Fix formatting cells with magic methods and starting or trailing empty lines

[AleksMat]

Description

This PR fixes an issue where Jupyter cells containing any magic method and at least 1 empty line at the start or at the end of the cell don't get formatted at all.

This is an example of a cell that doesn't get formatted:

%time
a=1

But if there are no magic methods then formatting works. The following cell:

#No magic methods
a=1

gets correctly reformatted into:

# No magic methods
a = 1

It turns out the problem is in black.handle_ipynb_magics.mask_cell function:

1. The call to TransformerManager.transform_cell returns a result without empty lines at the beginning of the cell. Because this part is implemented in ipython I didn't investigate why this is the case.

2. The call to replace_magics returns a result without empty lines at the end of the cell. This is because the implementation of replace_magics assumes that

   "\n".join(src.splitlines())

   will always produce src again. This is not the case, e.g. "\n".join("\n\n".splitlines()) returns "\n".

3. Because of 1. and 2. the check len(transformed.splitlines()) != len(src.splitlines()) is triggered and NothingChanged is raised.

Solution

1. A comment about TransformerManager.transform_cell removing empty lines. Future improvements in ipython are possible.
2. Minor update in replace_magics so that it won't be removing empty lines at the end anymore.
3. The number of lines in transformed and src should be compared without starting and ending empty lines. Hence len(transformed.strip().splitlines()) != len(src.strip().splitlines())

I also added a few unit tests that test such cases.

Checklist - did you ...

• Add an entry in CHANGES.md if necessary?
• Add / update tests if necessary?
• Add new / update outdated documentation?

[JelleZijlstra]

Thanks!