Support for percent format notebooks

[Conchylicultor]

In IPython / Jupyter notebook, trailing ; has a specific effect of suppressing the last output: https://ipython.org/ipython-doc/dev/interactive/tips.html#suppress-output

Other Colab extensions like https://github.com/google/etils also use ; to pretty display the line (as alias of the verbose IPython.display.display): https://etils.readthedocs.io/en/latest/ecolab.html#pretty-display-with-trailing

Colab/IPython notebook are meant for fast iteration. As such, those hack are very useful for fast visualization to be able to pretty display in Colab:

def fn():
  return x;  # Will display the return value

x = a + other_fn();  # Display `x`

Currently, when formatting a notebook file (e.g. a .py file with # %% cells), all ; get removed.

It would be great if there was a directive to force black keep the trailing ; locally on a file. Something like:

# fmt: disable=trailing-semicolon

Similar to # pylint: disable= directives.

As formatting is applied on the full codebase, but this behavior should only be ignored on some specific Colab/IPython files, Globally disabling this behavior is not an option. Instead, this would have to be disabled on a per-file basis.

[Conchylicultor]

It looks keeping trailing ; is already implemented for .ipynb files: #3577

However this do not seems to work for .py files following the # %% format (.py files meant to be loaded in IPython):
https://jupytext.readthedocs.io/en/latest/formats-scripts.html#the-percent-format

Maybe this bug should be instead about supporting the # %% format on .py files, so that src/black/handle_ipynb_magics.py is applied in addition of the standard Python formatting.