baffled classes within a function namespace. Evaluation order.

[Steven D'Aprano]

On Fri, 26 Apr 2013 07:39:32 +0200, Peter Otten wrote:

> A class body is basically a function body that is executed once. To
> allow an assignment
> 
>>>> x = 42
>>>> class A:
> ...     x = x
> ...
> 
> which is not possible inside a function


As far as I can tell, the documentation says that this assignment should 
also be impossible inside a class. Unless I'm missing something, I think 
this is a bug.

http://docs.python.org/3/reference/executionmodel.html


[...]
> However, while the above gives some technical background it doesn't
> explain why this scheme was chosen. My guess is that when Python got
> closures nobody was willing to do the extra work to make class bodies
> namespace-aware. The alternative, disallowing x = x in classes, would
> have seriously broken backwards-compatibility.

Not so. x = x was not allowed in classes before there were closures:


[steve at ando ~]$ python1.5
Python 1.5.2 (#1, Aug 27 2012, 09:09:18)  [GCC 4.1.2 20080704 (Red Hat 
4.1.2-52)] on linux2
Copyright 1991-1995 Stichting Mathematisch Centrum, Amsterdam
>>>
>>>
>>> def f():
...     x = 1
...     class Test:
...             x = x
...     return Test
...
>>> T = f()
Traceback (innermost last):
  File "<stdin>", line 1, in ?
  File "<stdin>", line 3, in f
  File "<stdin>", line 4, in Test
NameError: x



-- 
Steven