Why do I get this NameError in a generator within a Python class definition?

Question

In Python 3.5.0 this code:

a = (1,2)
class Foo(object):
    b = (3,4)
    c = tuple((i,j) for j in b for i in a)
    d = tuple((i,j) for i in a for j in b)

produces:

Traceback (most recent call last):
  File "genexprtest.py", line 2, in <module>
    class Foo(object):
  File "genexprtest.py", line 5, in Foo
    d = tuple((i,j) for i in a for j in b)
  File "genexprtest.py", line 5, in <genexpr>
    d = tuple((i,j) for i in a for j in b)
NameError: name 'b' is not defined

Why do I get this error? And why do I not get this error on the previous line?

Answer 1

This is because the expression for i in a has a local variable scope, and expression for j in b is inside the scope, thus, no b is found.
Actually, if you write c = tuple((i, j) for i in a for j in b), it will throw the same exception.

The solution is put b into scope of class definition (as you already did) and refer it by self.b.

Answer 2

I spent ages experimenting and I have a theory about why you're getting this error. I'm not certain but this does explain why it works for c and not for d. I hope this helps you, comment if you disagree :)

def Tuple(this):
    print(a) # this always works
    try:
        print(b) # this always gives an error
    except NameError:
        print("...b is not defined")
    try:
        return tuple(this) # this only gives an error for d and e
    except NameError:
        print("...couldn't make it a tuple")


a = (1,2)     
class Foo(object):
    b = (3,4)
    c = Tuple((i,j) for j in b for i in a)
    d = Tuple((i,j) for i in a for j in b)
    e = Tuple((i,j,k) for i in a for j in b for k in (5, 6))
    f = Tuple((i,j,k) for j in b for i in (5, 6) for k in a)

    print("\nc:", c,"\nd:", d,"\ne:", e,"\nf:", f)

What happened: every time I called the Tuple() function, b was not defined, but a was always defined. This explains why you get an error for d and e but it doesn't explain why c and f work even though b is 'not defined'

My theory: The first for loop is calculated before the whole thing is converted into a tuple. For example, if you tried to do this: Tuple((a, b, c) for a in loop1, for b in loop2 for c in loop3), in the Foo class it would calculate for a in loop1 first, then it would move to the foo and calculate the loops 2 and 3.

In summary:

1. does first for loop
2. moves to tuple function
3. does the remaining loops
4. the error occurs if a variable in the 2nd or 3rd loop is in class Foo