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

Project description

flynt - string formatting converter

Actions Status License: MIT PyPI Downloads Code style: black

flynt is a command line tool to automatically convert a project's Python code from old "%-formatted" and .format(...) strings into Python 3.6+'s "f-strings".


Not only are they more readable, more concise, and less prone to error than other ways of formatting, but they are also faster!


pip install flynt. It requires Python version 3.7+.


Flynt will modify the files it runs on. Add your project to version control system before using flynt.

To run: flynt {source_file_or_directory}

  • Given a single file, it will 'f-stringify' it: replace all applicable string formatting in this file (file will be modified).
  • Given a folder, it will search the folder recursively and f-stringify all the .py files it finds. It skips some hard-coded folder names: blacklist = {'.tox', 'venv', 'site-packages', '.eggs'}.

It turns the code it runs on into Python 3.6+, since 3.6 is when "f-strings" were introduced.

Command line options

From the output of flynt -h:

usage: flynt [-h] [-v | -q] [--no-multiline | -ll LINE_LENGTH]
             [-d | --stdout] [-s] [--no-tp] [--no-tf] [-tc] [-tj]
             [-f] [-a] [-e EXCLUDE [EXCLUDE ...]] [--version]
             [src ...]

flynt v.0.78

positional arguments:
  src                   source file(s) or directory (or a single `-`
                        to read stdin and output to stdout)

optional arguments:
  -h, --help            show this help message and exit
  -v, --verbose         run with verbose output
  -q, --quiet           run without outputting statistics to stdout
  --no-multiline        convert only single line expressions
  -ll LINE_LENGTH, --line-length LINE_LENGTH
                        for expressions spanning multiple lines,
                        convert only if the resulting single line
                        will fit into the line length limit. Default
                        value is 88 characters.
  -d, --dry-run         Do not change the files in-place and print
                        the diff instead. Note that this must be
                        used in conjunction with '--fail-on-change'
                        when used for linting purposes.
  --stdout              Do not change the files in-place and print
                        the result instead. This argument implies
                        --quiet, i.e. no statistics are printed to
                        stdout, only the resulting code. It is
                        incompatible with --dry-run and --verbose.
  -s, --string          Interpret the input as a Python code snippet
                        and print the converted version. The snippet
                        must use single quotes or escaped double
  --no-tp, --no-transform-percent
                        Don't transform % formatting to f-strings
                        (default: do so)
  --no-tf, --no-transform-format
                        Don't transform .format formatting to
                        f-strings (default: do so)
  -tc, --transform-concats
                        Replace string concatenations (defined as +
                        operations involving string literals) with
                        f-strings. Available only if flynt is
                        installed with 3.8+ interpreter.
  -tj, --transform-joins
                        Replace static joins (where the joiner is a
                        string literal and the joinee is a static-
                        length list) with f-strings. Available only
                        if flynt is installed with 3.8+ interpreter.
  -f, --fail-on-change  Fail when changing files (for linting
  -a, --aggressive      Include conversions with potentially changed
  -e EXCLUDE [EXCLUDE ...], --exclude EXCLUDE [EXCLUDE ...]
                        ignore files with given strings in it's
                        absolute path.
  --version             Print the current version number and exit.

Sample output of a successful run:

38f9d3a65222:~ ikkamens$ git clone
Cloning into 'flask'...
Resolving deltas: 100% (12203/12203), done.

38f9d3a65222:open_source ikkamens$ flynt flask
Running flynt v.0.40

Flynt run has finished. Stats:

Execution time:                            0.789s
Files modified:                            21
Character count reduction:                 299 (0.06%)

Per expression type:
Old style (`%`) expressions attempted:     40/42 (95.2%)
`.format(...)` calls attempted:            26/33 (78.8%)
F-string expressions created:              48
Out of all attempted transforms, 7 resulted in errors.
To find out specific error messages, use --verbose flag.

Please run your tests before committing. Did flynt get a perfect conversion? give it a star at:
~ ~
Thank you for using flynt. Upgrade more projects and recommend it to your colleagues!

38f9d3a65222:~ ikkamens$

Pre-commit hook

To make sure all formatted strings are always converted to f-strings, you can add flynt to your pre-commit hooks.

Add a new section to .pre-commit-config.yaml:

-   repo:
    rev: ''
    -   id: flynt

This will run flynt on all modified files before committing.

You can skip conversion of certain lines by adding # noqa [: anything else] flynt [anything else]

Configuration files

Since v0.71 flynt can be configured using pyproject.toml file on a per-project basis. Use same arguments as in CLI, and add them to [tool.flynt] section. CLI arguments takes precedence over the config file. It can also be configured globally with a toml file located in ~/.config/flynt.toml on Unix / ~/.flynt.toml on Windows.


Read up on f-strings here:

After obsessively refactoring a project at work, and not even covering 50% of f-string candidates, I realized there was some place for automation. Also it was very interesting to work with ast module.

Dangers of conversion

It is not guaranteed that formatted strings will be exactly the same as before conversion.

'%s' % var is converted to f'{var}'. There is a case when this will behave different from the original - if var is a tuple of one element. In this case, %s displays the element, and f-string displays the tuple. Example:

foo = (1,)
print('%s' % foo) # prints '1'
print(f'{foo}')   # prints '(1,)'

Furthermore, some arguments cause formatting of strings to throw exceptions. One example where f-strings are inconsistent with previous formatting is %d vs {:d} - new format no longer accepts floats. While most cases are covered by taking the formatting specifiers to the f-strings format, the precise exception behaviour might differ as well. Make sure you have sufficient test coverage.

Other Credits / Dependencies / Links

  • astor is used to turn the transformed AST back into code.
  • Thanks to folks from pyddf for their support, advice and participation during spring hackathon 2019, in particular Holger Hass, Farid Muradov, Charlie Clark.
  • Logic finding the pyproject.toml and parsing it was partially copied from black

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