---
id: flynt
version: "1.0.6"
license: MIT
license_treatment: permissive
maintenance: active
---

# flynt — CLI tool to convert a python project's %-formatted strings to f-strings.

License: permissive · Maintenance: active · Downloads: 134.5K/mo

## 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 above — verify before relying.

Flynt is a command-line tool that automatically converts Python string formatting from %-style and .format() syntax to f-strings, modernizing codebases to Python 3.6+ syntax.

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

pip install flynt
uv add flynt
poetry add flynt

## Installing flynt

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.

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/project

Requires Python 3.9 or later. Files will be modified in-place; version control is strongly recommended before running.

Verify 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_current
- Install friction: low
- Maintenance: active
- Downloads: 134.5K/month (top 15,000 on PyPI)
- Known vulnerabilities: none known

## Tags

python string formatting converter, f-string migration tool, convert format strings to f-strings, python code modernization, automatic string refactoring, %-format to f-string, python formatting automation, code-modernization, refactoring-tool, cli

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