False negative ``unused-variable`` for catched exception when used in the inner context

[Pierre-Sassoulas]

Steps to reproduce

Given a file a.py:

#pylint: disable=missing-docstring

def function():
    unused = 1  # [unused-variable]
    try:
        1 / 0
    except ZeroDivisionError as error:  # [unused-variable]
        try:
            1 / 0
        except ZeroDivisionError as error:
            raise Exception("") from error

Current behavior

************* Module a
a.py:4:4: W0612: Unused variable 'unused' (unused-variable)

------------------------------------------------------------------
Your code has been rated at 8.89/10 (previous run: 8.89/10, +0.00)

Expected behavior

************* Module a
a.py:4:4: W0612: Unused variable 'unused' (unused-variable)
a.py:8:30: W0612: Unused variable 'error' (unused-variable)

pylint --version output

pylint 2.7.4
astroid 2.5.3
Python 3.8.5 (default, Jan 27 2021, 15:41:15) 
[GCC 9.3.0]

[ksaketou]

Hello, I think that pylint considers the error variable as used due to the raise ... from error. I ran pylint on this code and I got the correct output.

#pylint: disable=missing-docstring

def function():
    unused = 1  # [unused-variable]
    try:
        1 / 0
    except ZeroDivisionError as error:  # [unused-variable]
        try:
            1 / 0
        except ZeroDivisionError as error:
            raise Exception("")

Output:

unused-variable.py:4:4: W0612: Unused variable 'unused' (unused-variable)
unused-variable.py:7:4: W0612: Unused variable 'error' (unused-variable)

[Pierre-Sassoulas]

Good point, this is certainely part of it, but I think that without the raise Exception("") from error the correct output would become:

unused-variable.py:4:4: W0612: Unused variable 'unused' (unused-variable)
unused-variable.py:7:4: W0612: Unused variable 'error' (unused-variable)
unused-variable.py:10:4: W0612: Unused variable 'error' (unused-variable)

[ksaketou]

Without the raise Exception("") from error the output stays the same. However, the output you suggest makes sense. I could give this a try if there is no one working on it

[Pierre-Sassoulas]

I assigned you :) Thanks for handling this !

[ksaketou]

Thank you:) One question, are there separate test files for the unused-variable error somewhere?

[Pierre-Sassoulas]

Separate test files ? I don't know if I understood but there is some functional tests here: https://github.com/PyCQA/pylint/tree/master/tests/functional/u/unused

[ksaketou]

okay great thanks

[ksaketou]

Now that I am looking at it a bit better, I notice that the same exception has been named as error twice, which I don't think that makes any sense. The ZeroDivisionError could be named once and then call it through the error value.

[Pierre-Sassoulas]

The second error shadows the first one, but this is valid code, we could imagine printing the first error for example:

#pylint: disable=missing-docstring

def function():
    unused = 1  # [unused-variable]
    try:
        1 / 0
    except ZeroDivisionError as error:
        print(f"First error : {error}")
        try:
            1 / 0
        except ZeroDivisionError as error:
            raise Exception("") from error

[ksaketou]

In the print statement, we consider that error is being used or not?

[Pierre-Sassoulas]

Yes the error would be used in the print then so no error should appear if there is a print. I think flake8 has a check for unused variable that works for this use case.