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A tool to automatically convert old string literal formatting to f-strings
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ikkamens bump version
Latest commit bd7e849 Sep 7, 2019

flynt - string formatting converter

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This is a beta release. Do NOT use on uncommitted code!

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+.


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

usage: flynt [-h] [--verbose | --quiet]
             [--no_multiline | --line_length LINE_LENGTH]

positional arguments:
  src                   source file or directory

optional arguments:
  -h, --help            show this help message and exit
  --verbose             run with verbose output
  --quiet               run without output
  --no_multiline        convert only single line expressions
  --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 79 characters.
  --upgrade             run pyupgrade on .py files

Sample output of a successful run:

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

38f9d3a65222:~ ikkamens$ flynt flask

Flynt run has finished. Stats:

Execution time: 0.623s
Files modified: 18
Expressions transformed: 43
Character count reduction: 241 (0.04%)


Please run your tests before commiting. Report bugs as github issues at:
Thank you for using flynt! Fstringify 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: local
         - id: flynt
           name: flynt
           entry: flynt
           args: [--fail-on-change]
           types: [python]
           language: python
               - flynt

This will run flynt on all modified files before commiting.


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, e.g. print('%d' % 'bla'). While most cases are covered by taking the formatting specifiers to the f-strings format, the precise exception behaviour might differ as well.

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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