flynt
CLI tool to convert a python project's %-formatted strings to f-strings.
Decision gist · record as of 2026-08-14
Yes. Flynt is a low-friction, actively maintained tool with no security vulnerabilities, permissive licensing, and a clear, single purpose: automating string formatting modernization. It's well-suited for teams wanting to standardize on f-strings or for one-time codebase refactoring. The documented edge cases (tuple formatting, format specifier differences) are manageable with adequate test coverage and are not blockers for most projects.AI-flagged interpretation of the facts on this page — verify before relying
Before you install
- Requires Python 3.9 or later.
- Files will be modified in-place; version control is strongly recommended before running.
- Installation is straightforward with low friction—a single pure-Python wheel with one lightweight dependency (tomli).
License · maintenance · safety
MIT (permissive) — Flynt is released under the MIT license, a permissive open-source license that allows free use, modification, and distribution with minimal restrictions—suitable for both commercial and personal projects.
last release 2025-07-29 (381 days) · last repo commit 2026-07-19 · 733 stars
0 known vulnerabilities (OSV.dev, 2026-08-14) · 134,527 downloads/mo, #11,472 on PyPI
Alternatives
Verify before relying
pip install flynt
# Convert all .py files in a directory recursively
flynt /path/to/project
# Or convert a single file
flynt script.py
# Dry-run to see changes without modifying
flynt --dry-run /path/to/project- Whether the tool correctly handles all edge cases in tuple formatting (the fact sheet documents one known case where behavior differs).
- Performance characteristics on very large codebases or projects with thousands of files.
- Maturity and reliability of the Jupyter notebook transformation feature, which is noted as alpha.
What it is and what it does
Flynt is a command-line refactoring tool that scans Python source files and automatically rewrites string formatting expressions into f-strings. It processes %-style formatting (e.g., `'%s' % var`) and .format() calls (e.g., `'{}'.format(var)`) into their f-string equivalents (e.g., `f'{var}'`), modernizing code to use Python 3.6+ syntax. The tool can operate on single files or recursively traverse directories, skipping common non-source folders like venv and site-packages.
Flynt is designed as a safe, automated refactoring pass for existing projects. It offers dry-run and linting modes, configuration via pyproject.toml, and integration with pre-commit hooks. The tool depends only on tomli for configuration parsing and uses Python's built-in ast module for code transformation. While it handles the vast majority of formatting conversions, the fact sheet documents a known edge case where single-element tuples format differently under f-strings, and some format specifiers (like %d with floats) may raise different exceptions than before.
Use it for
- Modernize a legacy Python codebase by converting all string formatting to f-strings in a single automated pass.
- Enforce f-string usage in a project via a pre-commit hook to ensure consistent formatting style across all commits.
- Dry-run conversion on a project to review what changes would be made before applying them to version control.
- Integrate into a CI/CD pipeline as a linting check to fail builds if non-f-string formatting is detected.
- Convert string concatenations or static joins to f-strings when using Python 3.9+ with the --transform-concats or --transform-joins flags.
Worth the install?
AI-flagged interpretation of the facts on this page. Verify before relying on it.
Yes.
Flynt is a low-friction, actively maintained tool with no security vulnerabilities, permissive licensing, and a clear, single purpose: automating string formatting modernization. It's well-suited for teams wanting to standardize on f-strings or for one-time codebase refactoring. The documented edge cases (tuple formatting, format specifier differences) are manageable with adequate test coverage and are not blockers for most projects.
Install
flynt on PyPI
Before you install
Installation is straightforward with low friction—a single pure-Python wheel with one lightweight dependency (tomli). The project is actively maintained with recent commits and has been stable since its initial release in 2019.
Requires Python 3.9 or later. Files will be modified in-place; version control is strongly recommended before running.
License in practice
Flynt is released under the MIT license, a permissive open-source license that allows free use, modification, and distribution with minimal restrictions—suitable for both commercial and personal projects.
Quickstart
pip install flynt
# Convert all .py files in a directory recursively
flynt /path/to/project
# Or convert a single file
flynt script.py
# Dry-run to see changes without modifying
flynt --dry-run /path/to/projectVerify before relying
- Whether the tool correctly handles all edge cases in tuple formatting (the fact sheet documents one known case where behavior differs).
- Performance characteristics on very large codebases or projects with thousands of files.
- Maturity and reliability of the Jupyter notebook transformation feature, which is noted as alpha.
Package facts
| License | MIT permissive |
| Python support | Supports the current Python release >=3.9 |
| Install friction | Low. Pure-Python wheel |
| Runtime dependencies | 1 packagetomli |
| Maintenance | Actively maintained 381 days since the last release |
| Last repo commit | |
| First released | |
| Downloads | 134,527 / month, #11,472 on PyPI 30-day window, as of 2026-08-14 |
| Known vulnerabilities | None known OSV.dev, checked 2026-08-14 |
| Classifiers | Programming Language :: PythonProgramming Language :: Python :: 3Programming Language :: Python :: 3.10Programming Language :: Python :: 3.11Programming Language :: Python :: 3.12Programming Language :: Python :: 3.13Programming Language :: Python :: 3.9Topic :: Software Development :: Libraries :: Python ModulesTopic :: Utilities |
Evidence: flynt-1.0.6-py3-none-any.whl
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