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To: python-list@python.org
From: Terry Reedy <tjreedy@udel.edu>
Subject: Re: Generarl programming question.
Date: Sat, 11 Apr 2015 17:12:14 -0400
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On 4/11/2015 3:19 PM, Thomas 'PointedEars' Lahn wrote:
> Terry Reedy wrote:
>
>> On 4/11/2015 12:23 PM, Thomas 'PointedEars' Lahn wrote:
>>> Chris Angelico wrote:
>>>> The 'x' inside each function is completely separate, no matter how
>>>> many times they get called. They're usually stored on something call=
ed
>>>> a "call stack" - you put another sheet of paper on top of the stack
>>>> every time you call a function, local variables are all written on
>>>> that paper, and when you return from a function, you discard the top=

>>>> sheet and see what's underneath.
>>>
>>> Thank you for that description; I shall use it from now on when teach=
ing
>>> laymen about the call stack.
>>
>> What Chris is describing is one local namespace (sheet of paper) per
>> function *call*.

> I *know* what he is describing: the *call* stack.

My comment above was directed not at you specifically but at the OP,=20
Jonas, who appears to have had a mental model (like the following) in=20
which recursion is not possible.  I think this mental model is fairly=20
common among programming newbies.  And it is not crazy, just obsolete=20
and superceded.  And, we constantly talk about a function's local names, =

which is correct, without constantly adding the caveat that in Python=20
(and most modern languages) they are instanced per call.

I think of functions as being something like a class, in that each call=20
gives a new instance with a new set of named values.

>> In early Fortran (at least the first version I used),
>> there was one local namespace (sheet) per *function*.
>
> The names in such namespaces are now called static variables.  AFAIK, P=
ython
> does not have them, but PHP, for example, has:
>
>    function foo ()
>    {
>      static $bar =3D 1;
>      $bar *=3D 2;
>      return $bar;
>    }
>
> The variable $bar then keeps its last value for subsequent calls of foo=
().

In Python, one can do something similar with attributes, except that=20
attributes are easily accessible from outside the function.  Mutable=20
defaults probably come closer.

def doubler(_val=3D[1])"
   _val[0] *=3D 2
   return _val[0]

print(doubler(), doubler(), doubler())
# 2 4 8

>> The call stack was a stack of (pointers to) functions.
>
> It would appear that the commonly used definition of =E2=80=9Ccall stac=
k=E2=80=9D has
> considerably changed since then, since I have been programming computer=
s for
> more than two decades now (not including FORTRAN, though) and never hea=
rd of
> your definition before.

I don't know what the stack required for returns was called in Fortran=20
or how it was implemented in any particular compiler.

>> It has been proposed that Python use a hybrid model.  Function objects=

>
> Interesting.  I did not know that functions are objects in Python, too.=


In Python, everything you can bind a name to is an object, and in 3.x,=20
an instance of the base class 'object'.

--=20
Terry Jan Reedy