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Re: scope of function parameters

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On May 29, 1:30 pm, Henry Olders <henry.old...@mcgill.ca> wrote:
> I just spent a considerable amount of time and effort debugging a program. The made-up code snippet below illustrates the problem I encountered:
>
> def main():
>         a = ['a list','with','three elements']
>         print a
>         print fnc1(a)
>         print a
>
> def fnc1(b):
>         return fnc2(b)
>
> def fnc2(c):
>         c[1] = 'having'
>         return c
>
> This is the output:
> ['a list', 'with', 'three elements']
> ['a list', 'having', 'three elements']
> ['a list', 'having', 'three elements']
>
> I had expected the third print statement to give the same output as the first, but variable a had been changed by changing variable c in fnc2.
>
> It seems that in Python, a variable inside a function is global unless it's assigned. This rule has apparently been adopted in order to reduce clutter by not having to have global declarations all over the place.
>
> I would have thought that a function parameter would automatically be considered local to the function. It doesn't make sense to me to pass a global to a function as a parameter.
>
> One workaround is to call a function with a copy of the list, eg in fnc1 I would have the statement "return fnc2(b[:]". But this seems ugly.
>
> Are there others who feel as I do that a function parameter should always be local to the function? Or am I missing something here?
>
> Henry

You want a functional language.
You can simulate that in python by using tuples in place of lists.

fnc2(c):
  c[1] = 'having'
  return c
will of course then give you an error that tuples are not assignable
(which seems to be what you want?)

So you then use (something like)

fnc2(c):  return c[0:1] + c[2:]

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scope of function parameters Henry Olders <henry.olders@mcgill.ca> - 2011-05-29 04:30 -0400
  Re: scope of function parameters Mel <mwilson@the-wire.com> - 2011-05-29 07:59 -0400
    Re: scope of function parameters Terry Reedy <tjreedy@udel.edu> - 2011-05-29 16:27 -0400
  Re: scope of function parameters Peter Pearson <ppearson@nowhere.invalid> - 2011-05-29 17:21 +0000
    Re: scope of function parameters Ben Finney <ben+python@benfinney.id.au> - 2011-05-30 07:42 +1000
      Re: scope of function parameters Laurent Claessens <moky.math@gmail.com> - 2011-05-30 09:12 +0200
        Re: scope of function parameters Chris Rebert <clp2@rebertia.com> - 2011-05-30 00:27 -0700
        Re: scope of function parameters Laurent <moky.math@gmail.com> - 2011-05-30 09:38 +0200
        Re: scope of function parameters Daniel Kluev <dan.kluev@gmail.com> - 2011-05-30 19:10 +1100
        Re: scope of function parameters Terry Reedy <tjreedy@udel.edu> - 2011-05-30 05:02 -0400
          Re: scope of function parameters Laurent Claessens <moky.math@gmail.com> - 2011-05-30 11:08 +0200
            Re: scope of function parameters Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2011-05-30 09:14 +0000
            Re: scope of function parameters Ben Finney <ben+python@benfinney.id.au> - 2011-05-30 19:17 +1000
            Re: scope of function parameters Peter Otten <__peter__@web.de> - 2011-05-30 11:41 +0200
              Re: scope of function parameters Laurent Claessens <moky.math@gmail.com> - 2011-05-30 12:10 +0200
              Re: scope of function parameters Laurent Claessens <moky.math@gmail.com> - 2011-05-30 12:10 +0200
            Re: scope of function parameters Jussi Piitulainen <jpiitula@ling.helsinki.fi> - 2011-05-30 13:40 +0300
            Re: scope of function parameters Terry Reedy <tjreedy@udel.edu> - 2011-05-30 12:49 -0400
        Re: scope of function parameters Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2011-05-30 09:16 +0000
  Re: scope of function parameters Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2011-05-29 23:46 +0000
    Re: scope of function parameters Ben Finney <ben+python@benfinney.id.au> - 2011-05-30 11:31 +1000
      Re: scope of function parameters Chris Angelico <rosuav@gmail.com> - 2011-05-30 11:56 +1000
        Re: scope of function parameters Ben Finney <ben+python@benfinney.id.au> - 2011-05-30 12:08 +1000
          Re: scope of function parameters Chris Angelico <rosuav@gmail.com> - 2011-05-30 12:37 +1000
      Re: scope of function parameters Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2011-05-30 04:23 +0000
  Re: scope of function parameters rusi <rustompmody@gmail.com> - 2011-05-31 09:46 -0700
    Re: scope of function parameters rusi <rustompmody@gmail.com> - 2011-05-31 10:33 -0700

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