PEP 255: Simple Generators, Revised Posting

[Tim Peters]

[Jeremy Hylton]
> Actually, they are in the parse tree.

Yikes!  That's what I get for reading the code instead of typing what I
thought was the case <wink>.

> The compiler ignores "if 0" blocks by explicitly calling
> is_constant_false() in com_if_stmt and in the if_stmt case of
> symtable_node().  So it's easy enough to examine that part of the tree.

> ...
> This would be easy to fix in the case of yield for generators, except
> that I'm not sure it's easy to fix for names in general.  There are
> lots of other strange possibilities:
>
> def f():
>     if 0:
>         exec foo
>     def g(y):
>         return x + y
>
> Should this get a SyntaxError about the bare exec making resolution
> of free variable x ambiguous?

Yes.

> def g(x):
>     if 0:
>         global x
>
> Should this get a SyntaxError about an argument being declared global?

Yes.

The answers can only be "no" if you think of "if 0:" as a C "#if 0"
preprocessor gimmick.  The specific value after a Python "if" should have no
effect on statically-determined properties of Python code.

> I'm inclined not to worry about them, because they'd be a PITA to
> deal with and the "right" choice is far from obvious

Agreed it's a PITA, but disagree the right outcomes are unclear:  change "0"
to "42" or "hmm" and the answers are obvious, yes?

> -- unless Guido wants to offer some pronouncements on just what
> the heck they mean.

I may not be able to read our code, but I can still channel Guido <wink>.