Need help with Python scoping rules

Jan Kaliszewski zuo at chopin.edu.pl
Wed Aug 26 23:54:16 CEST 2009


26-08-2009 o 17:45:54 kj <no.email at please.post> wrote:

> In <02a54597$0$20629$c3e8da3 at news.astraweb.com> Steven D'Aprano  
> <steve at REMOVE-THIS-cybersource.com.au> writes:
>
>> On Wed, 26 Aug 2009 10:57:32 +0000, kj wrote:
>
>>> Recursion!  One of the central concepts in the theory of
>>> functions!  This is shown most clearly by the following elaboration of
>>> my original example:
>>>
>>> class Demo(object):
>>>     def fact_rec(n):
>>>         if n < 2:
>>>             return 1
>>>         else:
>>>             return n * fact_rec(n - 1)
>
>> Why are you defining a method without a self parameter?
>
> Because, as I've explained elsewhere, it is not a method: it's a
> "helper" function, meant to be called only once, within the class
> statement itself.
>
> Well, this is not strictly true, because the function is recursive,
> so it will also call itself a few times.  But still, all these
> calls happen (loosely speaking) "within the class statement".
>
> In fact the only reason to use a function for such initialization
> work is when one needs recursion; otherwise there's no point in
> defining a function that will only be invoked exactly once.

1. I don't understand then... Why do you desire to both define and
run it *within* that class statement as if it was this class's
method?

2. Could you please show me how it could be done in C++ or Java?
(Or you want to say that that languages also are not fully valuable
OO languages?)

3. Python makes function bodies "agnostic" about the context of their
definition -- generally any non-global information must be passed
explicitly to their interior. *It has nothing to do with recursion.*

If you really must both define and use such a function within the
class definition, pass function object to itself explicitly, and
everybody will be happy:


     class Demo(object):

         def fact(fact, n):
             if n < 2:
                 return 1
             else:
                 return n * fact(fact, n - 1)

         fact(fact, 3)


*j

-- 
Jan Kaliszewski (zuo) <zuo at chopin.edu.pl>



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