pylint-dev/pylint

Expand to ``use-implicit-booleaness-not-len`` to catch `len(iterable) == 0` and `>0`

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#10,281 opened on Mar 17, 2025

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False Negative 🦋Good first issueHacktoberfestNeeds decision :lock:

Description

Current problem

C1802 currently catches this

Examples 1&2 (currently caught by rule C1802)

fruits = ["orange", "apple"]
vegetables = []

if len(fruits):
    print(fruits)

if not len(vegetables):
    print(vegetables)

I come across the following sometimes as well (potentially in botched attempt to fix the the flagged violation above), which I think should be equally flagged as a violation.

Examples 3&4 (not caught by PLC1802)

fruits = ["orange", "apple"]
vegetables = []

if len(fruits) > 0:
    print(fruits)

if len(vegetables) == 0:
    print(vegetables)

Examples 5&6 (recommended formulation)

fruits = ["orange", "apple"]
vegetables = []

if fruits:
    print(fruits)

if not vegetables:
    print(vegetables)

Desired solution

Examples 3&4 to be flagged as well

Additional context

See our previous discussion on this issue.

Contributor guide

Tech stack
python
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toolingdeveloper experience
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feature
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3
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1-2 days
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active
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clear
Prerequisites
Pythonpylint internalsAST knowledge
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
45
Research direction
The issue requests extending the existing rule C1802 (use implicit booleaness not len) to also flag cases like `len(iterable) > 0` and `len(iterable) == 0`. Start by examining the current implementation of the checker in pylint's source code, likely in the `checkers/` directory. Review the referenced discussion at https://github.com/astral sh/ruff/issues/16798#issuecomment 2729812152 for additional context. The solution involves adding AST pattern matching for these additional patterns in the rule's logic, similar to how the existing `len()` usage is detected.