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Date: Fri, 5 Dec 2014 23:48:24 +1100
Subject: Re: The binding operator, and what gets bound to what
From: Chris Angelico <rosuav@gmail.com>
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On Fri, Dec 5, 2014 at 11:34 PM, Steven D'Aprano
<steve+comp.lang.python@pearwood.info> wrote:
>> I don't think this is implementation-dependent.
>
> The docs say that it is:
>
> https://docs.python.org/3/reference/simple_stmts.html#the-global-statement
>
>     Names listed in a global statement MUST NOT [emphasis added] be
>     defined as formal parameters or in a for loop control target,
>     class definition, function definition, or import statement.
>
>     CPython implementation detail: The current implementation does not
>     enforce the two restrictions, but programs should not abuse this
>     freedom, as future implementations may enforce them or silently
>     change the meaning of the program.

Interesting, I didn't know that either.

But the first restriction _is_ enforced:

rosuav@sikorsky:~$ python3
Python 3.5.0a0 (default:23ab1197df0b, Nov 20 2014, 12:57:44)
[GCC 4.7.2] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> def x(y):
...     global y
...
  File "<stdin>", line 2
SyntaxError: name 'y' is parameter and global

Likewise in CPython 2.7:

rosuav@sikorsky:~$ python
Python 2.7.3 (default, Mar 13 2014, 11:03:55)
[GCC 4.7.2] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> def x(y):
...     global y
...
  File "<stdin>", line 1
SyntaxError: name 'y' is local and global

ChrisA