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Back-port compiler for Python 3.6 f-string literals.

Project description


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Write f-string in Python 3.6 flavour, and let f2format worry about back-port issues :beer:

  Since PEP 498, Python introduced f-string literals in version 3.6. Though released ever since December 23, 2016, Python 3.6 is still not widely used as expected. For those who are now used to f-strings, f2format provides an intelligent, yet imperfect, solution of a backport compiler by converting f-strings to str.format expressions, which guarantees you to always write f-strings in Python 3.6 flavour then compile for compatibility later.

f2format is inspired and assisted by my good mate @gousaiyang. It functions by tokenising and parsing Python code into multiple abstract syntax trees (AST), through which it shall synthesise and extract expressions from f-string literals, and then reassemble the original string using str.format method. Besides conversion and format specification, f2format also considered and resolved string concatenation. Also, it always tries to maintain the original layout of source code, and accuracy of syntax.


Note that f2format only supports Python versions since 3.3 🐍

  For macOS users, f2format is now available through Homebrew:

brew tap jarryshaw/tap
brew install f2format
# or simply, a one-liner
brew install jarryshaw/tap/f2format

  Simply run the following to install the current version from PyPI:

pip install f2format

  Or install the latest version from the git repository:

git clone
cd f2format
pip install -e .
# and to update at any time
git pull

Basic Usage


  It is fairly straightforward to use f2format:

context in ${...} changes dynamically according to runtime environment

usage: f2format [options] <python source files and folders...>

Convert f-string to str.format for Python 3 compatibility.

positional arguments:
  SOURCE                python source files and folders to be converted (${CWD})

optional arguments:
  -h, --help            show this help message and exit
  -V, --version         show program's version number and exit
  -q, --quiet           run in quiet mode

archive options:
  duplicate original files in case there's any issue

  -na, --no-archive     do not archive original files
  -p PATH, --archive-path PATH
                        path to archive original files (${CWD}/archive)

convert options:
  compatibility configuration for non-unicode files

  -c CODING, --encoding CODING
                        encoding to open source files (${LOCALE_ENCODING})
  -v VERSION, --python VERSION
                        convert against Python version (${LATEST_VERSION})

f2format will read then convert all f-string literals in every Python file under this path. In case there might be some problems with the conversion, f2format will duplicate all original files it is to modify into archive directory ahead of the process, if -n not set.

  For instance, the code will be converted as follows.

# the original code
var = f'foo{(1+2)*3:>5}bar{"a", "b"!r}boo'
# after `f2format`
var = 'foo{:>5}bar{!r}boo'.format((1+2)*3, ("a", "b"))


Well... it's not published to the Docker Hub yet ;)

  Considering f2format may be used in scenarios where Python is not reachable. We provide also a Docker image for those poor little guys.

  See Dockerfile for more information.

Bundled Executable

Coming soooooooooooon...

  For the worst case, we also provide bundled executables of f2format. In such case, you may simply download it then, voilà, it's ready for you.

  Special thanks to PyInstaller ❤️

Developer Reference

Automator provides a demo script, which may help integrate f2format in your development and distribution circle.

NB: is not an integrated automation script. It should be revised by design.

  It assumes

  • all source files in /src directory
  • using GitHub for repository management
  • having release branch under /release directory
  • already installed f2format and twine
  • permission to these files and folders granted

  And it will

  • copy and src to release directory
  • run f2format for Python files under release
  • distribute to PyPI and TestPyPI using twine
  • upload to release branch on GitHub
  • upload original files to GitHub


f2format currently supports three environment arguments:

  • F2FORMAT_QUIET -- run in quiet mode (same as --quiet option in CLI)
  • F2FORMAT_VERSION -- convert against Python version (same as --python option in CLI)
  • F2FORMAT_ENCODING -- encoding to open source files (same as --encoding option in CLI)


f2format -- wrapper works for conversion



  • filename -- str, file to be converted


  • F2FORMAT_QUIET -- run in quiet mode (same as --quiet option in CLI)
  • F2FORMAT_ENCODING -- encoding to open source files (same as --encoding option in CLI)
  • F2FORMAT_VERSION -- convert against Python version (same as --python option in CLI)


  • ConvertError -- when source code contains syntax errors

convert -- the main conversion process

convert(string, source='<unknown>')


  • string -- str, context to be converted
  • source -- str, source of the context


  • F2FORMAT_VERSION -- convert against Python version (same as --python option in CLI)


  • str -- converted string


  • ConvertError -- when source code contains syntax errors

Internal exceptions

class ConvertError(SyntaxError):
    """Parso syntax error."""


NB: this project is now stalled, because I just cannot figure out how to play w/ codecs :)

f2format-codec registers a codec in Python interpreter, which grants you the compatibility to write directly in Python 3.6 f-string syntax even through running with a previous version of Python.


  The current test samples are under /test folder. is the main entry point for tests.

  For unittests, see

Known issues

  Since f2format is currently based on parso project, it had encountered several compatibility and parsing issues.

  • Parsing f-strings with nested format specifiers produces incorrect SyntaxError (#74) This issue has been resolved since parso version 0.5.0.

  • Parsing f-strings with invalid quotes in expression part does not raise SyntaxError (#86)

  • Parsing f-strings with seeming assignment expressions produces incorrect SyntaxError (#87)


  Contributions are very welcome, especially fixing bugs and providing test cases, which @gousaiyang is to help with, so to speak. Note that code must remain valid and reasonable.

See Also

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