Cannot use `match case` to destructure any object using `case object(x=y)`

[RenDelaCruz]

Bug Report

In the global scope, mypy allows destructuring any object in match case with object(x=y):

@dataclass
class A:
    id: int

# SUCCESS
match A(id=5):
    case object(id=object_id):
        print(object_id)

However, in a method, mypy errors out trying to use object(x=y) in match case:

# FAILURE
def get_id_from_any_object(obj: object) -> int | None:
    match obj:
        # main.py:19: error: Class "builtins.object" has no attribute "id"
        case object(id=object_id):
            return object_id
        case _:
            return None

---

Edit: Turns out, it is not due to scoping, but due to the type of the match <subject>. See https://github.com/python/mypy/issues/16912#issuecomment-1939638659

To Reproduce

mypy playground: https://mypy-play.net/?mypy=latest&python=3.12&gist=c5799b1b128c52894f2879f9fc8d0115

Expected Behavior

Should allow using case object(x=y) in match case syntax.

Actual Behavior

Errors out trying case object(x=y) in match case syntax, inside a method. Does not error out in global scope.

Your Environment

• Mypy version used: 1.8.0
• Mypy command-line flags:
• Mypy configuration options from mypy.ini (and other config files):
• Python version used: 3.12

[Hnasar]

I poked at it a little and there is a bug here but I wanted to clarify that this is not influenced by the scope: it has to do with the type of the match "subject".

In your global scope example the subject has type A but in the function, the subject, obj has type object.

If we remove the match statement:

• Normally at runtime if you do python3 -c 'o = object(); o.id' you'll get an AttributeError. ✅
• Mypy also complains in this case: python3 -m mypy -c 'o = object(); o.id': ✅
  error: "object" has no attribute "id" [attr-defined]

However, with a match statment it's a bit different

match object():
    case object(id=object_id):
        print("match")
    case _:
        print("no match")

• No error at runtime: ✅
• Unfortunately mypy errors: 🐞🐞🐞
  error: Class "builtins.object" has no attribute "id"

[TeamSpen210]

The match statement is more like a hasattr() check, which mypy does have the ability to handle. But changing the behaviour to do that then opens up false negatives where mypy fails to warn you about typos etc.

[erictraut]

A similar question was recently posted to the pyright issue tracker. From my perspective, this is a legitimate use of pattern matching. Pyright therefore does not generate a type error for the code at the top of this thread. As @TeamSpen210 notes, this comes at the expense of some potential false negatives.

[RenDelaCruz]

I agree, using object() in this case is a legitimate use of pattern matching. Since this works with regular Python, and reduces excessive use of hasattr checks.

Perhaps mypy should make an exception for subjects with an object type, when used in match case statements only