Erroneous unused-variable (false positive) in lambda with *args and default argument

[pganssle]

When running pylint over the following function, it warns that the variable variable (the one defined in the function, not the parameter), is unused:

def lambda_with_args():
    variable = 1
    return lambda *_, variable=variable: variable + 1

I see the same issue when you add **kwargs and/or other parameters to the lambda:

def lambda_with_args_kwargs():
    variable = 1
    return lambda *_, variable=variable, **_kwargs: variable + 1

The expected behavior is that variable should be considered used, since it is used to set the default argument for variable in the lambda. This is actually a fairly common idiom to get "early-binding" behavior in lambdas (though having *args in a lambda is considerably less common, so I'm not entirely shocked that — as far as I saw — this hasn't been reported before).

With some further explorations, I have determined:

1. Multiple non-*args arguments, or **kwargs alone is not enough to trigger this bug.

2. Replacing the lambda with the equivalent def statement also won't trigger the bug:

   def func_def():
       variable = 1
       def f(*_, variable=variable):
           return variable + 1
       return f

3. If you change the name of the variable that variable gets bound to, it also won't trigger the bug (lambda *args, var=variable: var + 1 works fine).

Here is a minimal Python file that shows all of these properties:

# pylint: disable=missing-module-docstring, missing-docstring
# pylint: disable=invalid-name

# These trigger the bug
def lambda_with_args():
    variable = 1
    return lambda *_, variable=variable: variable + 1

def lambda_with_args_kwargs():
    variable = 1
    return lambda *_, variable=variable, **_kwargs: variable + 1

def lambda_with_args_and_multi_args():
    variable = 1
    return lambda *_, a, variable=variable: variable + a


# The rest of these do not trigger the bug
def lambda_with_multi_args():
    variable = 1
    return lambda a, variable=variable: variable + a

def lambda_without_args():
    variable = 1
    return lambda variable=variable: variable + 1

def lambda_with_kwargs():
    variable = 1
    return lambda variable=variable, **_: variable + 1

def func_def():
    variable = 1
    def f(*_, variable=variable):
        return variable + 1

    return f

def different_name():
    variable = 1
    return lambda *args, var=variable: var + 1

pylint --version output

pylint 2.6.0
astroid 2.4.2
Python 3.8.2 (default, Apr 18 2020, 12:47:51) 
[GCC 9.3.0]

Doing a bit more digging, it seems that prior to pylint==2.6.0, this bug also affected the def f(*_, variable=variable) variant, and in earlier versions even more erroneous warnings were raised as well. Not entirely sure, but I suspect #3713 fixed the issue for the def version of this.