---
id: flake8-pep3101
version: "3.0.0"
license: GNU GENERAL PUBLIC LICENSE Version 2, June 1991 Copyright (C) 1989, 1991 Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA Everyone is permitted to copy and… (full text in the JSON record)
license_treatment: copyleft
maintenance: aging
---

# flake8-pep3101 — Checks for old string formatting

License: copyleft · Maintenance: aging · Downloads: 98.0K/mo

## What it is and what it does

flake8-pep3101 is a linter plugin that integrates with flake8 to detect and report old-style percent-operator string formatting in Python code. It encourages developers to use the modern .format() method or f-strings instead, aligning with PEP 3101 recommendations. The plugin works as a flake8 extension, running automatically when you invoke flake8 on your codebase without requiring separate configuration.

The package has been in production since 2015 and supports modern Python versions (3.10 through 3.14, including PyPy). It is a lightweight, single-purpose tool with only flake8 as a runtime dependency. While maintenance is aging—no updates in 293 days—the repository remains active and unarchived, and the package carries no known security vulnerabilities.

Use it for:

- Enforce consistent string formatting style across a codebase during CI/CD by running flake8 with this plugin enabled.
- Identify legacy string formatting in older Python projects to guide modernization efforts toward .format() or f-strings.
- Integrate into pre-commit hooks to catch old-style formatting before code review.
- Audit a large repository for formatting inconsistencies as part of a code quality initiative.

## Worth the install?

AI-flagged interpretation of the facts above — verify before relying.

A flake8 plugin that detects and flags old-style string formatting (percent operator) in Python code, encouraging migration to the .format() method or f-strings.

Yes, if you want automated enforcement of modern string formatting in your codebase. The plugin is low-friction to install, carries no security vulnerabilities, and integrates seamlessly into flake8 workflows. The copyleft license is not a barrier for development tools used in-house. The aging maintenance status is a minor concern but not a blocker—the plugin solves a narrow, stable problem and does not require frequent updates. Install it if string formatting consistency is a priority in your project.

## Install

pip install flake8-pep3101
uv add flake8-pep3101
poetry add flake8-pep3101

## Installing flake8-pep3101

Before you install: Low install friction with a single runtime dependency on flake8. Maintenance status is aging—last commit was 2025-10-25 and the package has not been updated in 293 days, though the repository remains active and unarchived.

License in practice: Licensed under GPLv2 (copyleft). If you distribute code or modifications, you must make source available under the same license terms. For private use or as a development tool in a closed environment, this poses minimal constraint.

Quickstart:

pip install flake8-pep3101

Then run flake8 on your code:
flake8 your_file.py

The plugin will automatically flag old-style string formatting like:
result = "Hello %s" % name

Requires flake8 to be installed; the plugin does not work standalone. Requires Python 3.10 or later.

Verify before relying:

- Whether the plugin's error codes and configuration options are documented or discoverable
- Performance impact when checking large codebases
- Whether the plugin handles edge cases in string formatting detection correctly

## Package facts

- License: GNU GENERAL PUBLIC LICENSE Version 2, June 1991 Copyright (C) 1989, 1991 Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA Everyone is permitted to copy and… (full text in the JSON record) (copyleft)
- Python support: supports_current
- Install friction: low
- Maintenance: aging
- Downloads: 98.0K/month (top 15,000 on PyPI)
- Known vulnerabilities: none known

## Tags

flake8 string formatting checker, detect old string formatting, percent operator linter, pep 3101 compliance, python code style enforcement, format method migration, flake8 plugin, linting, code-style, flake8-plugin

[View on SkillFed](https://skillfed.io/packages/flake8-pep3101) · [View on PyPI](https://pypi.org/project/flake8-pep3101/)