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))