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Subject: Re: global and loop control variable
To: python-list@python.org
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From: Lorenzo Sutton <lorenzofsutton@gmail.com>
Date: Thu, 23 Jul 2015 15:58:55 +0200
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On 23/07/2015 14:31, Steven D'Aprano wrote:
> On Thu, 23 Jul 2015 09:20 pm, Lorenzo Sutton wrote:
>
>> On 23/07/2015 12:24, candide wrote:
>>> Now, global declaration has another restriction, as PLR explains:
>>>
> [https://docs.python.org/3.4/reference/simple_stmts.html#the-global-statement]
>>> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>>> Names listed in a global statement must not be defined as formal
>>> parameters or in a for loop control target,
>>> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>>>
>>> What I understand is that the following is a must-not-code:
>>>
>>> def f():
>>>       global i
>>>       for i in range(1,3):
>>>           print(10*i)
> [...]
>>> So my question is: what is the restriction about global as loop control
>>> variable the docs is referring to?
>
> You are correct. The above example is exactly the restriction mentions. The
> very next paragraph in the docs says:
>
> "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."
>
> In other words, the behaviour of global loop variables is not guaranteed,
> and you should not use it even if the compiler/interpreter fails to raise a
> syntax error.
>
>
>> I think for situations like this one?
>>
>> def f():
>>       global temperature
>>       for temperature in range(1,3):
>>           print "In f temperature is:", temperature
>
>
> There's no meaningful difference between the example Candide gave (for i in
> range) and the example you give (for temperature in range). They both use a
> global for the loop variable. Only the names differ.

Of course... it was just to highlight that it could be potentially, 
especially if your programme is going to launch a rocket - eventually 
(see my entire code example) :-)

Lorenzo.