Improve match statement union narrowing/inference by bergkvist · Pull Request #17600 · python/mypy · GitHub

Improve inference/narrowing support for union types in match statements.

Before:

var: tuple[int, int] | tuple[str, str]

# TODO: we can infer better here.
match var:
    case (42, a):
        reveal_type(a)  # N: Revealed type is "Union[builtins.int, builtins.str]"
    case ("yes", b):
        reveal_type(b)  # N: Revealed type is "Union[builtins.int, builtins.str]"

After:

var: tuple[int, int] | tuple[str, str]

match var:
    case (42, a):
        reveal_type(a)  # N: Revealed type is "builtins.int"
    case ("yes", b):
        reveal_type(b)  # N: Revealed type is "builtins.str"

Support for python 3.10 match statement #10191

…tures

Nice!
Does your fix address any of the other match statement issues?
https://github.com/python/mypy/issues?q=sort%3Aupdated-desc+is%3Aopen+label%3Atopic-match-statement

Co-authored-by: Hashem <Hnasar@users.noreply.github.com>

Nice! Does your fix address any of the other match statement issues? https://github.com/python/mypy/issues?q=sort%3Aupdated-desc+is%3Aopen+label%3Atopic-match-statement

From reading through the issues, I believe it should at least solve these:

I notice there are a bunch of match exhaustiveness issues with narrowing of the type passed on to the next match branches, which this PR does not fix.

maybe it would address this too? #16835

maybe it would address this too? #16835

I don't think so, as I return the "current type" as the "rest type", which means the remainder/rest type (which is what gets passed on to the next match case) doesn't get narrowed properly yet. I have yet to figure out how to do this.

for more information, see https://pre-commit.ci

Hi @bergkvist @Hnasar are there any blockers for landing this PR?
even though it might not address all match narrowing issues it moves it towards a better place.

I don't think this is solving the issue in my use case involving TypedDicts:

$ uvx --from 'mypy @ git+https://github.com/bergkvist/mypy@tobias/match-union-narrowing' mypy
    Updated https://github.com/bergkvist/mypy (92d3c91aaf2757706f9ef923650eb342d19af2fb)
      Built mypy @ git+https://github.com/bergkvist/mypy@92d3c91aaf2757706f9ef923650eb342d19af2fb
Installed 3 packages in 17ms
src/pack.py:150: error: "object" has no attribute "encode"  [attr-defined]
src/pack.py:151: error: "object" has no attribute "encode"  [attr-defined]

I have:

match node:
    case {"type": ("assign" | "cond_assign") as op, "key": key, "value": value}:
        kbs = key.encode()
        vbs = value.encode()
        ...

where reveal_type(node) shows

src/pack.py:148: note: Revealed type is "Union[TypedDict('propsast.AssignmentOperation', {'type': Union[Literal['assign'], Literal['cond_assign']], 'key': builtins.str, 'value': builtins.str, 'lineno': builtins.int}), TypedDict('propsast.IncludeOperation', {'type': Union[Literal['include'], Literal['tryinclude']], 'filename': builtins.str, 'lineno': builtins.int})]"

I tried removing the disjunction in the case statement and having a case branch for each literal value but that didn't help.

@bergkvist Sorry for the late reply, but can you rebase this on the latest mypy, then we can ping one of the real maintainers to get someone with commit privileges to take a look.

Thanks for working on this! Added two comments, would also be great to add test cases that can detect the effect of those two suggestions

hmm shouldn't the values in union_captures be a union of all the things, instead of potentially clobbering?

iiuc this should use conditional_types_with_intersection

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅

I realised there is a simpler and more complete approach than the one taken in python#19600. This adds the new "Step 2" to the original code. Best way to review the diff is probably to check it out and review it squashed with python#19600 , but really the net logic change is just "Step 2" Fixes python#15190 Fixes python#17549 Fixes python#17600 Mostly fixes python#18039 Helps with things in python#19081

bergkvist added 2 commits July 28, 2024 01:18

Improve match statement union narrowing/inference

100c659

Fix lint errors: Use get_proper_type and rename captures to union_cap…

c62587a

…tures