False positive used-before-assignment in pattern matching with a guard.

[pgoy-dod]

Bug description

When using pattern matching with a guard, pylint claims that the variable the guard uses is used before assignment.

Example:

match ("example", "one"):
    case (x, y) if x == "example":  # <-- pylint complains about 'x' here
        print("woo!")
    case _:
        print("boo...")

Configuration

No response

Command used

pylint a.py

Pylint output

************* Module a
a.py:2:19: E0601: Using variable 'x' before assignment (used-before-assignment)

-------------------------------------
Your code has been rated at -10.00/10

Expected behavior

Pattern matching with guards is correctly parsed and supported.

Pylint version

pylint 2.11.1
astroid 2.8.4
Python 3.10.0 (default, Oct 13 2021, 06:45:00) [Clang 13.0.0 (clang-1300.0.29.3)]

OS / Environment

Mac OSX Big Sur 11.6 (20G165)

Additional dependencies

No response

[pilattebe]

This bug is still present in:

pylint 2.12.2
astroid 2.9.3
Python 3.10.2 (main, Feb  1 2022, 16:44:51) [Clang 12.0.5 (clang-1205.0.22.9)]

Here my example code

import sys
match sys.argv[1:]:         # vvvvvvvvv Using variable 'direction' before assignment
    case ["go", direction] if direction in ["up", "down", "left", "right"]:
        print(f"going {direction}")
    case command:
        print(f"unknown command: {' '.join(command)}")

[jborman-exos]

Also hitting this false-positive, is there a plan to include this in an upcoming release?

[spagh-eddie]

I too suffer from this issue

[mthuurne]

I'm running into the same issue using:

pylint 2.14.4
astroid 2.11.6
Python 3.10.4 (main, Mar 26 2022, 21:00:27) [GCC]

My testcase:

def describe(i: int) -> str:
    match i:
        case n if n % 2 == 0:
            return "even"
        case _:
            return "odd"

[maujim]

If the same variable is used as a match guard twice, pylint won't complain the second time.

l = [7, 7]
match l:
    case [_, n] if n >= 4: 		# E0601: Using variable 'n' before assignment
        print("hi")

t = (4, "abc")
match t:
    case (n, _) if n < 12:		# not triggered here
        print("hi")

If the line case (n, _) if n < 12: is changed to use x instead of n, the false positive triggers again.

version:

pylint 2.14.5
astroid 2.12.5
Python 3.10.6 (main, Aug  3 2022, 17:39:45) [GCC 12.1.1 20220730]