function prototyping?

[Peter Otten]

Burton Samograd wrote:

> Duncan Booth <duncan.booth at invalid.invalid> writes:
> 
>> Burton Samograd wrote:
>> > Is there any way to 'prototype' functions in python, as you would in
>> > C?  Would that be what the 'global' keyword is for, or is there a more
>> > elegant or 'pythonic' way of doing forward references?
>> > 
>> There isn't really such a thing as a forward reference in Python. Always
>> remember that 'def' and 'class' are executable statements:
> 
> Ok, we'll here's what I'm trying to do.  I have a dictionary that I
> would like to initialize in a module file config.py:
> 
> -- config.py -------------------------
> global a_fun, b_fun
> dict = {
> 'a': a_fun,
> 'b': b_fun
> }
> --------------------------------------
> 
> where a_fun and b_fun are in fun.py:
> 
> -- fun.py ----------------------------
> def a_fun(): pass
> def b_fun(): pass
> 
> import config
> def main():
>         config.dict['a']()
>         config.dict['b']()
> main()
> --------------------------------------
> 
> I like having the module/namespace seperation with the configuration
> variables but I would like to make them easily (re)defined in the
> configuration file by the user.  Does python have the idea of a 'weak'
> reference or lazy style evaluation for the definition of the dict in
> the config file above so I can achive what i'm tryin to do?

I'd say Python has *only* that idea, but as a practical approach the
following might be easiest:

-- config.py --
a = a_fun # As a_fun is nowhere defined, config.py cannot be
          # used stand-alone and may be hard to test.
b = b_fun
def c():
    print "c-fun"

-- fun.py --
def a_fun(): print "a-fun"
def b_fun(): print "b-fun"

execfile("config.py") # Think #include <config.py>

def main():
    a()
    b()
    c()

if __name__ == "__main__":
    main()

A slightly stricter variant avoids cycles by using three modules:

-- fun.py --
def a_fun(): print "a-fun"
def b_fun(): print "b-fun"

-- config.py --
import fun

a = fun.a_fun
b = fun.b_fun

-- main.py --
import config

def main():
    config.a()
    config.b()

if __name__ == "__main__":
    main()

Thanks to the if... guard you could put the main.py code into fun.py, too,
but I suppose it's easier to understand with three files.

Peter