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

[Kirill Balunov]

2018-03-24 17:14 GMT+03:00 Nick Coghlan <ncoghlan at gmail.com>:
>
>
> They can be distinguished, just not at module or function scope. To give a
> concrete example:
>
> ==========
>     >>> class C:
>     ...     sequence = range(10)
>     ...     listcomp = [x for x in sequence]
>     ...     def works(data):
>     ...         return list(data)
>     ...     from_works = works(sequence)
>     ...     def fails():
>     ...         return list(sequence)
>     ...
>     >>> C.listcomp
>     [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
>     >>> C.from_works
>     [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
>     >>> C.fails()
>     Traceback (most recent call last):
>       File "<stdin>", line 1, in <module>
>       File "<stdin>", line 8, in fails
>     NameError: name 'sequence' is not defined
> ==========
>
> I think I did have an implementation that didn't do the "outermost
> iterator is evaluated in the outer scope" dance early on (back when I was
> still working on the initial version of the iteration variable hiding,
> before it was merged to the Py3k branch), but quickly changed it because
> relying solely on closures broke too much code (in addition to the class
> namespace case, you can create a situation with similar constraints by
> passing a separate locals namespace to exec).
>
> Cheers,
> Nick.
>

But is the example with the class appropriate in this context, it seems
that it is well understood why this example will fail, and  `C.fails()`
should at least be a `@classmethod`:) From the docs:

> Class definition blocks and arguments to exec()
> <https://docs.python.org/3/library/functions.html#exec> and eval()
> <https://docs.python.org/3/library/functions.html#eval> are special in
> the context of name resolution. A class definition is an executable
> statement that may use and define names. These references follow the normal
> rules for name resolution with an exception that unbound local variables
> are looked up in the global namespace. The namespace of the class
> definition becomes the attribute dictionary of the class. The scope of
> names defined in a class block is limited to the class block; it does not
> extend to the code blocks of methods – this includes comprehensions and
> generator expressions since they are implemented using a function scope.

And if we cross out class, exec and eval cases - they don't work now, they
will not be affected in future. Are not these examples equivalent?

With kind regards,
-gdg
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