[Python-ideas] PEP 572 version 2: Statement-Local Name Bindings

[Nick Coghlan]

On 25 March 2018 at 02:34, Eric Fahlgren <ericfahlgren at gmail.com> wrote:

> On Sat, Mar 24, 2018 at 7:14 AM, Nick Coghlan <ncoghlan at gmail.com> wrote:
>
>> ​​
>>     >>> class C:
>>     ...     sequence = range(10)
>>     ...     listcomp = [x for x in sequence]
>>
>
> >>> class C:
> ...     y = 1
> ...     sequence = range(10)
> ...     listcomp = [x+y for x in sequence] ​
> ...
> ​Traceback (most recent call last):
>   File "<stdin>", line 1, in <module>
>   File "<stdin>", line 4, in C
>   File "<stdin>", line 4, in <listcomp>
> NameError: name 'y' is not defined​
>
> ​Ok, so how does 'y' fit into these scoping rules?​
>

Everything except the outermost iterator is evaluated in the implicitly
nested scope, so comprehensions at class scope have the restriction that
only the outermost iterator can access names defined in the class scope. It
turned out that was enough to preserve compatibility with most of the
comprehensions that folks actually use at class scope.

For those rare cares where it isn't, the typical resolution is to either
define a helper function, or else switch to a regular for loop.

Cheers,
Nick.

-- 
Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia
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