Black produces a different output on the second pass on VS Code

[sgleisner]

Some files produce an extra newline at the end when running black. On a second pass, that line gets removed. I have not been able to reproduce it outside VS Code.

To Reproduce:

1. Download example.py and open it on a VS Code instance with the ms-python.python extension enabled (tested on v2020.5.86806, under VS Code 1.46.0. Possibly reproducible on other versions, too).

2. Check that black is set as the default Python formatter on settings.json:

...
"python.formatting.provider": "black",
...

3. Back in example.py's tab, open the command palette and select Format Document.
   It will leave 2 empty lines at the end of the file.

4. Repeat step 3. It will remove one of the empty lines.

Expected behavior
Step 3 should result in an output that wouldn't change on a second pass of black (i.e., the output after step 4).

Environment:
Version: 19.10b0

Note: This is the version that black -- version outputs. However, I'm not 100% sure if ms-python.python is using that version.

• OS and Python version:
  Linux, Python 3.6.9

Does this bug also happen on master?
I'm not sure (see note above)

Additional context
I believe this might be a duplicate of #1006, or it could be related to this VS Code issue. Since I'm not quite sure about that, I present this as a new issue, providing a simpler minimal reproducible example

[ichard26]

Thanks for the bug report! These are the more subtle kinds of bugs that harder to spot. Although IMO it seems like a VS Code issue.

With a quick test, I cannot reproduce the bug running the same version of VS Code and the Python extension, but I'll try again harder later. A little note: version 19.10b0 is not master. It is in fact stable. Stable is the release branch that gets updated on every release. As I am writing, the stable branch is 115 commits behind master (we're getting close to being due for a release 😃 ).

[sgleisner]

Oops! I totally confused stable with master. Fixed! Thanks @ichard26 for pointing it out.

Still, I'm not sure on how to check which version of black is using the ms-python.python extension.

I'll try to check if I can reproduce it in another environment. In the meantime, I suggest anybody trying to reproduce the issue to download the file instead of copying and pasting, just to be sure that autoformat on paste/save is not getting in the way of reproducing the problem.

[ichard26]

Okay, so I can reproduce and, yes this is probably a duplicate of #1006. This is probably attributed to the bug report on the Visual Studio Code Python extension issue tracker.

[JelleZijlstra]

Can't reproduce this with Black by itself. It may be an editor bug as @ichard26 mentioned above.