python - Variable Scope In Generators In Classes

Question

I think that I know how variables and generators work in Python well.
However, the following code makes me confused.

from __future__ import print_function

class A(object):
    x = 4
    gen = (x for _ in range(3))

a = A()
print(list(a.gen))

When run the code （Python 2), it says:

Traceback (most recent call last):
  File "Untitled 8.py", line 10, in <module>
    print(list(a.gen))
  File "Untitled 8.py", line 6, in <genexpr>
    gen = (x for _ in range(3))
NameError: global name 'x' is not defined

In Python 3, it says NameError: name 'x' is not defined
but, when I run:

from __future__ import print_function

class A(object):
    x = 4
    lst = [x for _ in range(3)]

a = A()
print(a.lst)

The code doesn't work in Python 3, but it does in Python 2, or in a function like this

from __future__ import print_function

def func():
    x = 4
    gen = (x for _ in range(3))
    return gen

print(list(func()))

This code works well in Python 2 and Python 3 or on the module level

from __future__ import print_function

x = 4
gen = (x for _ in range(3))

print(list(gen))

The code works well in Python 2 and Python 3 too.

Why is it wrong in class?

Answer 1

Because x is a class attribute (static variable), which you access like,

Example

>>> class A(object):
...     x = 4
...     gen = (A.x for _ in range(3))
...
>>> a = A()
>>> list(a.gen)
[4, 4, 4]

---

Here even gen is another class attribute, which means that,

>>> b = A()
>>> list(b.gen)
[]

This gives empty because the generator has already exhausted.

---

This happens because the generator is evaluated only when you issue a.gen, when it won't be able to resolve the name x.