Black formatting leaves an extra line at the end of the file

[Tinche]

Environment data

• VS Code version: 1.23.1
• Extension version (available under the Extensions sidebar): 2018.5.0
• OS and version: macOs High Sierra
• Python version (& distribution if applicable, e.g. Anaconda): CPython 3.6.5
• Type of virtual environment used (N/A | venv | virtualenv | conda | ...): venv
• Relevant/affected Python packages and their versions: black-18.6b1

Actual behavior

Formatting using black leaves two empty lines at the end of the file, if the file has more than 1 empty line already.

Expected behavior

Formatting should leave just one line, and this is what happens when I run black manually from the terminal.

Steps to reproduce:

1. Open Python file
2. Insert 10 empty lines at the end
3. Activate "Format document" with black set as the formatter

Logs

Output for Python in the Output panel (View→Output, change the drop-down the upper-right of the Output panel to Python)

##########Linting Output - flake8##########
20,1,W,W391:blank line at end of file

Output from Console under the Developer Tools panel (toggle Developer Tools on under Help)

[brettcannon]

I can reproduce and the expected behaviour that @Tinche found and I can verify that using black --diff does what is expected:

--- tpn\tpn\tests\test_npm.py   2018-06-06 22:04:15.383923 +0000
+++ tpn\tpn\tests\test_npm.py   2018-06-06 22:05:29.227218 +0000
@@ -71,16 +71,5 @@

 def test_find_license():
     # XXX
     pass

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[brettcannon]

@DonJayamanne so when I tested this I added the blank lines, we just weren't stripping them out. So I don't think it's a case of adding as much as not removing.

[DonJayamanne]

I think its best to use an npm package (e.g. https://www.npmjs.com/package/diff) rather than parsing diff patches manually.
Moving to July

This will need to be done as part of #1950 as parsing of diff patches is a shared block of code (used by refactoring and sorting of imports).