flake8-string-format
string format checker, plugin for flake8
Decision gist · record as of 2026-08-14
Yes, if you use Flake8 and want to catch format string errors statically. The plugin is lightweight, actively maintained, permissively licensed, and has no known vulnerabilities. Install it if format string correctness matters in your codebase; skip it only if you never use str.format() or already rely on type checking and runtime testing to catch these errors.AI-flagged interpretation of the facts on this page — verify before relying
Before you install
- Requires flake8 to be installed; Python >= 3.8 required.
- Low friction: pure Python wheel with a single runtime dependency on flake8.
- Actively maintained as of 2026-06-09 with recent Python 3.14 compatibility fixes.
License · maintenance · safety
MIT (permissive) — MIT license (permissive) — no restrictions on use, modification, or distribution in proprietary or open-source projects.
last release 2026-06-09 (66 days) · last repo commit 2026-08-01 · 20 stars
0 known vulnerabilities (OSV.dev, 2026-08-14) · 170,838 downloads/mo, #10,385 on PyPI
Alternatives
Verify before relying
pip install flake8-string-format
Then run flake8 as usual; the plugin auto-registers:
$ flake8 your_file.py
your_file.py:1:1: FMT101 format string contains unindexed parameters- Whether the plugin correctly handles all modern f-string edge cases beyond str.format()
- Performance impact on large codebases with many format strings
- Compatibility with recent Flake8 versions beyond 3.0
What it is and what it does
flake8-string-format is a Flake8 linter plugin that analyzes all string format operations in your code and reports errors when format calls don't match their arguments. It detects unindexed parameters (which fail in Python 2.6), mismatched argument counts, unused parameters, and mixing of implicit and explicit indexing. The plugin integrates directly into Flake8's error reporting and can be controlled via standard Flake8 ignore flags.
It works by parsing format strings and their corresponding format() calls, checking whether the parameters referenced in the string actually exist in the call's arguments. It distinguishes between format strings (where format() is called), docstrings, and plain strings, issuing different error codes for each context. The plugin handles variable arguments gracefully by skipping checks that would be unreliable when *args or **kwargs are present.
Use it for
- Catch format string bugs during CI/CD by detecting mismatched argument counts before runtime
- Enforce Python 2.6 compatibility by flagging unindexed parameters like '{}'.format(x)
- Identify unused parameters passed to format() calls, reducing confusion and maintenance burden
- Detect typos in keyword argument names in format strings like '{name}'.format(nmae=value)
- Validate docstrings and other string literals for consistent format parameter usage
Worth the install?
AI-flagged interpretation of the facts on this page. Verify before relying on it.
Yes, if you use Flake8 and want to catch format string errors statically.
The plugin is lightweight, actively maintained, permissively licensed, and has no known vulnerabilities. Install it if format string correctness matters in your codebase; skip it only if you never use str.format() or already rely on type checking and runtime testing to catch these errors.
Install
flake8-string-format on PyPI
Before you install
Low friction: pure Python wheel with a single runtime dependency on flake8. Actively maintained as of 2026-06-09 with recent Python 3.14 compatibility fixes.
Requires flake8 to be installed; Python >= 3.8 required.
License in practice
MIT license (permissive) — no restrictions on use, modification, or distribution in proprietary or open-source projects.
Quickstart
pip install flake8-string-format
Then run flake8 as usual; the plugin auto-registers:
$ flake8 your_file.py
your_file.py:1:1: FMT101 format string contains unindexed parametersVerify before relying
- Whether the plugin correctly handles all modern f-string edge cases beyond str.format()
- Performance impact on large codebases with many format strings
- Compatibility with recent Flake8 versions beyond 3.0
Package facts
| License | MIT permissive |
| Python support | Supports the current Python release >=3.8 |
| Install friction | Low. Pure-Python wheel |
| Runtime dependencies | 1 packageflake8 |
| Maintenance | Actively maintained 66 days since the last release |
| Last repo commit | |
| First released | |
| Downloads | 170,838 / month, #10,385 on PyPI 30-day window, as of 2026-08-14 |
| Known vulnerabilities | None known OSV.dev, checked 2026-08-14 |
| Classifiers | Development Status :: 3 - AlphaEnvironment :: ConsoleFramework :: Flake8Intended Audience :: DevelopersLicense :: OSI Approved :: MIT LicenseOperating System :: OS IndependentProgramming Language :: PythonProgramming Language :: Python :: 3Topic :: Software Development :: Libraries :: Python ModulesTopic :: Software Development :: Quality Assurance |
Evidence: flake8_string_format-0.4.0-py3-none-any.whl
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See also flake8-pep3101 · flake8-implicit-str-concat · flake8-no-implicit-concat · flake8-use-fstring · flake8-noqa · flake8-broken-line · flake8-rst-docstrings · flake8-annotations · flake8-print · flake8-mutable