possibly-used-before-assignment / E0606

Message emitted:

Possibly using variable %r before assignment

Description:

Emitted when a local variable is accessed before its assignment took place in both branches of an if/else switch.

Problematic code:

def check_lunchbox(items: list[str]):
    if not items:
        empty = True
    print(empty)  # [possibly-used-before-assignment]

Correct code:

def check_lunchbox(items: list[str]):
    empty = False
    if not items:
        empty = True
    print(empty)

Additional details:

You can use assert_never to mark exhaustive choices:

from typing import assert_never

def handle_date_suffix(suffix):
    if suffix == "d":
        ...
    elif suffix == "m":
        ...
    elif suffix == "y":
        ...
    else:
        assert_never(suffix)

if suffix in "dmy":
    handle_date_suffix(suffix)

Or, instead of assert_never(), you can call a function with a return annotation of Never or NoReturn. Unlike in the general case, where by design pylint ignores type annotations and does its own static analysis, here, pylint treats these special annotations like a disable comment.

Pylint currently allows repeating the same test like this, even though this lets some error cases through, as pylint does not assess the intervening code:

if guarded():
    var = 1

if guarded():
    print(var)  # emits possibly-used-before-assignment

you may be concerned that possibly-used-before-assignment is not totally useful in this instance. However, consider that pylint, as a static analysis tool, does not know if guarded() is deterministic or talks to a database. (Likewise, for guarded instead of guarded(), any other part of your program may have changed its value in the meantime.)

Created by the variables checker.