Black formatter doesn't format anything with --diff argument

[decastromonteiro]

Environment data

• VS Code version: 1.49.2
• Extension version (available under the Extensions sidebar): v2020.9.111407
• OS and version: Windows 10 - 10.0.18363
• Python version (& distribution if applicable, e.g. Anaconda): 3.7.6
• Type of virtual environment used (N/A | venv | virtualenv | conda | ...): venv
• Relevant/affected Python packages and their versions: black version 20.8b1
• Relevant/affected Python-related VS Code extensions and their versions: Python v2020.9.111407
• Value of the python.languageServer setting: Microsoft

Expected behaviour

When saving file, black formatter should format the file.

Sample file is:

a = {"1": 2, 
            "3": 3, 

"5": 4}

b = [
    1,
    2,
    3,
    4,
    5,
    6,
    7,8,9,10,10,101,10,101,0,131,
    2
]

Actual behaviour

On save, nothing changes on the file, the file remains unformatted.

Steps to reproduce:

1. Select "black" as Python code formatter
2. Copy the contents of the provided python code
3. Save the file and see if anything happens

Logs

I see that vs code Python extensions uses the --diff key argument when using black. From black documentation it seems --diff doens't format the code, rather it only makes a diff.

--diff Don't write the files back, just output a diff for each file on stdout.

e:\VirtualEnviroments\Scripts\python.exe "c:/Users/Leonardo Monteiro/.vscode/extensions/ms-python.python-2020.9.111407/pythonFiles/pyvsc-run-isolated.py" black --diff --quiet e:\thepacketwizards\web\blog\views\BlogCategory.py
cwd: e:\thepacketwizards
e:\VirtualEnviroments\Scripts\python.exe "c:/Users/Leonardo Monteiro/.vscode/extensions/ms-python.python-2020.9.111407/pythonFiles/pyvsc-run-isolated.py" black --diff --quiet e:\thepacketwizards\web\blog\views\BlogCategory.py
cwd: e:\thepacketwizards
e:\VirtualEnviroments\Scripts\python.exe "c:/Users/Leonardo Monteiro/.vscode/extensions/ms-python.python-2020.9.111407/pythonFiles/pyvsc-run-isolated.py" flake8 --format=%(row)d,%(col)d,%(code).1s,%(code)s:%(text)s e:\thepacketwizards\web\blog\views\BlogCategory.py
cwd: e:\thepacketwizards
e:\VirtualEnviroments\Scripts\python.exe "c:/Users/Leonardo Monteiro/.vscode/extensions/ms-python.python-2020.9.111407/pythonFiles/pyvsc-run-isolated.py" flake8 --format=%(row)d,%(col)d,%(code).1s,%(code)s:%(text)s e:\thepacketwizards\web\blog\views\BlogCategory.py
cwd: e:\thepacketwizards
##########Linting Output - flake8##########

When running black without the diff argument, everything works as expected. Therefore I'm guessing this --diff argument should not be present when calling black.

[karthiknadig]

Duplicate of #14078