The following code throws a NameError which seems to be a bug existing
since Python 2.4 up to the current 2.5.2.

class A:
    a = 'test'
    [c for c in a]
    (c for c in a)
    tuple(c for c in a)
    [c for c in a if c in a]
    (c for c in a if c in a)
    tuple(c for c in a if c in a) # --> NameError

This may seem odd, but is correct as per spec.

The "if c in a" part is executed in a scope of its own, and class scopes
don't contribute to nested scoping.

Thanks for the quick explanation. I understand that class scopes don't
extend and this is documented behavior.

However, the question is why the if clause is executed in a scope of its
own and where this is documented.

You would expect that the problematic generator expression is equivalent
to the following code which works fine:

class A:
    a = 'test'
    def __g(a):
        for c in a:
            if c in a:
                yield c
    tuple(__g(a))

The actual equivalent would be

class A:
    a = 'test'
    def __g(_x):
        for c in _x:
            if c in a:
                yield c
    tuple(__g(a))

i.e. the outmost iterator is evaluated in the enclosing scope; not the
if clause is in its own scope, but the whole genexp.

Thanks, this now makes sense to me. You're right, it's rather an ugly
wart than a bug.

But I think the Python reference needs to be improved to make this clear
enough.

How about the following proposed addtions (in square brackets) to
section 5.2.5 (http://docs.python.org/ref/genexpr.html):

"Variables used in the generator expression are evaluated lazily [in the
scope of the generator function] when the next() method is called for
[the] generator object (in the same fashion as normal generators).
However, the leftmost for clause is immediately evaluated [in the
current scope] so that [an] error produced by it can be seen before any
other possible error in the code that handles the generator expression.
Subsequent for [and if] clauses cannot be evaluated immediately since
they may depend on the previous for loop."

Good idea! I've committed these changes in r61212.