False positive undefined-variable when variable name in decoration matches function argument

[mthuurne]

Steps to reproduce

Run pylint on the following code:

def decorate(seq):
    return lambda f: f

@decorate(x*x for x in range(3))
def f(x):
    pass

@decorate(x*x for x in range(3))
def g(y):
    pass

Current behavior

pylint reports:

4:12: E0602: Undefined variable 'x' (undefined-variable)

Line 4 being the @decorate preceding f().

Note that no error is reported for g(): the problem only occurs if the variable name inside the comprehension matches one of the decorated function's arguments. Which is strange, since they should be in separate scopes.

Expected behavior

No error is reported.

pylint --version output

pylint 2.6.0
astroid 2.4.2
Python 3.8.4 (default, Jul 14 2020, 09:08:56) [GCC]

This problem did not occur until I upgraded from pylint 2.5.3 to 2.6.0.

[stpierre]

We're hitting this as well. The other requirement of this is that the variable needs to be used twice in a list comprehension in the decorator. (This is really remarkably specific!) So for instance, these all give the false-positive undefined-variable:

@decorate(i * i for i in [])
def thing1(i):
    pass

@decorate(dict(i=i, j=i) for i in [])
def thing2(i):
    pass

@decorate((i, i) for i in [])
def thing3(i):
    pass

But this does not:

@decorate(i for i in [])
def thing4(i):
    pass

Neither do other ways of declaring and introducing new variables, for instance lambdas. These are both fine:

@decorate(lambda i: i * i)
def thing5(i):
    pass

@decorate(lambda i: [i * i for i in []])
def thing6(i):
    pass

This is also broken in pylint 2.7.

[rr-]

I believe this is relevant place to patch
pylint/checkers/variables.py:992

            # if the name node is used as a function default argument's value or as
            # a decorator, then start from the parent frame of the function instead
            # of the function frame - and thus open an inner class scope
            if (
                current_consumer.scope_type == "function"
                and self._defined_in_function_definition(node, current_consumer.node)
            ):
                # ignore function scope if is an annotation/default/decorator, as not in the body
                continue

Relevant issue: #1731