flake8-pep3101

library3.0.0pypypi✓ verified 86d ago

A flake8 plugin that checks for use of old-style % string formatting, enforcing PEP 3101 (str.format and f-strings). Current version 3.0.0, supports Python >=3.10. Maintained on GitHub.

testing

pip install flake8-pep3101

github ↗package ↗changelog ↗

INSTALL

IMPORT

SIG · FLAKE8-PEP3101

flake8-pep3101

testingpythonv3.0.0

Install

1.8s avg

Import

42ms

Disk

17MB

Pass rate

10/ 10

Env Coverage10 / 10

glibc

3.9–3.13

musl

3.9–3.13

Install & Compatibility

Where this runs

tested against v3.0.0 · pip install

no network on importno background threads

Install × environment matrix

Each cell = how many times install + import succeeded across repeated harness runs. Partial = flaky.
glibc = Debian/Ubuntu slim · musl = Alpine Linux

musl

py 3.10–3.95 runs

installs and imports cleanly · install 0.0s · import 0.044s · 19.3MB

glibc

py 3.10–3.95 runs

installs and imports cleanly · install 1.8s · import 0.040s · 20MB

17MB installed

● package 17MB

Code

Verified usage

Verified import paths — ran on the pinned version, not inferred.

flake8_pep3101

✓ import flake8_pep3101

Imported as a flake8 plugin, not directly called. Flake8 auto-discovers.

Create a sample Python file with old-style % formatting and run flake8 to see the P101 warning.

# Create a file with old-style formatting code = "name = 'world'; print('Hello %s' % name)" with open('test.py', 'w') as f: f.write(code) # Run flake8 import subprocess result = subprocess.run(['flake8', 'test.py'], capture_output=True, text=True) print(result.stdout) # Expected: test.py:1:1: P101 % string formatting

Debug

Known issues

gotchaThe plugin only detects '%' formatting, not 'Template' strings or other patterns.

fix

Use str.format() or f-strings as per PEP 3101.

affects: all

gotchaThe plugin may produce false positives when '%' is used in non-string contexts (e.g., modulo operator) or inside comments.

fix

Review warnings manually or use # noqa: P101 to suppress if legitimate.

affects: all

deprecatedOld versions used import from flake8_pep3101; plugin auto-loads via flake8's built-in plugin discovery.

fix

Upgrade to 3.0.0. No code changes needed.

affects: <3.0.0

Errors

Common errors & fixes

flake8: P101 not found

Plugin not installed or not recognized by flake8.

fix

Run 'pip install flake8-pep3101' and ensure flake8 is installed in the same environment.

ModuleNotFoundError: No module named 'flake8_pep3101'

Trying to import the plugin directly in code instead of using it as a flake8 plugin.

fix

Do not import manually; just install the package and run 'flake8'.

P101 warning for modulo in bash-like string

Plugin might flag '%' in strings that are not Python formatting but intended as literal percent (e.g., SQL queries, URLs).

fix

Use '%%' for literal percent or use # noqa: P101.

Upgrade

Version history

3.0.0latest on PyPI · released Oct 25, 2025

Audit

Dependencies

No dependency data recorded yet.

Agent activity

7 hits · last 30 days

node

Resources

githubhttps://github.com/gforcada/flake8-pep3101 ↗

packagehttps://pypi.org/project/flake8-pep3101/ ↗

changeloghttps://github.com/gforcada/flake8-pep3101/blob/main/CHANGES.rst ↗