ruff/crates/ty_python_semantic/resources/mdtest/mro.md

Method Resolution Order tests

Tests that assert that we can infer the correct type for a class's __mro__ attribute.

This attribute is rarely accessed directly at runtime. However, it's extremely important for us to know the precise possible values of a class's Method Resolution Order, or we won't be able to infer the correct type of attributes accessed from instances.

For documentation on method resolution orders, see:

• https://docs.python.org/3/glossary.html#term-method-resolution-order
• https://docs.python.org/3/howto/mro.html#python-2-3-mro

No bases

class C: ...

reveal_type(C.__mro__)  # revealed: tuple[<class 'C'>, <class 'object'>]

The special case: object itself

reveal_type(object.__mro__)  # revealed: tuple[<class 'object'>]

Explicit inheritance from object

class C(object): ...

reveal_type(C.__mro__)  # revealed: tuple[<class 'C'>, <class 'object'>]

Explicit inheritance from non-object single base

class A: ...
class B(A): ...

reveal_type(B.__mro__)  # revealed: tuple[<class 'B'>, <class 'A'>, <class 'object'>]

Linearization of multiple bases

class A: ...
class B: ...
class C(A, B): ...

reveal_type(C.__mro__)  # revealed: tuple[<class 'C'>, <class 'A'>, <class 'B'>, <class 'object'>]

Complex diamond inheritance (1)

This is "ex_2" from https://docs.python.org/3/howto/mro.html#the-end

class O: ...
class X(O): ...
class Y(O): ...
class A(X, Y): ...
class B(Y, X): ...

reveal_type(A.__mro__)  # revealed: tuple[<class 'A'>, <class 'X'>, <class 'Y'>, <class 'O'>, <class 'object'>]
reveal_type(B.__mro__)  # revealed: tuple[<class 'B'>, <class 'Y'>, <class 'X'>, <class 'O'>, <class 'object'>]

Complex diamond inheritance (2)

This is "ex_5" from https://docs.python.org/3/howto/mro.html#the-end

class O: ...
class F(O): ...
class E(O): ...
class D(O): ...
class C(D, F): ...
class B(D, E): ...
class A(B, C): ...

# revealed: tuple[<class 'C'>, <class 'D'>, <class 'F'>, <class 'O'>, <class 'object'>]
reveal_type(C.__mro__)
# revealed: tuple[<class 'B'>, <class 'D'>, <class 'E'>, <class 'O'>, <class 'object'>]
reveal_type(B.__mro__)
# revealed: tuple[<class 'A'>, <class 'B'>, <class 'C'>, <class 'D'>, <class 'E'>, <class 'F'>, <class 'O'>, <class 'object'>]
reveal_type(A.__mro__)

Complex diamond inheritance (3)

This is "ex_6" from https://docs.python.org/3/howto/mro.html#the-end

class O: ...
class F(O): ...
class E(O): ...
class D(O): ...
class C(D, F): ...
class B(E, D): ...
class A(B, C): ...

# revealed: tuple[<class 'C'>, <class 'D'>, <class 'F'>, <class 'O'>, <class 'object'>]
reveal_type(C.__mro__)
# revealed: tuple[<class 'B'>, <class 'E'>, <class 'D'>, <class 'O'>, <class 'object'>]
reveal_type(B.__mro__)
# revealed: tuple[<class 'A'>, <class 'B'>, <class 'E'>, <class 'C'>, <class 'D'>, <class 'F'>, <class 'O'>, <class 'object'>]
reveal_type(A.__mro__)

Complex diamond inheritance (4)

This is "ex_9" from https://docs.python.org/3/howto/mro.html#the-end

class O: ...
class A(O): ...
class B(O): ...
class C(O): ...
class D(O): ...
class E(O): ...
class K1(A, B, C): ...
class K2(D, B, E): ...
class K3(D, A): ...
class Z(K1, K2, K3): ...

# revealed: tuple[<class 'K1'>, <class 'A'>, <class 'B'>, <class 'C'>, <class 'O'>, <class 'object'>]
reveal_type(K1.__mro__)
# revealed: tuple[<class 'K2'>, <class 'D'>, <class 'B'>, <class 'E'>, <class 'O'>, <class 'object'>]
reveal_type(K2.__mro__)
# revealed: tuple[<class 'K3'>, <class 'D'>, <class 'A'>, <class 'O'>, <class 'object'>]
reveal_type(K3.__mro__)
# revealed: tuple[<class 'Z'>, <class 'K1'>, <class 'K2'>, <class 'K3'>, <class 'D'>, <class 'A'>, <class 'B'>, <class 'C'>, <class 'E'>, <class 'O'>, <class 'object'>]
reveal_type(Z.__mro__)

Inheritance from Unknown

from does_not_exist import DoesNotExist  # error: [unresolved-import]

class A(DoesNotExist): ...
class B: ...
class C: ...
class D(A, B, C): ...
class E(B, C): ...
class F(E, A): ...

reveal_type(A.__mro__)  # revealed: tuple[<class 'A'>, Unknown, <class 'object'>]
reveal_type(D.__mro__)  # revealed: tuple[<class 'D'>, <class 'A'>, Unknown, <class 'B'>, <class 'C'>, <class 'object'>]
reveal_type(E.__mro__)  # revealed: tuple[<class 'E'>, <class 'B'>, <class 'C'>, <class 'object'>]
# revealed: tuple[<class 'F'>, <class 'E'>, <class 'B'>, <class 'C'>, <class 'A'>, Unknown, <class 'object'>]
reveal_type(F.__mro__)

Inheritance with intersections that include Unknown

An intersection that includes Unknown or Any is permitted as long as the intersection is not disjoint from type.

from does_not_exist import DoesNotExist  # error: [unresolved-import]

reveal_type(DoesNotExist)  # revealed: Unknown

if hasattr(DoesNotExist, "__mro__"):
    reveal_type(DoesNotExist)  # revealed: Unknown & <Protocol with members '__mro__'>

    class Foo(DoesNotExist): ...  # no error!
    reveal_type(Foo.__mro__)  # revealed: tuple[<class 'Foo'>, Unknown, <class 'object'>]

if not isinstance(DoesNotExist, type):
    reveal_type(DoesNotExist)  # revealed: Unknown & ~type

    class Foo(DoesNotExist): ...  # error: [unsupported-base]
    reveal_type(Foo.__mro__)  # revealed: tuple[<class 'Foo'>, Unknown, <class 'object'>]

Inheritance from type[Any] and type[Unknown]

Inheritance from type[Any] and type[Unknown] is also permitted, in keeping with the gradual guarantee:

from typing import Any
from ty_extensions import Unknown, Intersection

def f(x: type[Any], y: Intersection[Unknown, type[Any]]):
    class Foo(x): ...
    reveal_type(Foo.__mro__)  # revealed: tuple[<class 'Foo'>, Any, <class 'object'>]

    class Bar(y): ...
    reveal_type(Bar.__mro__)  # revealed: tuple[<class 'Bar'>, Unknown, <class 'object'>]

__bases__ lists that cause errors at runtime

If the class's __bases__ cause an exception to be raised at runtime and therefore the class creation to fail, we infer the class's __mro__ as being [<class>, Unknown, object]:

# error: [inconsistent-mro] "Cannot create a consistent method resolution order (MRO) for class `Foo` with bases list `[<class 'object'>, <class 'int'>]`"
class Foo(object, int): ...

reveal_type(Foo.__mro__)  # revealed: tuple[<class 'Foo'>, Unknown, <class 'object'>]

class Bar(Foo): ...

reveal_type(Bar.__mro__)  # revealed: tuple[<class 'Bar'>, <class 'Foo'>, Unknown, <class 'object'>]

# This is the `TypeError` at the bottom of "ex_2"
# in the examples at <https://docs.python.org/3/howto/mro.html#the-end>
class O: ...
class X(O): ...
class Y(O): ...
class A(X, Y): ...
class B(Y, X): ...

reveal_type(A.__mro__)  # revealed: tuple[<class 'A'>, <class 'X'>, <class 'Y'>, <class 'O'>, <class 'object'>]
reveal_type(B.__mro__)  # revealed: tuple[<class 'B'>, <class 'Y'>, <class 'X'>, <class 'O'>, <class 'object'>]

# error: [inconsistent-mro] "Cannot create a consistent method resolution order (MRO) for class `Z` with bases list `[<class 'A'>, <class 'B'>]`"
class Z(A, B): ...

reveal_type(Z.__mro__)  # revealed: tuple[<class 'Z'>, Unknown, <class 'object'>]

class AA(Z): ...

reveal_type(AA.__mro__)  # revealed: tuple[<class 'AA'>, <class 'Z'>, Unknown, <class 'object'>]

__bases__ includes a Union

We don't support union types in a class's bases; a base must resolve to a single ClassType. If we find a union type in a class's bases, we infer the class's __mro__ as being [<class>, Unknown, object], the same as for MROs that cause errors at runtime.

from typing_extensions import reveal_type

def returns_bool() -> bool:
    return True

class A: ...
class B: ...

if returns_bool():
    x = A
else:
    x = B

reveal_type(x)  # revealed: <class 'A'> | <class 'B'>

# error: 11 [unsupported-base] "Unsupported class base with type `<class 'A'> | <class 'B'>`"
class Foo(x): ...

reveal_type(Foo.__mro__)  # revealed: tuple[<class 'Foo'>, Unknown, <class 'object'>]

__bases__ is a union of a dynamic type and valid bases

If a dynamic type such as Any or Unknown is one of the elements in the union, and all other types would be valid class bases, we do not emit an invalid-base or unsupported-base diagnostic, and we use the dynamic type as a base to prevent further downstream errors.

from typing import Any

def _(flag: bool, any: Any):
    if flag:
        Base = any
    else:
        class Base: ...

    class Foo(Base): ...
    reveal_type(Foo.__mro__)  # revealed: tuple[<class 'Foo'>, Any, <class 'object'>]

__bases__ includes multiple Unions

def returns_bool() -> bool:
    return True

class A: ...
class B: ...
class C: ...
class D: ...

if returns_bool():
    x = A
else:
    x = B

if returns_bool():
    y = C
else:
    y = D

reveal_type(x)  # revealed: <class 'A'> | <class 'B'>
reveal_type(y)  # revealed: <class 'C'> | <class 'D'>

# error: 11 [unsupported-base] "Unsupported class base with type `<class 'A'> | <class 'B'>`"
# error: 14 [unsupported-base] "Unsupported class base with type `<class 'C'> | <class 'D'>`"
class Foo(x, y): ...

reveal_type(Foo.__mro__)  # revealed: tuple[<class 'Foo'>, Unknown, <class 'object'>]

__bases__ lists that cause errors... now with Unions

def returns_bool() -> bool:
    return True

class O: ...
class X(O): ...
class Y(O): ...

if returns_bool():
    foo = Y
else:
    foo = object

# error: 21 [unsupported-base] "Unsupported class base with type `<class 'Y'> | <class 'object'>`"
class PossibleError(foo, X): ...

reveal_type(PossibleError.__mro__)  # revealed: tuple[<class 'PossibleError'>, Unknown, <class 'object'>]

class A(X, Y): ...

reveal_type(A.__mro__)  # revealed: tuple[<class 'A'>, <class 'X'>, <class 'Y'>, <class 'O'>, <class 'object'>]

if returns_bool():
    class B(X, Y): ...

else:
    class B(Y, X): ...

# revealed: tuple[<class 'B'>, <class 'X'>, <class 'Y'>, <class 'O'>, <class 'object'>] | tuple[<class 'B'>, <class 'Y'>, <class 'X'>, <class 'O'>, <class 'object'>]
reveal_type(B.__mro__)

# error: 12 [unsupported-base] "Unsupported class base with type `<class 'B'> | <class 'B'>`"
class Z(A, B): ...

reveal_type(Z.__mro__)  # revealed: tuple[<class 'Z'>, Unknown, <class 'object'>]

__bases__ lists that include objects that are not instances of type

class Foo(2): ...  # error: [invalid-base]

A base that is not an instance of type but does have an __mro_entries__ method will not raise an exception at runtime, so we issue unsupported-base rather than invalid-base:

class Foo:
    def __mro_entries__(self, bases: tuple[type, ...]) -> tuple[type, ...]:
        return ()

class Bar(Foo()): ...  # error: [unsupported-base]

But for objects that have badly defined __mro_entries__, invalid-base is emitted rather than unsupported-base:

class Bad1:
    def __mro_entries__(self, bases, extra_arg):
        return ()

class Bad2:
    def __mro_entries__(self, bases) -> int:
        return 42

class BadSub1(Bad1()): ...  # error: [invalid-base]
class BadSub2(Bad2()): ...  # error: [invalid-base]

__bases__ lists with duplicate bases

from typing_extensions import reveal_type

class Foo(str, str): ...  # error: [duplicate-base] "Duplicate base class `str`"

reveal_type(Foo.__mro__)  # revealed: tuple[<class 'Foo'>, Unknown, <class 'object'>]

class Spam: ...
class Eggs: ...
class Bar: ...
class Baz: ...

# fmt: off

# error: [duplicate-base] "Duplicate base class `Spam`"
# error: [duplicate-base] "Duplicate base class `Eggs`"
class Ham(
    Spam,
    Eggs,
    Bar,
    Baz,
    Spam,
    Eggs,
): ...

# fmt: on

reveal_type(Ham.__mro__)  # revealed: tuple[<class 'Ham'>, Unknown, <class 'object'>]

class Mushrooms: ...
class Omelette(Spam, Eggs, Mushrooms, Mushrooms): ...  # error: [duplicate-base]

reveal_type(Omelette.__mro__)  # revealed: tuple[<class 'Omelette'>, Unknown, <class 'object'>]

# fmt: off

# error: [duplicate-base] "Duplicate base class `Eggs`"
class VeryEggyOmelette(
    Eggs,
    Ham,
    Spam,
    Eggs,
    Mushrooms,
    Bar,
    Eggs,
    Baz,
    Eggs,
): ...

# fmt: off

A type: ignore comment can suppress duplicate-bases errors if it is on the first or last line of the class "header":

# fmt: off

class A: ...

class B(  # type: ignore[duplicate-base]
    A,
    A,
): ...

class C(
    A,
    A
):  # type: ignore[duplicate-base]
    x: int

# fmt: on

But it will not suppress the error if it occurs in the class body, or on the duplicate base itself. The justification for this is that it is the class definition as a whole that will raise an exception at runtime, not a sub-expression in the class's bases list.

# fmt: off

# error: [duplicate-base]
class D(
    A,
    # error: [unused-ignore-comment]
    A,  # type: ignore[duplicate-base]
): ...

# error: [duplicate-base]
class E(
    A,
    A
):
    # error: [unused-ignore-comment]
    x: int  # type: ignore[duplicate-base]

# fmt: on

__bases__ lists with duplicate Unknown bases

We do not emit errors on classes where multiple bases are inferred as Unknown, Todo or Any. Usually having duplicate bases in a bases list like this would cause us to emit a diagnostic; however, for gradual types this would break the gradual guarantee: the dynamic base can usually be materialised to a type that would lead to a resolvable MRO.

from unresolvable_module import UnknownBase1, UnknownBase2  # error: [unresolved-import]

reveal_type(UnknownBase1)  # revealed: Unknown
reveal_type(UnknownBase2)  # revealed: Unknown

# no error here -- we respect the gradual guarantee:
class Foo(UnknownBase1, UnknownBase2): ...

reveal_type(Foo.__mro__)  # revealed: tuple[<class 'Foo'>, Unknown, <class 'object'>]

However, if there are duplicate class elements, we do emit an error, even if there are also multiple dynamic members. The following class definition will definitely fail, no matter what the dynamic bases materialize to:

# error: [duplicate-base] "Duplicate base class `Foo`"
class Bar(UnknownBase1, Foo, UnknownBase2, Foo): ...

reveal_type(Bar.__mro__)  # revealed: tuple[<class 'Bar'>, Unknown, <class 'object'>]

Unrelated objects inferred as Any/Unknown do not have special __mro__ attributes

from does_not_exist import unknown_object  # error: [unresolved-import]

reveal_type(unknown_object)  # revealed: Unknown
reveal_type(unknown_object.__mro__)  # revealed: Unknown

MROs of classes that use multiple inheritance with generic aliases and subscripted Generic

from typing import Generic, TypeVar, Iterator

T = TypeVar("T")

class peekable(Generic[T], Iterator[T]): ...

# revealed: tuple[<class 'peekable[Unknown]'>, <class 'Iterator[T]'>, <class 'Iterable[T]'>, typing.Protocol, typing.Generic, <class 'object'>]
reveal_type(peekable.__mro__)

class peekable2(Iterator[T], Generic[T]): ...

# revealed: tuple[<class 'peekable2[Unknown]'>, <class 'Iterator[T]'>, <class 'Iterable[T]'>, typing.Protocol, typing.Generic, <class 'object'>]
reveal_type(peekable2.__mro__)

class Base: ...
class Intermediate(Base, Generic[T]): ...
class Sub(Intermediate[T], Base): ...

# revealed: tuple[<class 'Sub[Unknown]'>, <class 'Intermediate[T]'>, <class 'Base'>, typing.Generic, <class 'object'>]
reveal_type(Sub.__mro__)

Unresolvable MROs involving generics have the original bases reported in the error message, not the resolved bases

from typing_extensions import Protocol, TypeVar, Generic

T = TypeVar("T")

class Foo(Protocol): ...
class Bar(Protocol[T]): ...
class Baz(Protocol[T], Foo, Bar[T]): ...  # error: [inconsistent-mro]

Classes that inherit from themselves

These are invalid, but we need to be able to handle them gracefully without panicking.

class Foo(Foo): ...  # error: [cyclic-class-definition]

reveal_type(Foo)  # revealed: <class 'Foo'>
reveal_type(Foo.__mro__)  # revealed: tuple[<class 'Foo'>, Unknown, <class 'object'>]

class Bar: ...
class Baz: ...
class Boz(Bar, Baz, Boz): ...  # error: [cyclic-class-definition]

reveal_type(Boz)  # revealed: <class 'Boz'>
reveal_type(Boz.__mro__)  # revealed: tuple[<class 'Boz'>, Unknown, <class 'object'>]

Classes with indirect cycles in their MROs

These are similarly unlikely, but we still shouldn't crash:

class Foo(Bar): ...  # error: [cyclic-class-definition]
class Bar(Baz): ...  # error: [cyclic-class-definition]
class Baz(Foo): ...  # error: [cyclic-class-definition]

reveal_type(Foo.__mro__)  # revealed: tuple[<class 'Foo'>, Unknown, <class 'object'>]
reveal_type(Bar.__mro__)  # revealed: tuple[<class 'Bar'>, Unknown, <class 'object'>]
reveal_type(Baz.__mro__)  # revealed: tuple[<class 'Baz'>, Unknown, <class 'object'>]

Classes with cycles in their MROs, and multiple inheritance

class Spam: ...
class Foo(Bar): ...  # error: [cyclic-class-definition]
class Bar(Baz): ...  # error: [cyclic-class-definition]
class Baz(Foo, Spam): ...  # error: [cyclic-class-definition]

reveal_type(Foo.__mro__)  # revealed: tuple[<class 'Foo'>, Unknown, <class 'object'>]
reveal_type(Bar.__mro__)  # revealed: tuple[<class 'Bar'>, Unknown, <class 'object'>]
reveal_type(Baz.__mro__)  # revealed: tuple[<class 'Baz'>, Unknown, <class 'object'>]

Classes with cycles in their MRO, and a sub-graph

class FooCycle(BarCycle): ...  # error: [cyclic-class-definition]
class Foo: ...
class BarCycle(FooCycle): ...  # error: [cyclic-class-definition]
class Bar(Foo): ...

# Avoid emitting the errors for these. The classes have cyclic superclasses,
# but are not themselves cyclic...
class Baz(Bar, BarCycle): ...
class Spam(Baz): ...

reveal_type(FooCycle.__mro__)  # revealed: tuple[<class 'FooCycle'>, Unknown, <class 'object'>]
reveal_type(BarCycle.__mro__)  # revealed: tuple[<class 'BarCycle'>, Unknown, <class 'object'>]
reveal_type(Baz.__mro__)  # revealed: tuple[<class 'Baz'>, Unknown, <class 'object'>]
reveal_type(Spam.__mro__)  # revealed: tuple[<class 'Spam'>, Unknown, <class 'object'>]

Other classes with possible cycles

[environment]
python-version = "3.13"

class C(C.a): ...
reveal_type(C.__class__)  # revealed: <class 'type'>
reveal_type(C.__mro__)  # revealed: tuple[<class 'C'>, Unknown, <class 'object'>]

class D(D.a):
    a: D
reveal_type(D.__class__)  # revealed: <class 'type'>
reveal_type(D.__mro__)  # revealed: tuple[<class 'D'>, Unknown, <class 'object'>]

class E[T](E.a): ...
reveal_type(E.__class__)  # revealed: <class 'type'>
reveal_type(E.__mro__)  # revealed: tuple[<class 'E[Unknown]'>, Unknown, typing.Generic, <class 'object'>]

class F[T](F(), F): ...  # error: [cyclic-class-definition]
reveal_type(F.__class__)  # revealed: type[Unknown]
reveal_type(F.__mro__)  # revealed: tuple[<class 'F[Unknown]'>, Unknown, <class 'object'>]