PyCQA/flake8-bugbear

B018 doesn't trigger for useless expressions involving multiple variables

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#452 opened on Jan 16, 2024

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Description

I recently ran into the following bug and was surprised it wasn't flagged by B018:

def some_funcion(a, b, c, d):
    result = a * b
    +c * d

    return result

that should have been

def some_funcion(a, b, c, d):
    result = (
        a * b
        + c * d
    )

    return result

It looks like B018 doesn't trigger for expressions containing multiple variables.

Here's some examples:

a * b     # doesn't trigger B018
+a * b    # doesn't trigger B018
+1 * 2    # doesn't trigger B018
-1 * 2    # doesn't trigger B018
1 * 2 + 3 # doesn't trigger B018
a + 1     # doesn't trigger B018
a         # triggers B018
(a, b)    # triggers B018
1         # triggers B018
+1        # triggers B018
-1        # triggers B018

Versions I'm using:

python version 3.10.13
flake8 version 7.0.0
flake8-bugbear version 23.12.2
ruff version 0.1.13

Contributor guide

Research direction
Inspect the B018 rule implementation in flake8 bugbear, specifically the `visit Expr` method or similar, to understand how it detects useless expressions. Identify why expressions involving multiple variables are not flagged, possibly due to missing recursion or handling of nested binary operations. Trace the AST visitor pattern and ensure that all expression types (e.g., BinOp, UnaryOp) are considered.
Tech stack
python
Domain
backendtesting qa
Issue type
Bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-3 hours
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
Active
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
Clear
Prerequisites
GitPythonflake8 bugbear codebase understanding
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
60