inconsistency in partial formatting with Darker

[Eyobkibret15]

Hello,
I've encountered an issue when using Darker to apply partial formatting to a Python file. following the documentation provided here https://pypi.org/project/darker/ my goal was to selectively format only specific lines of code.
the original Code which i made initial commit on is
if True: print('hi')
print()
if False: print('there)

• when i change only the first print string it works as expected only suggested to format the first line

• however, when I remove the empty print() from the 2nd line, Darker diff output includes lines that should not be formatted

• moreover, when i add only a new unformatted print statement if False: print('new line') to the original file darker diff still includes line that should not be formatted

i am seeking assistance to understand why Darker is recommending these additional changes and how i might configure it to respect the boundaries of partial formatting ?

[akaihola]

Hi @Eyobkibret15, thanks for your report!

Since Black doesn't support partial formatting natively, Darker takes a diff between the original and modified versions of the file and tries to track down which lines correspond to each other, and how they map to the reformatting made by Black. It then takes a conservative approach to apply minimal diff chunks which intersect with detected changes. Unfortunately it's very tricky to get this approach to really minimize the changes since we're not digging down to analyzing the Python code.

So unfortunately this is a known limitation which can't be approved using the current approach with reasonable effort.

Let me know if the documentation can be improved to explain this limitation better!

[Eyobkibret15]

Thanks @akaihola for the clarification. It would be great if the documentation could briefly mention this limitation and any possible workarounds. Appreciate your help!

[akaihola]

@Eyobkibret15 I'm working on improving the documentation in #566. Review and comments are welcome!

[akaihola]

@Eyobkibret15 I sent you an invitation to become a contributor on the Darker repository. If you accept that, I can assign you as a reviewer of #566 so you can double check that the documentation I added is understandable.

[Eyobkibret15]

Thank you for the invitation, @akaihola. I’ve accepted the contributor role and will review the proposed changes in #566.

[akaihola]

Oops @Eyobkibret15 I already merged #566, but feel free to propose further improvements to the text!