[Python-3000] Change to class construction? (original) (raw)

Guido van Rossum guido at python.org
Sat Jul 7 01:41:16 CEST 2007

On 7/7/07, Phillip J. Eby <pje at telecommunity.com> wrote:

At 12:32 AM 7/7/2007 +0200, Guido van Rossum wrote: >On 7/6/07, Phillip J. Eby <pje at telecommunity.com> wrote: > > At 05:00 PM 7/6/2007 +0200, Georg Brandl wrote: > > >Collin Winter schrieb: > > > > While experimenting with porting setuptools to py3k (as of r56155), I > > > > ran into this situation: > > > > > > > > class C: > > > > a = (4, 5) > > > > b = [c for c in range(2) if a] > > > > > > > > results in a "NameError: global name 'a' is not defined" error, while > > > > > > > > class C: > > > > a = (4, 5) > > > > b = [c for c in a] > > > > > > > > works fine. > > > > This looks like a bug to me. A list comprehension's local scope > > should be the locals of the enclosing code, even if its loop indexes > > aren't exposed to that scope. > >It's because the class scope is not made available to the methods.

The examples are in the class body, not in methods. The code is statically initializing the class contents, so using C.a isn't possible.

Understood, but a generator expression (and hence in 3.0 also a list comprehension) is treated the same as a method body.

I suppose it can be worked around by moving the static initialization code outside the class body; it's just not obvious why it happens.

Collin, where did you find this code in setuptools, btw? I've been looking around at other packages of mine where static class initialization uses data structures like this, and I haven't found any place where anything but the "in" clause of a comprehension depends on class-scope variables. So, if setuptools is the only one of my libraries that does this, I'd have to agree with Guido that it is indeed quite rare. :) If I had to hazard a guess, I'd guess that it's in one of the setuptools command classes that subclasses a distutils command, and proceeds to muck around with the original options in some fashion. I just don't want to check all of them if you know which one it is. :)

-- --Guido van Rossum (home page: http://www.python.org/~guido/)

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