Introduction to pyexpander

Macro substitution

Note: The examples below use “”, the python 3 implementation of pyexpander. You have to call “” if you installed the python 2 implementation of pyexpander.

For some projects there is the need to replace macros in a text file with values defined in other files or the command line. There are already other macro replacement tools that can do this but if you want to calculations or string operations and insert the results of this into your file, the list of possible tools becomes much shorter.

Pyexpander combines the python programming language with a simple macro replacement scheme in order to give the user both, ease of use and the power of a full featured scripting language.

Even without being familiar with python you can use pyexpander to perform calculations or string operations and use the results in your macro replacements.

Here is a very simple example, this is the content of file “letter.txt”:

Dear $(salutation) $(surname),

this is a simple pyexpander example.

Applying this command: --eval 'salutation="Mr";surname="Smith"' -f letter.txt

gives this result:

Dear Mr Smith,

this is a simple pyexpander example.

Here is more advanced example:

$py(start=0; end=5)\
 x |  x**2
$for(x in range(start,end+1))\
$("%2d | %3d" % (x,x*x))

Applying to a file with the content shown above gives the following result:

 x |  x**2
 0 |   0
 1 |   1
 2 |   4
 3 |   9
 4 |  16
 5 |  25

And here we show how pyexpander compares with the well known m4 macro processor. We have taken the m4 example with small modifications from Wikipedia:

# This starts the count at ONE as the incr is a preincrement.
define(`H2_COUNT', 0)
# The H2_COUNT macro is redefined every time the H2 macro is used.
        `define(`H2_COUNT', incr(H2_COUNT))<h2>H2_COUNT. $1</h2>')
divert(0)dnl Diversion to 0 means back to normal. dnl macro removes this line.
H2(First Section)
H2(Second Section)

Here is the same example formulated in pyexpander:

# This starts the count at ONE as the incr is a preincrement.
# H2_COUNT is incremented each time H2 is called.
def H2(st):
    global H2_COUNT
    return "<h2>%d. %s</h2>" % (H2_COUNT,st)
$# the following makes H2 callable without another pair of enclosing brackets:
$H2("First Section")
$H2("Second Section")

Both produce this output:

<h2>1. First Section</h2>
<h2>2. Second Section</h2>
<h2>3. Conclusion</h2>

The advantages of pyexpander are:

  • simple syntax definition, all expander commands start with a dollar (“$”) sign followed by word characters, parameters or python code enclosed in brackets or both.
  • the full power of the python programming language can be used, all operators, functions and modules.
  • any python expression can be used to insert text.
  • There is also a python library,, which you can use to develop other macro tools based on pyexpander.

If you are not familiar with the python programming language, you should have a look at python 3 introduction for pyexpander.

If you intend to use the python 2 version of pyexpander, you should look at python 2 introduction for pyexpander.

For a detailed description of the pyexpander language see reference.