Python: Weird "nameerror: Name ... Is Not Defined" In An 'exec' Environment

I was confident to have at least some basic understanding of Python's scope system. Now I get an error and unfortunately I'm not even able to write a good code snippet for reproduc

Solution 1:

This is just a guess, because you haven't shown us enough code, and what you've shown us doesn't actually reproduce the problem, but…

If you're doing this exec inside a function, then locals() and globals() are going to be different. In which case the code will be executed as if it were inside a class definition. So, it'll be (sort of) as if you did this:

class_:
    from x import X
    classY(X): # does not crash here, ...def__init__(self):
            X.__init__(self) # ... but here
    foo=Y()
del _

(I previously thought you'd have to also be doing something like Y() outside the exec, but user2357112's answer convinced me that isn't necessary.)

If that's your problem, you may be able to fix it by just calling exec(code, globals(), globals()) or exec(code, locals(), locals()). (Which one is appropriate, if either, depends on what you're actually trying to do, of course, which you haven't told us.)

Solution 2:

From the exec documentation:

If exec gets two separate objects as globals and locals, the code will be executed as if it were embedded in a class definition.

There are good reasons for this, which I won't go into here.

Functions defined in a class definition don't look in the scope of the class definition for variable resolution. When you exec your code, it's actually executed like this:

classDummy:
    from x import X
    ...
    classY(X):
        def__init__(self):
            X.__init__(self)
            ...
    foo=Y()

That means this function:

def__init__(self):
    X.__init__(self)

doesn't see this variable:

from x import X

even though this bit:

class Y(X):

does see it.