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

Project description

flynt - string formatting converter

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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 3.6+.


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:

positional arguments:
  src                   source file(s) or directory

optional arguments:
  -h, --help            show this help message and exit
  -v, --verbose         run with verbose output
  -q, --quiet           run without output
  --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.
  -s, --string          Interpret the input as a Python code snippet and print the converted version. The snippet must use single quotes or escaped double quotes.
  -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.
  -f, --fail-on-change  Fail when changing files (for linting purposes)
  -a, --aggressive      Include conversions with potentially changed behavior.
  -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 commiting. 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 commiting.

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


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.

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