ruff/crates/red_knot_python_semantic/resources/mdtest/call/methods.md

Methods

Background: Functions as descriptors

Note: See also this related section in the descriptor guide: Functions and methods.

Say we have a simple class C with a function definition f inside its body:

class C:
    def f(self, x: int) -> str:
        return "a"

Whenever we access the f attribute through the class object itself (C.f) or through an instance (C().f), this access happens via the descriptor protocol. Functions are (non-data) descriptors because they implement a __get__ method. This is crucial in making sure that method calls work as expected. In general, the signature of the __get__ method in the descriptor protocol is __get__(self, instance, owner). The self argument is the descriptor object itself (f). The passed value for the instance argument depends on whether the attribute is accessed from the class object (in which case it is None), or from an instance (in which case it is the instance of type C). The owner argument is the class itself (C of type Literal[C]). To summarize:

• C.f is equivalent to getattr_static(C, "f").__get__(None, C)
• C().f is equivalent to getattr_static(C, "f").__get__(C(), C)

Here, inspect.getattr_static is used to bypass the descriptor protocol and directly access the function attribute. The way the special __get__ method on functions works is as follows. In the former case, if the instance argument is None, __get__ simply returns the function itself. In the latter case, it returns a bound method object:

from inspect import getattr_static

reveal_type(getattr_static(C, "f"))  # revealed: Literal[f]

reveal_type(getattr_static(C, "f").__get__)  # revealed: <method-wrapper `__get__` of `f`>

reveal_type(getattr_static(C, "f").__get__(None, C))  # revealed: Literal[f]
reveal_type(getattr_static(C, "f").__get__(C(), C))  # revealed: <bound method `f` of `C`>

In conclusion, this is why we see the following two types when accessing the f attribute on the class object C and on an instance C():

reveal_type(C.f)  # revealed: Literal[f]
reveal_type(C().f)  # revealed: <bound method `f` of `C`>

A bound method is a callable object that contains a reference to the instance that it was called on (can be inspected via __self__), and the function object that it refers to (can be inspected via __func__):

bound_method = C().f

reveal_type(bound_method.__self__)  # revealed: C
reveal_type(bound_method.__func__)  # revealed: Literal[f]

When we call the bound method, the instance is implicitly passed as the first argument (self):

reveal_type(C().f(1))  # revealed: str
reveal_type(bound_method(1))  # revealed: str

When we call the function object itself, we need to pass the instance explicitly:

C.f(1)  # error: [missing-argument]

reveal_type(C.f(C(), 1))  # revealed: str

When we access methods from derived classes, they will be bound to instances of the derived class:

class D(C):
    pass

reveal_type(D().f)  # revealed: <bound method `f` of `D`>

If we access an attribute on a bound method object itself, it will defer to types.MethodType:

reveal_type(bound_method.__hash__)  # revealed: <bound method `__hash__` of `MethodType`>

If an attribute is not available on the bound method object, it will be looked up on the underlying function object. We model this explicitly, which means that we can access __kwdefaults__ on bound methods, even though it is not available on types.MethodType:

reveal_type(bound_method.__kwdefaults__)  # revealed: @Todo(generics) | None

Basic method calls on class objects and instances

class Base:
    def method_on_base(self, x: int | None) -> str:
        return "a"

class Derived(Base):
    def method_on_derived(self, x: bytes) -> tuple[int, str]:
        return (1, "a")

reveal_type(Base().method_on_base(1))  # revealed: str
reveal_type(Base.method_on_base(Base(), 1))  # revealed: str

Base().method_on_base("incorrect")  # error: [invalid-argument-type]
Base().method_on_base()  # error: [missing-argument]
Base().method_on_base(1, 2)  # error: [too-many-positional-arguments]

reveal_type(Derived().method_on_base(1))  # revealed: str
reveal_type(Derived().method_on_derived(b"abc"))  # revealed: tuple[int, str]
reveal_type(Derived.method_on_base(Derived(), 1))  # revealed: str
reveal_type(Derived.method_on_derived(Derived(), b"abc"))  # revealed: tuple[int, str]

Method calls on literals

Boolean literals

reveal_type(True.bit_length())  # revealed: int
reveal_type(True.as_integer_ratio())  # revealed: tuple[int, Literal[1]]

Integer literals

reveal_type((42).bit_length())  # revealed: int

String literals

reveal_type("abcde".find("abc"))  # revealed: int
reveal_type("foo".encode(encoding="utf-8"))  # revealed: bytes

"abcde".find(123)  # error: [invalid-argument-type]

Bytes literals

reveal_type(b"abcde".startswith(b"abc"))  # revealed: bool

Method calls on LiteralString

from typing_extensions import LiteralString

def f(s: LiteralString) -> None:
    reveal_type(s.find("a"))  # revealed: int

Method calls on tuple

def f(t: tuple[int, str]) -> None:
    reveal_type(t.index("a"))  # revealed: int

Method calls on unions

from typing import Any

class A:
    def f(self) -> int:
        return 1

class B:
    def f(self) -> str:
        return "a"

def f(a_or_b: A | B, any_or_a: Any | A):
    reveal_type(a_or_b.f)  # revealed: <bound method `f` of `A`> | <bound method `f` of `B`>
    reveal_type(a_or_b.f())  # revealed: int | str

    reveal_type(any_or_a.f)  # revealed: Any | <bound method `f` of `A`>
    reveal_type(any_or_a.f())  # revealed: Any | int

Method calls on KnownInstance types

[environment]
python-version = "3.12"

type IntOrStr = int | str

reveal_type(IntOrStr.__or__)  # revealed: <bound method `__or__` of `typing.TypeAliasType`>

Error cases: Calling __get__ for methods

The __get__ method on types.FunctionType has the following overloaded signature in typeshed:

from types import FunctionType, MethodType
from typing import overload

@overload
def __get__(self, instance: None, owner: type, /) -> FunctionType: ...
@overload
def __get__(self, instance: object, owner: type | None = None, /) -> MethodType: ...

Here, we test that this signature is enforced correctly:

from inspect import getattr_static

class C:
    def f(self, x: int) -> str:
        return "a"

method_wrapper = getattr_static(C, "f").__get__

reveal_type(method_wrapper)  # revealed: <method-wrapper `__get__` of `f`>

# All of these are fine:
method_wrapper(C(), C)
method_wrapper(C())
method_wrapper(C(), None)
method_wrapper(None, C)

# Passing `None` without an `owner` argument is an
# error: [missing-argument] "No argument provided for required parameter `owner`"
method_wrapper(None)

# Passing something that is not assignable to `type` as the `owner` argument is an
# error: [invalid-argument-type] "Object of type `Literal[1]` cannot be assigned to parameter 2 (`owner`) of method wrapper `__get__` of function `f`; expected type `type`"
method_wrapper(None, 1)

# Passing `None` as the `owner` argument when `instance` is `None` is an
# error: [invalid-argument-type] "Object of type `None` cannot be assigned to parameter 2 (`owner`) of method wrapper `__get__` of function `f`; expected type `type`"
method_wrapper(None, None)

# Calling `__get__` without any arguments is an
# error: [missing-argument] "No argument provided for required parameter `instance`"
method_wrapper()

# Calling `__get__` with too many positional arguments is an
# error: [too-many-positional-arguments] "Too many positional arguments to method wrapper `__get__` of function `f`: expected 2, got 3"
method_wrapper(C(), C, "one too many")

Fallback to metaclass

When a method is accessed on a class object, it is looked up on the metaclass if it is not found on the class itself. This also creates a bound method that is bound to the class object itself:

from __future__ import annotations

class Meta(type):
    def f(cls, arg: int) -> str:
        return "a"

class C(metaclass=Meta):
    pass

reveal_type(C.f)  # revealed: <bound method `f` of `Literal[C]`>
reveal_type(C.f(1))  # revealed: str

The method f can not be accessed from an instance of the class:

# error: [unresolved-attribute] "Type `C` has no attribute `f`"
C().f

A metaclass function can be shadowed by a method on the class:

from typing import Any, Literal

class D(metaclass=Meta):
    def f(arg: int) -> Literal["a"]:
        return "a"

reveal_type(D.f(1))  # revealed: Literal["a"]

If the class method is possibly unbound, we union the return types:

def flag() -> bool:
    return True

class E(metaclass=Meta):
    if flag():
        def f(arg: int) -> Any:
            return "a"

reveal_type(E.f(1))  # revealed: str | Any

@classmethod

Basic

When a @classmethod attribute is accessed, it returns a bound method object, even when accessed on the class object itself:

from __future__ import annotations

class C:
    @classmethod
    def f(cls: type[C], x: int) -> str:
        return "a"

reveal_type(C.f)  # revealed: <bound method `f` of `Literal[C]`>
reveal_type(C().f)  # revealed: <bound method `f` of `type[C]`>

The cls method argument is then implicitly passed as the first argument when calling the method:

reveal_type(C.f(1))  # revealed: str
reveal_type(C().f(1))  # revealed: str

When the class method is called incorrectly, we detect it:

C.f("incorrect")  # error: [invalid-argument-type]
C.f()  # error: [missing-argument]
C.f(1, 2)  # error: [too-many-positional-arguments]

If the cls parameter is wrongly annotated, we emit an error at the call site:

class D:
    @classmethod
    def f(cls: D):
        # This function is wrongly annotated, it should be `type[D]` instead of `D`
        pass

# error: [invalid-argument-type] "Object of type `Literal[D]` cannot be assigned to parameter 1 (`cls`) of bound method `f`; expected type `D`"
D.f()

When a class method is accessed on a derived class, it is bound to that derived class:

class Derived(C):
    pass

reveal_type(Derived.f)  # revealed: <bound method `f` of `Literal[Derived]`>
reveal_type(Derived().f)  # revealed: <bound method `f` of `type[Derived]`>

reveal_type(Derived.f(1))  # revealed: str
reveal_type(Derived().f(1))  # revealed: str

Accessing the classmethod as a static member

Accessing a @classmethod-decorated function at runtime returns a classmethod object. We currently don't model this explicitly:

from inspect import getattr_static

class C:
    @classmethod
    def f(cls): ...

reveal_type(getattr_static(C, "f"))  # revealed: Literal[f]
reveal_type(getattr_static(C, "f").__get__)  # revealed: <method-wrapper `__get__` of `f`>

But we correctly model how the classmethod descriptor works:

reveal_type(getattr_static(C, "f").__get__(None, C))  # revealed: <bound method `f` of `Literal[C]`>
reveal_type(getattr_static(C, "f").__get__(C(), C))  # revealed: <bound method `f` of `Literal[C]`>
reveal_type(getattr_static(C, "f").__get__(C()))  # revealed: <bound method `f` of `type[C]`>

The owner argument takes precedence over the instance argument:

reveal_type(getattr_static(C, "f").__get__("dummy", C))  # revealed: <bound method `f` of `Literal[C]`>

Classmethods mixed with other decorators

When a @classmethod is additionally decorated with another decorator, it is still treated as a class method:

from __future__ import annotations

def does_nothing[T](f: T) -> T:
    return f

class C:
    @classmethod
    @does_nothing
    def f1(cls: type[C], x: int) -> str:
        return "a"

    @does_nothing
    @classmethod
    def f2(cls: type[C], x: int) -> str:
        return "a"

# TODO: We do not support decorators yet (only limited special cases). Eventually,
# these should all return `str`:

reveal_type(C.f1(1))  # revealed: @Todo(return type of decorated function)
reveal_type(C().f1(1))  # revealed: @Todo(return type of decorated function)

reveal_type(C.f2(1))  # revealed: @Todo(return type of decorated function)
reveal_type(C().f2(1))  # revealed: @Todo(return type of decorated function)