Macro expansion: intercept statement interpretation

Caleb Hattingh caleb1 at telkomsa.net
Wed Aug 25 04:56:20 CEST 2004


Hi

Here is a script I want to be able to write (explanation appears after):

*** start of script ***

import MyCustomMacroLib	 # This does the magic I would like help for.

# This is not python, but the module imported above
# will use this block internally and prevent it
# getting to the interpreter.
defmacro MyMacro1:
   form = """for <#1> := <#2> to <#3> do begin
               <#4multiline>
             end"""
   subsituteWith = """for <#1> in range(<#2>,<#3>+1):
                        <#4multiline>"""

for i := 0 to 3 do begin # This is not python syntax, but the module
   print i                # imported above makes it so that this
   print 'hello'          # is first checked against defined macros
end                      # and substitutions are made if syntax
                          # matches
""" Should give:
0
hello
1
hello
2
hello
3
hello
"""

*** end of script ***

I want the module "MyCustomMacroLib" to set up something where I can have  
write code that checks each of the statements of this script, and does  
macro expansion before passing the result to the python interpreter.

When the (undefined in python) "defmacro" statement is encountered, the  
module sets this up in memory as some kind of "macro" object, and as the  
remainder of the lines is parsed, a syntax check is made to see if a macro  
fits, and if so the expansion is performed and the result is passed to the  
interpreter/compiler.

This particular (useless) example sets up a macro where loops can be  
written in objectpascal form.  I actually have no particular need for this  
kind of functionality (I think?), but it looks cool!.  I got interested  
after reading a couple of Paul Grahams essays about lisp.  I also suspect  
there actually may be some real problem domains where extending the  
language syntax may be beneficial - I just cannot think of any right now :)

I can figure out the minor practical details of things like handling  
indention, code objects and the like myself, but I don't know where to  
start to try and intercept statements before they get to the interpreter.   
Some of the docs I read today appear to indicate that there may also be  
differences in the way interactive interpreter and command-line execution  
deal with statements.  I am lost in the python internals!

Note that I could easily write a script that takes another script as an  
argument, preprocesses it, and evaluates the resulting list of python  
statments.  I do however, prefer a drop-in module as shown above.  Plus, I  
would also like to do this dynamically in the interpreter.

Any ideas, or is this impossible? (gratuitous guru-bait)

Thanks
Caleb




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