Class attribute evaluation and generators

Question

How exactly does Python evaluate class attributes? I\'ve stumbled across an interesting quirk (in Python 2.5.2) that I\'d like explained.

I have a class with some a

Answer 1

Yeah, it's a bit dodgy, this. A class doesn't really introduce a new scope, it just sort of looks a little bit like it does; constructs like this expose the difference.

The idea is that when you're using a generator expression it's equivalent to doing it with a lambda:

class Brie(object):
    base= 2
    powers= map(lambda i: base**i, xrange(5))

or explicitly as a function statement:

class Brie(object):
    base= 2

    def __generatePowers():
        for i in xrange(5):
            yield base**i

    powers= list(__generatePowers())

In this case it's clear that base isn't in scope for __generatePowers; an exception results for both (unless you were unlucky enough to also have a base global, in which case you get a wrongness).

This doesn't happen for list comprehensions due to some internal details on how they're evaluated, however that behaviour goes away in Python 3 which will fail equally for both cases. Some discussion here.

A workaround can be had using a lambda with the same technique we relied on back in the bad old days before nested_scopes:

class Brie(object):
    base= 2
    powers= map(lambda i, base= base: base**i, xrange(5))

Answer 2

From PEP 289:

After exploring many possibilities, a consensus emerged that binding issues were hard to understand and that users should be strongly encouraged to use generator expressions inside functions that consume their arguments immediately. For more complex applications, full generator definitions are always superior in terms of being obvious about scope, lifetime, and binding [6].

[6] (1, 2) Patch discussion and alternative patches on Source Forge http://www.python.org/sf/872326

It's how generator expressions are scoped as far as I can make out.