Why is there a NameError in this list comprehension?

Question

Could you please explain, why this code works in Python 3.6

class B:
    a = 'a_val'
    b = {'a': 'a_val: {val}'.format(val=a)}

but this doesn't work?

class C:
    a = 'a_val'
    b = {i: 'a_val: {val}'.format(val=a) for i in range(0,2)}

NameError: name 'a' is not defined

and both classes work in Python 2.6.