Enable pattern matching syntax by default

[thisisrandy]

Describe the bug
PEP 634 introduces structural pattern matching in python 3.10. Black at 21.11b1 (latest) does not seem to support this new syntax.

To Reproduce

Here is a basic example of the new syntax, which we will save in match_test.py:

foo = "str"
match foo:
    case str():
        print("string!")
    case int():
        print("int!")
    case _:
        print("something else!")

> python --version
Python 3.10.0

> python match_test.py
string!

> python -m black --version
python -m black, 21.11b1 (compiled: no)

> python -m black match_test.py
error: cannot format match_test.py: Cannot parse: 2:6: match foo:
Oh no! 💥 💔 💥
1 file failed to reformat.

Expected behavior

Structural pattern matching (and all other new python 3.10 syntax) should be fully supported.

Environment

• Black's version: 21.11b1
• OS and Python version: Ubuntu 20.04.3 LTS/Python 3.10.0

[ichard26]

Black 21.11b1 does have experimental support for pattern matching syntax, but since it's experimental and is / was quite buggy we decided to lock it under the --target-version py310 option, please try that first, thank you!

[JelleZijlstra]

It is supported, but you need to explicitly set --target-version py310 for now. We did this because the new parser is potentially unstable.

I think we should consider turning it on by default, so I'll rename this issue to reflect that suggestion. cc @isidentical

[thisisrandy]

Thanks for the quick replies. I'll try out --target-version py310 and look forward to seeing it stable and on by default soon.

[thisisrandy]

Confirmed that at least this basic example works with --target-version py310. Also for fellow VS Code pylance users, it took me a moment to figure out how to configure things, so I'll detail here. In Settings, search for @ext:ms-python.python black, then add two items to Python>Formatting: Black Args, namely --target-version and py310.

[Kos]

FWIW would be great experience if error: cannot format match_test.py: Cannot parse: 2:6: match foo: would also include hint: support for match statements is experiemental, enable it using --target-version py310

[lpil]

Is there a way to specify this in a configuration file? I would like the editors for all the members of my team to automatically have this flag set.

[JelleZijlstra]

@Kos's suggestion is implemented in 21.12b0. @lpil you can write target_version = ['py310'] in your pyproject.toml.

[ichard26]

• Implemented on main and will be coming out soon hopefully later today: Enable pattern matching by default #2758 🎉