ruff/crates/ty_python_semantic/resources/mdtest/subscript/instance.md

Instance subscript

__getitem__ unbound

class NotSubscriptable: ...

a = NotSubscriptable()[0]  # error: "Cannot subscript object of type `NotSubscriptable` with no `__getitem__` method"

__getitem__ not callable

class NotSubscriptable:
    __getitem__ = None

# TODO: this would be more user-friendly if the `call-non-callable` diagnostic was
# transformed into a `not-subscriptable` diagnostic with a subdiagnostic explaining
# that this was because `__getitem__` was possibly not callable
#
# error: [call-non-callable] "Method `__getitem__` of type `Unknown | None` may not be callable on object of type `NotSubscriptable`"
a = NotSubscriptable()[0]

Valid __getitem__

class Identity:
    def __getitem__(self, index: int) -> int:
        return index

reveal_type(Identity()[0])  # revealed: int

__getitem__ union

def _(flag: bool):
    class Identity:
        if flag:
            def __getitem__(self, index: int) -> int:
                return index
        else:
            def __getitem__(self, index: int) -> str:
                return str(index)

    reveal_type(Identity()[0])  # revealed: int | str

__getitem__ with invalid index argument

class Identity:
    def __getitem__(self, index: int) -> int:
        return index

a = Identity()
# error: [invalid-argument-type] "Method `__getitem__` of type `bound method Identity.__getitem__(index: int) -> int` cannot be called with key of type `Literal["a"]` on object of type `Identity`"
a["a"]

__setitem__ with no __getitem__

class NoGetitem:
    def __setitem__(self, index: int, value: int) -> None:
        pass

a = NoGetitem()
a[0] = 0

Subscript store with no __setitem__

class NoSetitem: ...

a = NoSetitem()
a[0] = 0  # error: "Cannot assign to object of type `NoSetitem` with no `__setitem__` method"

__setitem__ not callable

class NoSetitem:
    __setitem__ = None

a = NoSetitem()
a[0] = 0  # error: "Method `__setitem__` of type `Unknown | None` may not be callable on object of type `NoSetitem`"

Valid __setitem__ method

class Identity:
    def __setitem__(self, index: int, value: int) -> None:
        pass

a = Identity()
a[0] = 0

__setitem__ with invalid index argument

class Identity:
    def __setitem__(self, index: int, value: int) -> None:
        pass

a = Identity()
# error: [invalid-assignment] "Method `__setitem__` of type `bound method Identity.__setitem__(index: int, value: int) -> None` cannot be called with a key of type `Literal["a"]` and a value of type `Literal[0]` on object of type `Identity`"
a["a"] = 0