un-fstring

Sometimes, unfortunately, you need to write code that is compatible with Python 3.5 (e.g., to run on Ubuntu 16 or Debian 9, which will be supported until April 2021 and June 2022, respectively). Sometimes, even more unfortunately, you didn't know it needed to be when you wrote it, and you need to make it compatible post-facto.

The biggest syntax change going from 3.5 to 3.6 was the addition of f-strings. Many packages can convert old-style string formatting methods into f-strings. un-fstring does the opposite: it converts f-strings into .format() calls to preserve compatibility with Python 3.5.

To convert your code, first install un-fstring (it itself requires Python 3.6 or later):

$ pip install un-fstring

Then run it over your source code:

$ un-fstring path/to/source/directory another_file.py

un-fstring will replace f-strings with .format() calls in-place.

The --dry-run option will show a contextual diff of what un-fstring would do to your code without actually overwriting it. Run un-fstring --help to see what other options are available.

un-fstring is available as a pre-commit hook:

- repo: https://github.com/JoshKarpel/un-fstring
  rev: v0.1.2
  hooks:
    - id: un-fstring

un-fstring is not a code formatter; I recommend running black over your code afterwards with the --target-version py35 option enabled.

Though potentially useful, this is mostly a toy project based on some problems I ran into at work. If you're looking for a more robust implementation, check out f2format.