Understanding variable scope in Python classes

Question

Can someone please explain why this will fail:

class A:
   a = 42
   b = list(a + i for i in range(10))

Answer 1

It fails because a isn't in a namespace that the generator expression can see.

This works for Python2 and Python3. The lambda with the default argument is a trick to get a reference of a in a namespace that the generator expression can see.

>>> class A:
...    a = 42
...    b = (lambda a=a:list(a + i for i in range(10)))()
... 
>>> A.b
[42, 43, 44, 45, 46, 47, 48, 49, 50, 51]

Using a list comprehension for b is clearer in my opinion

...    b = (lambda a=a:[a + i for i in range(10)])()