on
its values. All validation features
.B require
the limitations to be specified by the calling program ahead of time.
.IP \(bu 4
Similarly, you cannot change any of the enforcement options from
within a configuration file. These features are only available
under program control, presumably by the application program that
is calling \fCtconfpy\fP.
.IP \(bu 4
There is no way to know what the limitations are on a particular
variable from within the configuration file. Programmers
who use these features should document the variable restrictions
they've employed as a part of the documentation for the application
in question.
.SS Some Further Notes On Boolean Variables
One last note here concerns Boolean variables. Booleans are actually
stored in the symbol table as the Python boolean values, \fCTrue\fP or
\fCFalse\fP.
However, \fCtconfpy\fP accepts user statements that set the value of the
boolean in a number of formats:
.ft C \" Courier
.nf
Boolean True Boolean False
------------ -------------
foo = 1 foo = 0
foo = True foo = False
foo = Yes foo = No
foo = On foo = Off
.fi
.ft \" revert
This is the one case where \fCtconfpy\fP is insensitive to case -
\fCtRUE\fP, \fCTRUE\fP, and \fCtrue\fP are all accepted, for example.
.B NOTE HOWEVER:
If the user wants to do a conditional test on the value of
a boolean they
.B must
observe case and test for either \fCTrue\fP or \fCFalse\fP:
.ft C \" Courier
.nf
boolvar = No
.if [boolvar] == False # This works fine
.if [boolvar] == FALSE # This does not work - Case is not being observed
.if [boolvar] == Off # Neither does this - Only True/False can be tested
.fi
.ft \" revert
.SS The \fC.include\fP Directive
At any point in a configuration file, you can "include" another configuration file
like this:
.ft C \" Courier
.nf
.include filename
.fi
.ft \" revert
In fact, you can use all the variable substitution and string concatenation features we've
already discussed to do this symbolically:
.ft C \" Courier
.nf
Base = MyConfig
Ver = 1.01
.include [Base]-[Ver].cfg
.fi
.ft \" revert
The whitespace after the \fC.include\fP directive is mandatory to separate it from
the file name. You can have as many \fC.include\fP statements in your configuration file
as you wish, and they may appear anywhere. The only restriction is that they must
appear on a line by themselves (with an optional comment).
Why bother? There are several reasons:
.IP \(bu 4
This makes it easy to break up large, complex configurations into simpler (smaller)
pieces. This generally makes things easier to maintain.
.IP \(bu 4
This makes is easy to "factor" common configuration information into separate
files which can then be used by different programs as needed.
.IP \(bu 4
The most common use for \fC.include\fP is to load a "standard" configuration for
your program. Recall that the last assignment of a variable's value "wins".
Suppose you want all the standard settings for a program, but you just want to
change one or two options. Instead of requiring each user to have the whole
set of standard settings in their own configuration file, the system administrator
can make them available as a common configuration. You then \fC.include\fP
that file and override any options you like:
.ft C \" Courier
.nf
# Get the standard options
.include /usr/local/etc/MyAppStandardConfig.cfg
# Override the ones you like
ScreenColor = Blue
Currency = Euros
.fi
.ft \" revert
This makes maintenance of complex configuration files much simpler.
There is only one master copy of the configuration that needs to be
edited when system-wide changes are required.
.P
One last thing needs to be noted here. \fCtconfpy\fP does not detect
so-called "circular" inclusions. If file \fCa\fP \fC.include\fPs file
\fCb\fP and file \fCb\fP \fC.include\fPs file \fCa\fP, you will have an
infinite loop of inclusion, which, uh ..., is a Bad Thing...
.SS Conditional Directives
One of the most powerful features of \fCtconfpy\fP is its "conditional
processing" capabilities. The general idea is to test some
condition and
.B include or exclude configuration information based on the outcome of the test.
What's the point? You can build large, complex configurations that
test things like environment variables, one of the Predefined
Variables, or even a variable you've set previously in the
configuration file. In other words, resulting configuration is then
produced in a way that is appropriate for that particular system, on
that particular day, for that particular user, ...
By using conditional directives, you can create a single configuration file that
works for every user regardless of operating system, location, and so on.
There are two kinds of conditional directives. "Existential Conditionals" test to
see if a configuration or environment variable
.B exists.
Existential Conditionals pay no attention to the
.B value
of the variables in question, merely whether or not those variables have been defined.
"Comparison Conditionals" actually
.B compare
two strings. Typically, one or more variable references appear in the compared strings.
In this case, the
.B value of the variable
is important.
The general structure of any conditional looks like this:
.ft C \" Courier
.nf
ConditionalDirective Argument(s)
This is included if the conditional was True
.else # Optional
This is included if the conditional was False
.endif # Required
.fi
.ft \" revert
Except for the whitespace after the conditional directive itself, whitespace is not
significant. You may indent as you wish.
Conditionals may also be "nested". You can have a conditional within another conditional
or \fC.else\fP block:
.ft C \" Courier
.nf
ConditionalDirective Argument(s)
stuff
ConditionalDirective Argument(s)
more stuff
.endif
interesting stuff
.else
yet more stuff
ConditionalDirective Argument(s)
other stuff
.endif
ending stuff
.endif
.fi
.ft \" revert
There are no explicit limits to how deeply you can nest a configuration. However,
you must have an \fC.endif\fP that terminates each conditional test. Bear in mind that
\fCtconfpy\fP pays no attention to your indentation. It associates an \fC.endif\fP
.B with the last conditional it encountered.
That's why it's a really good idea to use some consistent indentation style so
.B you
can understand the logical structure of the conditions. It's also a
good idea to put comments throughout such conditional blocks so it's
clear what is going on.
There are a few general rules to keep in mind when dealing with conditionals:
.IP \(bu 4
There must be whitespace between the conditional directive and its
arguments (which may- or may not have whitespace in them).
.IP \(bu 4
As with any other kind of \fCtconfpy\fP statement, you may place comments
anywhere at the end of a conditional directive line or within the
conditional blocks.
.IP \(bu 4
Each conditional directive must have a corresponding \fC.endif\fP. If
you have more conditionals than \fC.endif\fPs or vice-versa, \fCtconfpy\fP will
produce an error message to that effect. It can get complicated to
keep track of this, especially with deeply nested conditionals. It is
therefore recommended that you always begin and end conditional blocks
within the same file. i.e., Don't start a conditional in one file and
then \fC.include\fP another file that has the terminating \fC.endif\fP in
it.
.IP \(bu 4
The \fC.else\fP clause is optional. However, it can only appear after
some preceding conditional directive.
.IP \(bu 4
As in other parts of the \fCtconfpy\fP language, variable names and references
in conditional directives are always relative to the currently active
namespace unless they are escaped with a leading period. Similarly,
in this context, Environment Variables, Predefined Variables,
and the NAMESPACE Variable are always relative to the root namespace,
no matter what namespace is currently active.
.SS Existential Conditional Directives
There are three Existential Conditionals: \fC.ifall\fP, \fC.ifany\fP, and \fC.ifnone\fP.
Each has the same syntax:
.ft C \" Courier
.nf
ExistentialDirective varname ...
included if test was True
.else # optional
included if test was False
.fi
.ft \" revert
.fi
In other words, existential conditionals require one or more
.B variable names.
In each case, the actual content of that variable is ignored. The test merely
checks to see if a variable by that name
.B exists.
Nothing else may appear on an existential conditional line, except,
perhaps, a comment.
The three forms of existential conditional tests implement three different kinds of
logic:
.ft C \" Courier
.nf
.ifall var1 var2 ...
This is a logical "AND" operation. ALL of the variables, var1, var2 ...
must exist for this test to be True.
.ifany var1 var2 ...
This is a logical "OR" operation. It is True of ANY of the variables,
var1, var2 ... exist.
.ifnone var1 var2 ...
This is a logical "NOR" operation. It is True only if NONE of the variables,
var1, var2 ... exist.
.fi
.ft \" revert
Here is an example:
.ft C \" Courier
.nf
FOO = 1
BAR = 2
z = 0
.ifall FOO BAR
x = 1
.endif
.ifany FOO foo fOo
y = 2
.endif
.ifnone BAR bar Bar SOmething
z=3
.endif
.fi
.ft \" revert
When \fCtconfpy\fP finishes processing this, x=1, y=2, and z=0.
You can also use references to environment variables in an existential
conditional test:
.ft C \" Courier
.nf
.ifany $MYPROGOPTIONS
options = [$MYPROGOPTIONS]
.else
options = -b20 -c23 -z -r
.endif
.fi
.ft \" revert
Finally, you can use variable references here to get the name of a
variable to test by "indirection" (as we saw in the previous section
on accessing/setting variables indirectly). This should be used
sparingly since it can be kind of obscure to understand, but it is
possible to do this:
.ft C \" Courier
.nf
foo = MyVarName
.ifany [FOO]
...
.endif
.fi
.ft \" revert
This will test to see if either the variable \fCMyVarName\fP exists.
You can also do indirection through an environment variable. Use this
construct with restraint - it can introduce serious obscurity into
your configuration file. Still, it has a place. Say the \fCTERM\fP
environment variable is set to \fCvt100\fP:
.ft C \" Courier
.nf
.ifany [$TERM]
...
.endif
.fi
.ft \" revert
This will test to see if a variable called \fCvt100\fP exists in the
symbol table. This is a handy way to see if you have a local variable
defined appropriate for the currently defined terminal, for instance.
.SS Comparison Conditional Directives
There are two Comparison Conditionals:
.ft C \" Courier
.nf
.if string1 == string2 # True if string1 and string2 are identical
.if string1 != string2 # True if string1 and string2 are different
.fi
.ft \" revert
As a general matter, you can put literal strings on both sides of such a
test, but the real value of these tests comes when you use variable
references within the tested strings. In this case, the value of the
variable
.B does matter.
It is the variable's value that is replaced in the string to test for
equality or inequality:
.ft C \" Courier
.nf
MyName = Tconfpy
.if [MyName] == Tconfpy
MyAge = 100.1
.else
MyAge = Unknown
.fi
.ft \" revert
.fi
These are particularly useful when used in combination with the \fCtconfpy\fP
Predefinded Variable or environment variables. You can build configurations
that "sense" what system is currently running and "adapt" accordingly:
.ft C \" Courier
.nf
AppFiles = MyAppFiles
.if [.OSNAME] == FreeBSD
files = [$HOME]/[AppFiles]
.endif
.if [.OSNAME] == Windows
files = [$USERPROFILE]\\[AppFiles]
.endif
.ifnone [files]
ErrorMessage = I don't know what kind of system I am running!
.endif
.fi
.ft \" revert
.SS The \fC.literal\fP Directive
By default, \fCtconfpy\fP only permits statements it "recognizes" in the
configuration file. Anything else is flagged as an unrecognized
statement or "syntax error". However, it is possible to "embed"
arbitrary text in a configuration file and have \fCtconfpy\fP pass it back
to the calling program without comment by using the \fC.literal\fP
directive. It works like this:
.ft C \" Courier
.nf
.literal
This is literal text that will be passed back.
.endliteral
.fi
.ft \" revert
This tells \fCtconfpy\fP to ignore everything between \fC.literal\fP and \fC.endliteral\fP
and just pass it back to the calling program (in \fCretval.Literals\fP - see
previous section on the \fCtconfpy\fP API). Literal text is returned in the order
it is found in the configuration file.
What good is this? It is a nifty way to embed plain text or even
programs written in other languages within a configuration file and
pass them back to the calling program. This is especially handy
when used in combination with \fCtconfpy\fP conditional features:
.ft C \" Courier
.nf
.if [.PLATFORM] == posix
.literal
We're Running On A Unix-Like System
.endliteral
.else
.literal
We're Not Running On A Unix-Like System
.endliteral
.endif
.fi
.ft \" revert
In other words, we can use \fCtconfpy\fP as a "preprocessor" for other text or
computer languages. Obviously, the program has to be written to understand
whatever is returned as literal text.
By default, \fCtconfpy\fP leaves text within the literal block completely untouched.
It simply returns it as it finds it in the literal block. However, the
programmer can invoke \fCtconfpy\fP with an option (\fCLiteralVars=True\fP) that
allows
.B variable substitution within literal blocks.
This allows you to combine the results of your configuration into the
literal text that is returned to the calling program. Here is how it
works:
.ft C \" Courier
.nf
.ifall $USER
Greeting = Hello [$USER]. Welcome to [.MACHINENAME]!
.else
Greeting = Hello Random User. Welcome To Random Machine!
.endif
# Now embed the greeting in a C program
.literal
#include <stdio.h>
main()
{
printf("[Greeting]");
}
.endliteral
.fi
.ft \" revert
If the calling program sets \fCLiteralVars=True\fP, the literal block
will return a C program that prints the greeting defined at the top of
this example. If they use the default \fCLiteralVars=False\fP, the C
program would print \fC[Greeting]\fP.
In other words, it is possible to have your literal blocks make
reference to other configuration variables (and Predefined or
Environment Variables). This makes it convenient to
combine both configuration information for the program,
.B and
other, arbitrary textual information that the program may need,
all in a single configuration file.
Notice too that the \fC#\fP character can be freely included within a
literal block. You don't have to use a Reserved Variable reference
like \fC[HASH]\fP here because
.B everything
(including whitespace) inside a literal block is left untouched.
If you fail to provide a terminating \fC.endliteral\fP, the program will
treat everthing as literal until it reaches the end of the
configuration file. This will generate an appropriate warning, but
will work as you might expect. Everything from the \fC.literal\fP
directive forward will be treated literally. As a matter of good
style, you should always insert an explicit \fC.endliteral\fP, even if
it is at the end of file.
Placing an \fC.endliteral\fP in the configuration file without a
preceding \fC.literal\fP will also generate a warning message, and the
statement will be ignored.
.SS GOTCHAS
\fCtconfpy\fP is a "little language". It is purpose-built to do one
and only one thing well: process configuration options. Even
so, it is complex enough that there are a few things that can
"bite" you when writing these configuration files:
.IP \(bu 4
Probably the most common problem is attempting to do this:
.ft C \" Courier
.nf
foo = bar
.if foo == bar
...
.endif
.fi
.ft \" revert
But this will not work. \fCtconfpy\fP is very strict about requiring you to
explicitly distinguish between
.B variable names
and
.B variable references.
The example above checks to see if the string \fCfoo\fP equals
the string \fCbar\fP - which, of course, it never does.
What you probably want is to compare the value of variable \fCfoo\fP with
some string:
.ft C \" Courier
.nf
foo = bar
.if [foo] == bar
...
.endif
.fi
.ft \" revert
Now you're comparing the
.B value
of the variable \fCfoo\fP with the string \fCbar\fP.
This was done for a very good reason. Because you have to explicitly
note whether you want the name or value of a variable (instead of
having \fCtconfpy\fP infer it from context), you can mix both literal text and
variable values on either side of a comparison or assignment:
.ft C \" Courier
.nf
foo = bar
foo[foo]foo = bar # Means: foobarfoo = bar
.if foo[foo] == foobar # Means: .if foobar == foobar
.fi
.ft \" revert
.IP \(bu 4
Namespaces are a handy way to keep configuration options organized,
especially in large or complex configurations. However, you need to
keep track of the current namespace when doing things:
.ft C \" Courier
.nf
foo = bar
....
[NS-NEW]
.if [foo] == something # Checks value of NS-NEW.foo - will cause error
# since no such variable exists
.fi
.ft \" revert
.IP \(bu 4
Remember that "last assignment wins" when setting variable values:
.ft C \" Courier
.nf
myvar = 100
... a long configuration file
myvar = 200
.fi
.ft \" revert
At the end of all this, \fCmyvar\fP will be set to 200. This can be
especially annoying if you \fC.include\fP a configuration file after
you've set a value and the included file resets it. As a matter of
style, it's best to do all the \fC.include\fPs at the top of the master
configuration file so you won't get bitten by this one.
.IP \(bu 4
Remember that case matters. \fCFoo\fP, \fCfoo\fP, and \fCfoO\fP are all
different variable names.
.IP \(bu 4
Remember that all variable references are
.B string replacements
no matter what the type of the variable actually is. \fCtconfpy\fP
type and value enforcement is used to return the proper value and
type to the calling program. But within the actual processing
of a configuration file, variable references (i.e., the values of
variables) are always treated as
.B strings.
.IP \(bu 4
It is possible to load your own, user-defined, type in the variable
descriptor object when you pre-define a symbol table to be passed to
the parser. The problem is that this is more-or-less useless. The
parser attempts to coerce data assignments in the configuration into
the specified type. But, using only the assignment statements
available in this language, you cannot define values in a meaningful
way for user-defined types. So, assignment of user-defined variable
types will always fail with a type error. Again, \fCtconfpy\fP is designed as
a small configuration processing language, not as a general purpose
programming language. In short, user-defined types are not supported
in the variable descriptor processing and will always cause a type
error to occur.
.SH ADVANCED TOPICS FOR PROGRAMMERS
Here are some ideas on how you might combine \fCtconfpy\fP features
to enhance your own applications.
.SS Guaranteeing A Correct Base Configuration
While it is always nice to give users lots of "knobs" to turn, the
problem is that the more options you give them, the more they can
misconfigure a program. This is especially a problem when you are
doing technical support. You'd really like to get them to a
"standard" configuration and then work from there to help solve their
problem. If your write your program with this in mind, \fCtconfpy\fP gives
you several ways to easily do this:
.IP \(bu 4
Provide a "standard" system-, or even, enterprise-wide configuration
file for your application. This file presumably has all the program
options set to "sane" values. All the user has to do is create
a configuration file with one line in it:
.ft C \" Courier
.nf
.include /wherever/the/standard/config/file/is
.fi
.ft \" revert
.IP \(bu 4
Predefine every option variable the program will support. Populate the
initial symbol table passed to \fCParseConfig()\fP with these definitions.
By properly setting the \fCType\fP, \fCLegalVals\fP, and \fCMin/Max\fP for
each of these variables ahead of time, you can prevent the user from
ever entering option values that make no sense or are dangerous.
.IP \(bu 4
Make sure ever program option has a reasonable \fCDefault\fP value in
its variable descriptor. Recall that this attribute is provided for
the programmer's convenience. (When a variable descriptor is first
instantiated, it defaults to a string type and sets the default
attribute to an empty string. However, you can change both type and
default value under program control.) If you predefine a variable in the
initial symbol table passed to the parser, \fCtconfpy\fP will leave this
attribute alone. However, variables that are created for the first
time in the configuration file will have this attribute set to the
first value assigned to the variable. Now provide a "reset" feature
in your application. All it has to do is scan through the symbol
table and set each option to its default value.
.SS Enforcing Mandatory Configurations
The \fCtconfpy\fP type and value validation features give you a handy way to
enforce what the legal values for a particular option may be.
However, you may want to go further than this. For instance, you may
only want to give certain classes of users the ability to change
certain options. This is easily done. First, predefine all the
options of interest in the symbol table prior to calling the \fCtconfpy\fP
parser. Next, have your program decide which options the current user
is permitted to change. Finally, mark all the options they may not
change as "Read Only", by setting the "Writeable" attribute for those
options to \fCFalse\fP. Now call the parser.
This general approach allows you to write programs that support a
wide range of options which are enabled/disabled on a per-user,
per-machine, per-domain, per-ip, per-company... basis.
.SS Iterative Parsing
There may be situations where one "pass" through a configuration
file may not be enough. For example, your program may need to
read an initial configuration to decide how to further process
the remainder of a configuration file. Although it sounds
complicated, it is actually pretty easy to do. The idea
is to have the program set some variable that selects
which part of the configuration file to process, and then call
the parser. When the parser returns the symbol table, the program
examines the results, makes whatever adjustments to the symbol
table it needs to, and passes it back to the parser for another
"go". You can keep doing this as often as needed. For instance:
.ft C \" Courier
.nf
# Program calls the parser with PASS set to 1
.if [PASS] == 1
# Do 1st Pass Stuff
.endif
# Program examines the results of the first pass, does
# what is has to, and sets PASS to 2
.if [PASS] == 2
# Do 2nd Pass Stuff
.endif
# And so on
.fi
.ft \" revert
In fact, you can even make this iterative parsing "goal driven".
The program can keep calling the parser, modifing the results,
and calling the parser again until some "goal" is met. The goal
could be that a particular variable gets defined (like \fCCONFIGDONE\fP).
The goal might be that a variable is set to a particular value
(like, \fCSYSTEMS=3\fP).
It might even be tempting to keep parsing iteratively until \fCtconfpy\fP no
longer returns any errors. This is not recommended, though. A
well-formed configuration file should have no errors on any pass.
Iterating until \fCtconfpy\fP no longer detects errors makes it hard to
debug complex configuration files. It is tough to distinguish
actual configuration errors from errors would be resolved in a
future parsing pass.
.SH INSTALLATION
There are three ways to install \fCtconfpy\fP depending on your preferences
and type of system. In each of these installation methods you must be
logged in with root authority on Unix-like systems or as the
Administrator on Win32 systems.
.SS Preparation - Getting And Extracting The Package
For the first two installation methods, you must first download the
latest release from:
.ft C \" Courier
.nf
http://www.tundraware.com/Software/tconfpy/
.fi
.ft \" revert
Then unpack the contents by issuing the following command:
.ft C \" Courier
.nf
tar -xzvf py-tconfpy-X.XXX.tar.gz (where X.XXX is the version number)
.fi
.ft \" revert
Win32 users who do not have tar installed on their system can find
a Windows version of the program at:
.ft C \" Courier
.nf
http://unxutils.sourceforge.net/
.fi
.ft \" revert
.SS Install Method #1 - All Systems (Semi-Automated)
Enter the directory created in the unpacking step above. Then issue
the following command:
.ft C \" Courier
.nf
python setup.py install
.fi
.ft \" revert
This will install the \fCtconfpy\fP module and compile it.
You will manually have to copy the 'test-tc.py' program to a directory
somewhere in your executable path. Similarly, copy the documentation
files to locations appropriate for your system.
.SS Install Method #2 - All Systems (Manual)
Enter the directory created in the unpacking step above. Then,
manually copy the tconfpy.py file to a directory somewhere in your
PYTHONPATH. The recommended location for Unix-like systems is:
.ft C \" Courier
.nf
.../pythonX.Y/site-packages
.fi
.ft \" revert
For Win32 systems, the recommended location is:
.ft C \" Courier
.nf
...\\PythonX.Y\\lib\\site-packages
.fi
.ft \" revert
Where X.Y is the Python release number.
You can precompile the \fCtconfpy\fP module by starting Python interactively
and then issuing the command:
.ft C \" Courier
.nf
import tconfpy
.fi
.ft \" revert
Manually copy the 'test-tc.py' program to a directory
somewhere in your executable path. Copy the documentation
files to locations appropriate for your system.
.SS Install Method #3 - FreeBSD Only (Fully-Automated)
Make sure you are logged in as root, then:
.ft C \" Courier
.nf
cd /usr/ports/devel/py-tconfpy
make install
.fi
.ft \" revert
This is a fully-automated install that puts both code and
documentation where it belongs. After this command has completed
you'll find the license agreement and all the documentation (in the
various formats) in:
.ft C \" Courier
.nf
/usr/local/share/doc/py-tconfpy
.fi
.ft \" revert
The 'man' pages will have been properly installed so either of these
commands will work:
.ft C \" Courier
.nf
man tconfpy
man test-tc
.fi
.ft \" revert
.SS Bundling \fCtconfpy\fP With Your Own Programs
If you write a program that depends on \fCtconfpy\fP you'll need to ensure
that the end-users have it installed on their systems. There are two
ways to do this:
.IP \(bu 4
Tell them to download and install the package as described above.
This is not recommended since you cannot rely on the technical
ability of end users to do this correctly.
.IP \(bu 4
Just include 'tconfpy.py' in your program distribution directory.
This ensures that the module is available to your program
regardless of what the end-user system has installed.
.SH THE \fCtconfpy\fP MAILING LIST
TundraWare Inc. maintains a mailing list to help you with your
\fCtconfpy\fP questions and bug reports. To join the list, send email
to
.B majordomo@tundraware.com
with a single line of text in the body (not the Subject line)
of the message:
.ft C \" Courier
.nf
subscribe tconfpy-users your-email-address-goes-here
.fi
.ft \" revert
You will be notified when your subscription has been approved. You
will also receive detailed information about how to use the list,
access archives of previous messages, unsubscribe, and so on.
.SH OTHER
\fCtconfpy\fP requires Python 2.3 or later.
.SH BUGS AND MISFEATURES
None known as of this release.
.SH COPYRIGHT AND LICENSING
\fCtconfpy\fP is Copyright (c) \*(CP TundraWare Inc. For terms of use, see
the tconfpy-license.txt file in the program distribution. If you
install \fCtconfpy\fP on a FreeBSD system using the 'ports' mechanism, you
will also find this file in /usr/local/share/doc/py-tconfpy.
.SH AUTHOR
.ft C \" Courier
.nf
Tim Daneliuk
tconfpy@tundraware.com
.fi
.ft \" revert
.SH DOCUMENT REVISION INFORMATION
$Id: tconfpy.3,v 1.152 2005/01/19 08:31:33 tundra Exp $
tundra