Varsub.py

Varsub.py is a python script to define and use variables in arbitrary files, also known as string interpolation with evaluation of embedded expressions (for a different approach see see this recipe) . It evolved from the wish to use easy recognizable names for colours in CSS stylesheets so its use is probably best illustrated with a CSS example.

Given a file like this: 

/* meaningfull names for often used colors
   $(themebackground)=#eeeeee
   $(thememain)=#dd9999
*/

h2 { background-color:$theme-background;
     color:$theme-main}
Varsub.py will produce this: 
/* meaningfull names for often used colors
   $(themebackground)='#eeeeee'
   $(thememain)='#dd9999'
*/
      
h2 { background-color:#eeeeee;
     color:#dd9999}
This way it it is quite easy to refer to colors or other entities by meaningfull names and changing a value is simpler and less error prone as well: changing the definition of a variable in a single place is all there is to it.

Expressions

Varsub.py is capable of far more than illustrated in the previous example: the value that is assigned to a variable in its definition is actually evaluated as a python expression. So almost anything is possible even refering to previously defined variables: 

$a=40+2
$b=2
$c="a + b"
$(d)=1*1*1=1

$a times $b equals $c .... $d
will produce 
$a=40+2
$b=2
$c="a + b"
$(d)=1*1*1=1

42 times 2 equals 44 .... 1*1*1=1
Note the use of quotes when the expression consists of words seperated by whitespace. The use of parentheses in the definition of $d prevents the right hand side portion from being evaluated.

Usage

Varsub.py is a python module and its primary use is as a reusable component in other programs, but a simple interface for commandline use is provided as well. 

Usage: Varsub.py [options] [[-]file1 [[-]file2 ...]]
    
    Filenames starting with a minussign are read but
    produce no output. If such filenames are used,
    preceed the list of filenames with '--' to
    signal the end-of-options.
     
    Example: Varsub.py -- -constants.def style.def > style.css

Options:
  -h, --help            show this help message and exit
  -o FILE, --outfile=FILE
                        print to FILE instead to stdout
  -t, --test            run testsuite and then abort

Installation

Installation is quite simple:

You now should be able to test it with:

python -m Varsub -t

Installation is so simple that I didn't bother to write a setup script. It is tested on python 2.6.2. and 3.0 and released as free software under GPL.

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