False positive for cell-var-from-loop / W0640

[tushar-deepsource]

Sounds crazy, but I think W0640 (cell-var-from-loop) is a lint for an issue that is non-existent.

Here's the code snippet from the example, which should supposedly cause i to be assigned to the last value in the list:

def foo(numbers):
    for i in numbers:
        def bar():
            print(i)
        bar()

Except, it works fine:

>>> foo([1, 2, 3])
1
2
3

Same is true for lambdas:

def foo(numbers):
    for num in numbers:
        print(list(filter(lambda x: x != num, numbers)))

>>> foo([1, 2, 3])
[2, 3]
[1, 3]
[1, 2]

Here's pylint output:

asd.py:3:41: W0640: Cell variable num defined in loop (cell-var-from-loop)

Pylint version

pylint 2.13.7
astroid 2.11.3
Python 3.10.4 (main, Apr 26 2022, 19:36:29) [Clang 13.1.6 (clang-1316.0.21.2)]

[Pierre-Sassoulas]

I think our example is wrong in the documentation because we wanted to make it small but in the process we made it meaningless. It works fine because you're using the function directly in the loop and you could simply do:

def foo(numbers):
    for i in numbers:
        print(i)

But https://stackoverflow.com/questions/25314547/cell-var-from-loop-warning-from-pylint explains why the check is useful. I'd welcome a less naive example for the documentation :)

[tushar-deepsource]

Documentation aside, it's still a false positive for this simple case:

def foo(numbers):
    for num in numbers:
        print(list(filter(lambda x: x != num, numbers)))

Do you think it's safe to not raise this issue for these cases?

[tushar-deepsource]

I think I get it now.

def get_functions(numbers):
    for num in numbers:
        function = lambda: print(num)
        yield function

for func in get_functions():
    func()

This one works:

1
2
3

This one also works:

def get_functions(numbers):
    for num in numbers:
        function = lambda: print(num)
        yield function

functions = get_functions([1, 2, 3])
for func in functions:
    func()

1
2
3

BUT, this one doesn't work:

def get_functions(numbers):
    for num in numbers:
        function = lambda: print(num)
        yield function

functions = list(get_functions([1, 2, 3]))
for func in functions:
    func()

3
3
3

It's because list(...) eagerly collects the functions, but for-loops lazily yield the functions. If you eagerly evaluate the functions, the value of num gets to 3 before the function is ever called.

[Pierre-Sassoulas]

#5263 is a near duplicate.