The Liskov Substitution Principle

The Liskov Substitution Principle provides the basis for many of the assumptions a type checker generally makes about types in Python:

Subtype Requirement: Let ⁠ϕ(x)⁠ be a property provable about objects ⁠x⁠ of type T. Then ⁠ϕ(y)⁠ should be true for objects ⁠y of type S where S is a subtype of T.

In order for a type checker's assumptions to be sound, it is crucial for the type checker to enforce the Liskov Substitution Principle on code that it checks. In practice, this usually manifests as several checks for a type checker to perform when it checks a subclass B of a class A:

Read-only attributes should only ever be overridden covariantly: if a property A.p resolves to int when accessed, accessing B.p should either resolve to int or a subtype of int.
Method return types should only ever be overridden covariantly: if a method A.f returns int when called, calling B.f should also resolve to int or a subtype ofint`.
Method parameters should only ever be overridden contravariantly: if a method A.f can be called with an argument of type bool, then the method B.f must also be callable with type bool (though it is permitted for the override to also accept other types)
Mutable attributes should only ever be overridden invariantly: if a mutable attribute A.attr resolves to type str, it can only be overridden on a subclass with exactly the same type.

Method return types

It is fine for a subclass method to return a subtype of the return type of the method it overrides:

pyi

class Super:
    def method(self) -> int: ...

class Sub1(Super):
    def method(self) -> int: ...  # fine

class Sub2(Super):
    def method(self) -> bool: ...  # fine: `bool` is a subtype of `int`

However, returning a supertype leads to an error:

pyi

class Sub3(Super):
    def method(self) -> object: ...  # snapshot: invalid-method-override

snapshot

error[invalid-method-override]: Invalid override of method `method`
  --> src/mdtest_snippet.pyi:10:9
   |
10 |     def method(self) -> object: ...  # snapshot: invalid-method-override
   |         ^^^^^^^^^^^^^^^^^^^^^^ Definition is incompatible with `Super.method`
   |
  ::: src/mdtest_snippet.pyi:2:9
   |
 2 |     def method(self) -> int: ...
   |         ------------------- `Super.method` defined here
   |
info: incompatible return types: `object` is not assignable to `int`
info: This violates the Liskov Substitution Principle

Returning a completely unrelated type also leads to an error:

pyi

class Sub4(Super):
    def method(self) -> str: ...  # snapshot: invalid-method-override

snapshot

error[invalid-method-override]: Invalid override of method `method`
  --> src/mdtest_snippet.pyi:12:9
   |
12 |     def method(self) -> str: ...  # snapshot: invalid-method-override
   |         ^^^^^^^^^^^^^^^^^^^ Definition is incompatible with `Super.method`
   |
  ::: src/mdtest_snippet.pyi:2:9
   |
 2 |     def method(self) -> int: ...
   |         ------------------- `Super.method` defined here
   |
info: incompatible return types: `str` is not assignable to `int`
info: This violates the Liskov Substitution Principle

Generic class typevars bounded by the parent return type should still be valid covariant overrides.

toml

[environment]
python-version = "3.12"

pyi

class Root: ...

class Base:
    def get_parent_id(self) -> Base | Root: ...

class Child[PT: Base | Root](Base):
    parent_id: PT

    def get_parent_id(self) -> PT: ...

class DataArray: ...

class Coordinates:
    def __getitem__(self, key: object) -> DataArray: ...

class DataArrayCoordinates[T_DataArray: DataArray](Coordinates):
    def __getitem__(self, key: object) -> T_DataArray: ...

Method parameters

A subclass method may provide a different parameter list to the superclass method, but all combinations of arguments accepted by the superclass method must continue to be accepted by the overriding method.

pyi

class Super:
    def method(self, x: int, /): ...

class Sub1(Super):
    def method(self, x: int, /): ...  # fine

class Sub2(Super):
    def method(self, x: object, /): ...  # fine: `method` still accepts any argument of type `int`

class Sub4(Super):
    def method(self, x: int | str, /): ...  # fine

class Sub5(Super):
    def method(self, x: int): ...  # fine: `x` can still be passed positionally

class Sub6(Super):
    # fine: `method()` can still be called with just a single argument
    def method(self, x: int, *args): ...

class Sub7(Super):
    def method(self, x: int, **kwargs): ...  # fine

class Sub8(Super):
    def method(self, x: int, *args, **kwargs): ...  # fine

class Sub9(Super):
    def method(self, x: int, extra_positional_arg=42, /): ...  # fine

class Sub10(Super):
    def method(self, x: int, extra_pos_or_kw_arg=42): ...  # fine

class Sub11(Super):
    def method(self, x: int, *, extra_kw_only_arg=42): ...  # fine

In the following cases, some calls permitted by the superclass are no longer allowed, so we emit an error.

This method can no longer be passed arguments:

pyi

class Sub12(Super):
    def method(self, /): ...  # snapshot: invalid-method-override

snapshot

error[invalid-method-override]: Invalid override of method `method`
  --> src/mdtest_snippet.pyi:35:9
   |
35 |     def method(self, /): ...  # snapshot: invalid-method-override
   |         ^^^^^^^^^^^^^^^ Definition is incompatible with `Super.method`
   |
  ::: src/mdtest_snippet.pyi:2:9
   |
 2 |     def method(self, x: int, /): ...
   |         ----------------------- `Super.method` defined here
   |
info: This violates the Liskov Substitution Principle

This method can no longer be passed exactly one argument:

pyi

class Sub13(Super):
    def method(self, x, y, /): ...  # snapshot: invalid-method-override

snapshot

error[invalid-method-override]: Invalid override of method `method`
  --> src/mdtest_snippet.pyi:37:9
   |
37 |     def method(self, x, y, /): ...  # snapshot: invalid-method-override
   |         ^^^^^^^^^^^^^^^^^^^^^ Definition is incompatible with `Super.method`
   |
  ::: src/mdtest_snippet.pyi:2:9
   |
 2 |     def method(self, x: int, /): ...
   |         ----------------------- `Super.method` defined here
   |
info: unexpected extra parameter `y`
info: This violates the Liskov Substitution Principle

Here, x can no longer be passed positionally:

pyi

class Sub14(Super):
    def method(self, /, *, x): ...  # snapshot: invalid-method-override

snapshot

error[invalid-method-override]: Invalid override of method `method`
  --> src/mdtest_snippet.pyi:39:9
   |
39 |     def method(self, /, *, x): ...  # snapshot: invalid-method-override
   |         ^^^^^^^^^^^^^^^^^^^^^ Definition is incompatible with `Super.method`
   |
  ::: src/mdtest_snippet.pyi:2:9
   |
 2 |     def method(self, x: int, /): ...
   |         ----------------------- `Super.method` defined here
   |
info: parameter `x` is keyword-only but must also accept positional arguments
info: This violates the Liskov Substitution Principle

Here, x can no longer be passed any integer -- it now requires a bool!

pyi

class Sub15(Super):
    def method(self, x: bool, /): ...  # snapshot: invalid-method-override

snapshot

error[invalid-method-override]: Invalid override of method `method`
  --> src/mdtest_snippet.pyi:41:9
   |
41 |     def method(self, x: bool, /): ...  # snapshot: invalid-method-override
   |         ^^^^^^^^^^^^^^^^^^^^^^^^ Definition is incompatible with `Super.method`
   |
  ::: src/mdtest_snippet.pyi:2:9
   |
 2 |     def method(self, x: int, /): ...
   |         ----------------------- `Super.method` defined here
   |
info: parameter `x` has an incompatible type: `int` is not assignable to `bool`
info: This violates the Liskov Substitution Principle

In this case, x can no longer be passed as a keyword argument:

pyi

class Super2:
    def method2(self, x): ...

class Sub16(Super2):
    def method2(self, x, /): ...  # snapshot: invalid-method-override

snapshot

error[invalid-method-override]: Invalid override of method `method2`
  --> src/mdtest_snippet.pyi:43:9
   |
43 |     def method2(self, x): ...
   |         ---------------- `Super2.method2` defined here
44 |
45 | class Sub16(Super2):
46 |     def method2(self, x, /): ...  # snapshot: invalid-method-override
   |         ^^^^^^^^^^^^^^^^^^^ Definition is incompatible with `Super2.method2`
   |
info: parameter `x` is positional-only but must also accept keyword arguments
info: This violates the Liskov Substitution Principle

In this case, x can no longer be passed as a positional argument:

pyi

class Sub17(Super2):
    def method2(self, *, x): ...  # snapshot: invalid-method-override

snapshot

error[invalid-method-override]: Invalid override of method `method2`
  --> src/mdtest_snippet.pyi:43:9
   |
43 |     def method2(self, x): ...
   |         ---------------- `Super2.method2` defined here
44 |
45 | class Sub16(Super2):
46 |     def method2(self, x, /): ...  # snapshot: invalid-method-override
47 | class Sub17(Super2):
48 |     def method2(self, *, x): ...  # snapshot: invalid-method-override
   |         ^^^^^^^^^^^^^^^^^^^ Definition is incompatible with `Super2.method2`
   |
info: parameter `x` is keyword-only but must also accept positional arguments
info: This violates the Liskov Substitution Principle

The reverse is fine:

pyi

class Super3:
    def method3(self, *, x): ...

class Sub18(Super3):
    def method3(self, x): ...  # fine: `x` can still be used as a keyword argument

This is an error because x can no longer be passed as a keyword argument:

pyi

class Sub19(Super3):
    def method3(self, x, /): ...  # snapshot: invalid-method-override

snapshot

error[invalid-method-override]: Invalid override of method `method3`
  --> src/mdtest_snippet.pyi:50:9
   |
50 |     def method3(self, *, x): ...
   |         ------------------- `Super3.method3` defined here
51 |
52 | class Sub18(Super3):
53 |     def method3(self, x): ...  # fine: `x` can still be used as a keyword argument
54 | class Sub19(Super3):
55 |     def method3(self, x, /): ...  # snapshot: invalid-method-override
   |         ^^^^^^^^^^^^^^^^^^^ Definition is incompatible with `Super3.method3`
   |
info: This violates the Liskov Substitution Principle

Accepting a wider type for *args and **kwargs is fine:

pyi

class Super4:
    def method(self, *args: int, **kwargs: str): ...

class Sub20(Super4):
    def method(self, *args: object, **kwargs: object): ...  # fine

Omitting **kwargs is an error:

pyi

class Sub21(Super4):
    def method(self, *args): ...  # snapshot: invalid-method-override

snapshot

error[invalid-method-override]: Invalid override of method `method`
  --> src/mdtest_snippet.pyi:57:9
   |
57 |     def method(self, *args: int, **kwargs: str): ...
   |         --------------------------------------- `Super4.method` defined here
58 |
59 | class Sub20(Super4):
60 |     def method(self, *args: object, **kwargs: object): ...  # fine
61 | class Sub21(Super4):
62 |     def method(self, *args): ...  # snapshot: invalid-method-override
   |         ^^^^^^^^^^^^^^^^^^^ Definition is incompatible with `Super4.method`
   |
info: This violates the Liskov Substitution Principle

Similarly, omitting *args is also an error:

pyi

class Sub22(Super4):
    def method(self, **kwargs): ...  # snapshot: invalid-method-override

snapshot

error[invalid-method-override]: Invalid override of method `method`
  --> src/mdtest_snippet.pyi:64:9
   |
64 |     def method(self, **kwargs): ...  # snapshot: invalid-method-override
   |         ^^^^^^^^^^^^^^^^^^^^^^ Definition is incompatible with `Super4.method`
   |
  ::: src/mdtest_snippet.pyi:57:9
   |
57 |     def method(self, *args: int, **kwargs: str): ...
   |         --------------------------------------- `Super4.method` defined here
   |
info: This violates the Liskov Substitution Principle

Finally, this is not a Liskov violation because this is a gradual callable. It contains both *args and **kwargs without annotations, so it is compatible with any signature of method on the superclass.

pyi

class Sub23(Super4):
    def method(self, x, *args, y, **kwargs): ...

`ClassVar` and instance variables

A pure class variable cannot override an inherited instance variable, and an instance variable cannot override an inherited pure class variable.

Direct overrides

An annotation without ClassVar declares an instance variable, even if the declaration also has a class-level default value. An explicit ClassVar declaration is a pure class variable. Overriding one with the other changes the places where the attribute is valid, so it violates Liskov substitution:

from typing import ClassVar

class Base:
    instance_attr: int
    instance_attr_with_default: int = 1
    class_attr: ClassVar[int] = 1

class Subclass(Base):
    # error: [invalid-attribute-override] "class variable cannot override instance variable `Base.instance_attr`"
    instance_attr: ClassVar[int]

    # error: [invalid-attribute-override] "class variable cannot override instance variable `Base.instance_attr_with_default`"
    instance_attr_with_default: ClassVar[int] = 1

    # error: [invalid-attribute-override] "instance variable cannot override class variable `Base.class_attr`"
    class_attr: int

class ValidSubclass(Base):
    instance_attr: int
    instance_attr_with_default: int = 1
    class_attr: ClassVar[int] = 1

Regular class-body assignments

An unannotated class-body assignment is an instance variable with a class-level default. This means it can replace another inherited instance-variable default. If it overrides an inherited ClassVar, it inherits that declaration and remains a class variable. However, an explicit ClassVar cannot override an inherited unannotated class-body assignment, because code using the base class can still write that attribute through an instance:

from typing import ClassVar

class Base:
    instance_attr_with_default: int = 1
    class_attr: ClassVar[int] = 1

class RegularClassAttributeOverride(Base):
    class_attr = 1

class AugmentedClassAttributeOverride(Base):
    class_attr = 1
    class_attr += 1

class IntermediateClassAttributeOverride(Base):
    class_attr = 1

class ExplicitClassVarOverrideAfterInheritedClassVar(IntermediateClassAttributeOverride):
    class_attr: ClassVar[int] = 1

class RegularClassAttributeBase:
    attr = 1

class ExplicitClassVarOverride(RegularClassAttributeBase):
    # error: [invalid-attribute-override] "class variable cannot override instance variable `RegularClassAttributeBase.attr`"
    attr: ClassVar[int] = 1

class ClassDefaultBase:
    class_default: int = 1
    declared_instance: bool

class ClassDefaultSubclass(ClassDefaultBase):
    class_default = 2
    declared_instance = True

Method definitions

Method definitions create descriptors in the class body. They are not instance variable declarations, so the class-variable vs. instance-variable override check does not apply to them:

from collections.abc import Callable
from typing import Any, ClassVar

class ClassVarBase:
    plain: ClassVar[Callable[..., Any]]
    static: ClassVar[Callable[..., Any]]
    class_: ClassVar[Callable[..., Any]]
    non_callable: ClassVar[int]

class MethodSubclass(ClassVarBase):
    def plain(self, x: int) -> int:
        return x

    @staticmethod
    def static(x: int) -> int:
        return x

    @classmethod
    def class_(cls, x: int) -> int:
        return x

    def non_callable(self) -> int:
        return 1

class PropertyBase:
    attr: ClassVar[int]

class PropertySubclass(PropertyBase):
    @property
    def attr(  # error: [invalid-attribute-override] "instance variable cannot override class variable `PropertyBase.attr`"
        self,
    ) -> int:
        return 1

Repeated inherited conflicts

If a parent class already made an invalid change from class variable to instance variable, a child that keeps the parent's kind should not receive a duplicate diagnostic. The same applies in the other direction:

from typing import ClassVar

class GrandparentClassVar:
    attr: ClassVar[int]

class ParentInstance(GrandparentClassVar):
    # error: [invalid-attribute-override] "instance variable cannot override class variable `GrandparentClassVar.attr`"
    attr: int

class ChildInstance(ParentInstance):
    attr: int

class GrandparentInstance:
    attr: int

class ParentClassVar(GrandparentInstance):
    # error: [invalid-attribute-override] "class variable cannot override instance variable `GrandparentInstance.attr`"
    attr: ClassVar[int]

class ChildClassVar(ParentClassVar):
    attr: ClassVar[int]

Descriptors

A descriptor can define different behavior when accessed on an instance. Because descriptor lookup is neither a pure class variable nor a normal instance variable, overriding it with an instance attribute is accepted:

from typing import ClassVar

class Descriptor:
    def __get__(self, instance: object, owner: type[object]) -> int:
        return 1

class DescriptorBase:
    descriptor_attr = Descriptor()

class DescriptorOverride(DescriptorBase):
    descriptor_attr: int

class DescriptorAnnotationBase:
    descriptor_attr: Descriptor

class DescriptorAnnotationOverride(DescriptorAnnotationBase):
    # error: [invalid-attribute-override] "class variable cannot override instance variable `DescriptorAnnotationBase.descriptor_attr`"
    descriptor_attr: ClassVar[Descriptor]

Multiple inheritance

The subclass must satisfy every base class. It is not enough for the first base in the MRO to agree with the subclass: an unrelated base that declares the same member as a pure class variable still makes an instance-variable override invalid.

from typing import ClassVar

class ClassVarBase:
    attr: ClassVar[int]

class InstanceBase:
    attr: int

class MultipleInheritanceSubclass(InstanceBase, ClassVarBase):
    # error: [invalid-attribute-override] "instance variable cannot override class variable `ClassVarBase.attr`"
    attr: int

Dataclasses

Dataclass fields are instance variables, even though they are usually declared in the class body. ClassVar fields remain pure class variables and are excluded from dataclass instance fields:

from dataclasses import dataclass
from typing import ClassVar

@dataclass
class DC6:
    x: int
    y: ClassVar[int] = 1

@dataclass
class DC7(DC6):
    # error: [invalid-attribute-override] "class variable cannot override instance variable `DC6.x`"
    x: ClassVar[int]

    # error: [invalid-attribute-override] "instance variable cannot override class variable `DC6.y`"
    y: int

Protocol implementations

Regular class-body assignments can implement protocol instance attributes. The ClassVar case below uses the same rule as normal classes: an unannotated class-body assignment over an inherited ClassVar provides a value while preserving the inherited declaration.

from typing import ClassVar, Protocol

class ProtocolBase(Protocol):
    class_attr: ClassVar[int]
    instance_attr: int
    instance_attr_with_default: int = 1

class ProtocolImpl(ProtocolBase):
    class_attr = 1
    instance_attr = 1
    instance_attr_with_default = 1

class ProtocolWithClassVarImpl(ProtocolBase):
    class_attr = 0
    instance_attr = 0

Inherited method conflicts in multiple inheritance

The effective method definition in a class's resolved MRO must be compatible with the later definitions of that method. This matters even when the class does not define the method itself.

Basic conflicts

ReturnsStr.method cannot satisfy the contract inherited from ReturnsInt. Returning bool is compatible because bool is a subtype of int.

pyi

class ReturnsStr:
    def method(self) -> str: ...

class ReturnsInt:
    def method(self) -> int: ...

class ReturnsBool:
    def method(self) -> bool: ...

class BasicConflict(ReturnsStr, ReturnsInt): ...  # snapshot: invalid-method-override
class Compatible(ReturnsBool, ReturnsInt): ...

snapshot

error[invalid-method-override]: Base classes for class `BasicConflict` define method `method` incompatibly
  --> src/mdtest_snippet.pyi:2:9
   |
 2 |     def method(self) -> str: ...
   |         ------ `ReturnsStr.method` defined here
 3 |
 4 | class ReturnsInt:
 5 |     def method(self) -> int: ...
   |         ------ `ReturnsInt.method` defined here
 6 |
 7 | class ReturnsBool:
 8 |     def method(self) -> bool: ...
 9 |
10 | class BasicConflict(ReturnsStr, ReturnsInt): ...  # snapshot: invalid-method-override
   |       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `ReturnsStr.method` is incompatible with `ReturnsInt.method`
   |
info: incompatible return types: `str` is not assignable to `int`
info: This violates the Liskov Substitution Principle

Empty bases do not hide conflicts

An empty base can appear before, between, or after the bases that define the conflicting methods.

pyi

class Empty: ...

class ReturnsStr:
    def method(self) -> str: ...

class ReturnsInt:
    def method(self) -> int: ...

class EmptyFirst(Empty, ReturnsStr, ReturnsInt): ...  # error: [invalid-method-override]
class EmptyMiddle(ReturnsStr, Empty, ReturnsInt): ...  # error: [invalid-method-override]
class EmptyLast(ReturnsStr, ReturnsInt, Empty): ...  # error: [invalid-method-override]

An earlier compatible method satisfies both contracts

SatisfiesBoth.method is compatible with both later definitions because it returns NoReturn. It is the effective method for Compatible, so the later definitions do not conflict.

pyi

from typing import NoReturn

class SatisfiesBoth:
    def method(self) -> NoReturn: ...

class ReturnsStr:
    def method(self) -> str: ...

class ReturnsInt:
    def method(self) -> int: ...

class Compatible(SatisfiesBoth, ReturnsStr, ReturnsInt): ...

A compatible subclass override satisfies both contracts

A subclass can provide an implementation that satisfies otherwise-incompatible base definitions.

pyi

from typing import NoReturn

class ReturnsStr:
    def method(self) -> str: ...

class ReturnsInt:
    def method(self) -> int: ...

class CompatibleReturn(ReturnsStr, ReturnsInt):
    def method(self) -> NoReturn: ...

class AcceptsStr:
    def accepts(self, value: str) -> None: ...

class AcceptsInt:
    def accepts(self, value: int) -> None: ...

class CompatibleParameter(AcceptsStr, AcceptsInt):
    def accepts(self, value: str | int) -> None: ...

An intermediate `Any` does not hide a conflict

AcceptsAny.accepts can accept either parameter type, so it is compatible with both adjacent definitions. That does not allow AcceptsStr.accepts to accept the int values required by AcceptsInt.accepts.

pyi

from typing import Any

class AcceptsInt:
    def accepts(self, value: int) -> None: ...

class AcceptsAny(AcceptsInt):
    def accepts(self, value: Any) -> None: ...

class AcceptsStr:
    def accepts(self, value: str) -> None: ...

class AnyConflict(AcceptsStr, AcceptsAny): ...  # error: [invalid-method-override]

Dynamic bases only hide later conflicts

An intermediate Any base cannot hide a conflict when the effective method is already known. An earlier Any base can define the effective method, so later base definitions cannot be checked.

toml

[environment]
python-version = "3.11"

pyi

from typing import Any

class ReturnsStr:
    def method(self) -> str: ...

class ReturnsInt:
    def method(self) -> int: ...

class DynamicMiddle(ReturnsStr, Any, ReturnsInt): ...  # error: [invalid-method-override]
class DynamicFirst(Any, ReturnsStr, ReturnsInt): ...

Generic bases

A method inherited from GenericReturn[int] returns int. A plain Generic[T] base adds no method, but it must not prevent the remaining bases from being checked.

pyi

from typing import Generic, TypeVar

T = TypeVar("T")

class ReturnsStr:
    def method(self) -> str: ...

class ReturnsInt:
    def method(self) -> int: ...

class GenericReturn(Generic[T]):
    def method(self) -> T: ...

class SpecializedConflict(ReturnsStr, GenericReturn[int]): ...  # error: [invalid-method-override]
class GenericBaseConflict(Generic[T], ReturnsStr, ReturnsInt): ...  # error: [invalid-method-override]

Methods inherited by a direct base

A direct base can inherit the method from one of its own bases.

pyi

class ReturnsStr:
    def method(self) -> str: ...

class Intermediate(ReturnsStr): ...

class ReturnsInt:
    def method(self) -> int: ...

class IndirectConflict(Intermediate, ReturnsInt): ...  # error: [invalid-method-override]

Properties

Incompatible properties inherited from different bases are not yet checked.

pyi

class ReturnsStr:
    @property
    def value(self) -> str: ...

class ReturnsInt:
    @property
    def value(self) -> int: ...

# TODO: Incompatible inherited properties should be reported here.
class PropertyConflict(ReturnsStr, ReturnsInt): ...

Synthesized members

Methods synthesized on an earlier base can conflict with methods defined on a later base.

pyi

from functools import total_ordering

@total_ordering
class Ordered:
    def __lt__(self, other: Ordered) -> bool: ...

class AcceptsObject:
    def __gt__(self, other: object) -> bool: ...

# TODO: The synthesized `Ordered.__gt__` method conflicts with `AcceptsObject.__gt__`.
class SynthesizedConflict(Ordered, AcceptsObject): ...

Enum mixins

Ordinary mixin methods continue to contribute inherited contracts when the resulting class is an enum, including methods with names that EnumType can replace.

pyi

from enum import Enum
from typing import Literal

class ReturnsStr:
    def method(self) -> str: ...

class ReturnsInt:
    def method(self) -> int: ...

class MixedEnum(ReturnsStr, ReturnsInt, Enum):  # error: [invalid-method-override]
    MEMBER = 1

class FirstString:
    def __str__(self) -> Literal["first"]: ...

class SecondString:
    def __str__(self) -> Literal["second"]: ...

class StringConflict(FirstString, SecondString, Enum):  # error: [invalid-method-override]
    MEMBER = 1

Instance, static, and class methods

Instance methods, static methods, and class methods are bound differently. Each pair can therefore conflict even when the callable signatures look compatible.

pyi

class InstanceMethod:
    def kind(self, value: int) -> int: ...

class StaticMethod:
    @staticmethod
    def kind(value: int) -> int: ...

class ClassMethod:
    @classmethod
    def kind(cls, value: int) -> int: ...

class InstanceStaticConflict(InstanceMethod, StaticMethod): ...  # error: [invalid-method-override]
class ClassInstanceConflict(ClassMethod, InstanceMethod): ...  # snapshot: invalid-method-override
class StaticClassConflict(StaticMethod, ClassMethod): ...  # error: [invalid-method-override]

snapshot

error[invalid-method-override]: Base classes for class `ClassInstanceConflict` define method `kind` incompatibly
  --> src/mdtest_snippet.pyi:10:9
   |
10 |     def kind(cls, value: int) -> int: ...
   |         ---- `ClassMethod.kind` defined here
11 |
12 | class InstanceStaticConflict(InstanceMethod, StaticMethod): ...  # error: [invalid-method-override]
13 | class ClassInstanceConflict(ClassMethod, InstanceMethod): ...  # snapshot: invalid-method-override
   |       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `ClassMethod.kind` is incompatible with `InstanceMethod.kind`
   |
  ::: src/mdtest_snippet.pyi:2:9
   |
 2 |     def kind(self, value: int) -> int: ...
   |         ---- `InstanceMethod.kind` defined here
   |
info: `ClassMethod.kind` is a classmethod but `InstanceMethod.kind` is an instance method
info: This violates the Liskov Substitution Principle

Class methods use the subclass as their receiver

The signature selected for a class method overload can depend on the class used as the receiver. Binding through Combined selects the int overload, which conflicts with Right.selected. Binding through Compatible selects the compatible str overload.

pyi

from typing import overload

class ReceiverBase:
    @overload
    @classmethod
    def selected(cls: type[Combined]) -> int: ...
    @overload
    @classmethod
    def selected(cls) -> str: ...

class Left(ReceiverBase): ...

class Right(ReceiverBase):
    @classmethod
    def selected(cls) -> str: ...

class Combined(Left, Right): ...  # error: [invalid-method-override]
class Compatible(Left, Right): ...

Metaclass descriptors do not replace class method contracts

Looking up a class method by name on a class object can invoke a same-named descriptor on its metaclass. The inherited contract still comes from the class method defined in the class body.

pyi

from typing import Any

class Meta(type):
    @property
    def class_method(cls) -> Any: ...

class ReturnsInt(metaclass=Meta):
    @classmethod
    def class_method(cls) -> int: ...

class ReturnsStr:
    @classmethod
    def class_method(cls) -> str: ...

class MetaclassConflict(ReturnsInt, ReturnsStr): ...  # error: [invalid-method-override]

The implicit `object` base does not repeat an existing error

Every class eventually inherits from object. An otherwise empty base does not add another contract for __str__: InvalidStr receives the ordinary override error, and Combined receives no second error.

pyi

class Empty: ...

class InvalidStr:
    def __str__(self) -> int: ...  # error: [invalid-method-override]

class Combined(Empty, InvalidStr): ...

Existing violations on generic parents are not repeated

An invalid override on a generic parent is already reported at its definition. Specializing that parent and adding another base to a descendant must not report the same violation again.

toml

[environment]
python-version = "3.12"

[rules]
missing-type-argument = "ignore"

pyi

from typing import Generic, TypeVar

T = TypeVar("T")

class Base(Generic[T]):
    def method(self, value: object) -> None: ...

class Parent[U](Base):
    def method(self, value: U) -> None: ...  # error: [invalid-method-override]

class Child(Parent[str], object): ...

Bases with the same name are distinguished

When two bases have the same name, their qualified names make the incompatible methods clear in both the primary message and the definition annotations.

left.pyi:

pyi

class Base:
    def method(self) -> str: ...

right.pyi:

pyi

class Base:
    def method(self) -> int: ...

main.pyi:

pyi

from left import Base as LeftBase
from right import Base as RightBase

class Combined(LeftBase, RightBase): ...  # snapshot: invalid-method-override

snapshot

error[invalid-method-override]: Base classes for class `Combined` define method `method` incompatibly
 --> src/main.pyi:4:7
  |
4 | class Combined(LeftBase, RightBase): ...  # snapshot: invalid-method-override
  |       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `left.Base.method` is incompatible with `right.Base.method`
  |
 ::: src/left.pyi:2:9
  |
2 |     def method(self) -> str: ...
  |         ------ `left.Base.method` defined here
  |
 ::: src/right.pyi:2:9
  |
2 |     def method(self) -> int: ...
  |         ------ `right.Base.method` defined here
  |
info: incompatible return types: `str` is not assignable to `int`
info: This violates the Liskov Substitution Principle

Inconsistent generic bases do not produce override errors

The type arguments are already inconsistent, so there is no resolved generic MRO to check. The same is true for a descendant of the invalid class.

toml

[environment]
python-version = "3.12"

pyi

class User: ...
class Member(User): ...

class Base[T]:
    def method(self) -> T: ...

class UserBase(Base[User]): ...
class MemberBase(UserBase, Base[Member]): ...  # error: [invalid-generic-class]
class MemberChild(MemberBase, object): ...

The entire class hierarchy is checked

If a child class's method definition is Liskov-compatible with the method definition on its parent class, Liskov compatibility must also nonetheless be checked with respect to the method definition on its grandparent class. This is because type checkers will treat the child class as a subtype of the grandparent class just as much as they treat it as a subtype of the parent class, so substitutability with respect to the grandparent class is just as important.

However, if the parent class itself already has an LSP violation with an ancestor, we do not report the same violation for the child class. This is because the child class cannot fix the violation without introducing a new, worse violation against its immediate parent's contract.

stub.pyi:

pyi

from typing import Any

class Grandparent:
    def method(self, x: int) -> None: ...

class Parent(Grandparent):
    def method(self, x: str) -> None: ...  # snapshot: invalid-method-override

class Child(Parent):
    # compatible with the signature of `Parent.method`, but not with `Grandparent.method`.
    # However, since `Parent.method` already violates LSP with `Grandparent.method`,
    # we don't report the same violation for `Child` -- it's inherited from `Parent`.
    def method(self, x: str) -> None: ...

class OtherChild(Parent):
    # compatible with the signature of `Grandparent.method`, but not with `Parent.method`:
    def method(self, x: int) -> None: ...  # snapshot: invalid-method-override

class ChildWithNewViolation(Parent):
    # incompatible with BOTH `Parent.method` (str) and `Grandparent.method` (int).
    # We report the violation against the immediate parent (`Parent`), not the grandparent.
    def method(self, x: bytes) -> None: ...  # snapshot: invalid-method-override

class GrandparentWithReturnType:
    def method(self) -> int: ...

class ParentWithReturnType(GrandparentWithReturnType):
    def method(self) -> str: ...  # snapshot: invalid-method-override

class ChildWithReturnType(ParentWithReturnType):
    # Returns `int` again -- compatible with `GrandparentWithReturnType.method`,
    # but not with `ParentWithReturnType.method`. We report against the immediate parent.
    def method(self) -> int: ...  # snapshot: invalid-method-override

class GradualParent(Grandparent):
    def method(self, x: Any) -> None: ...

class ThirdChild(GradualParent):
    # `GradualParent.method` is compatible with the signature of `Grandparent.method`,
    # and `ThirdChild.method` is compatible with the signature of `GradualParent.method`,
    # but `ThirdChild.method` is not compatible with the signature of `Grandparent.method`
    def method(self, x: str) -> None: ...  # snapshot: invalid-method-override

snapshot

error[invalid-method-override]: Invalid override of method `method`
 --> src/stub.pyi:4:9
  |
4 |     def method(self, x: int) -> None: ...
  |         ---------------------------- `Grandparent.method` defined here
5 |
6 | class Parent(Grandparent):
7 |     def method(self, x: str) -> None: ...  # snapshot: invalid-method-override
  |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Definition is incompatible with `Grandparent.method`
  |
info: parameter `x` has an incompatible type: `int` is not assignable to `str`
info: This violates the Liskov Substitution Principle


error[invalid-method-override]: Invalid override of method `method`
  --> src/stub.pyi:17:9
   |
17 |     def method(self, x: int) -> None: ...  # snapshot: invalid-method-override
   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Definition is incompatible with `Parent.method`
   |
  ::: src/stub.pyi:7:9
   |
 7 |     def method(self, x: str) -> None: ...  # snapshot: invalid-method-override
   |         ---------------------------- `Parent.method` defined here
   |
info: parameter `x` has an incompatible type: `str` is not assignable to `int`
info: This violates the Liskov Substitution Principle


error[invalid-method-override]: Invalid override of method `method`
  --> src/stub.pyi:22:9
   |
22 |     def method(self, x: bytes) -> None: ...  # snapshot: invalid-method-override
   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Definition is incompatible with `Parent.method`
   |
  ::: src/stub.pyi:7:9
   |
 7 |     def method(self, x: str) -> None: ...  # snapshot: invalid-method-override
   |         ---------------------------- `Parent.method` defined here
   |
info: parameter `x` has an incompatible type: `str` is not assignable to `bytes`
info: This violates the Liskov Substitution Principle


error[invalid-method-override]: Invalid override of method `method`
  --> src/stub.pyi:25:9
   |
25 |     def method(self) -> int: ...
   |         ------------------- `GrandparentWithReturnType.method` defined here
26 |
27 | class ParentWithReturnType(GrandparentWithReturnType):
28 |     def method(self) -> str: ...  # snapshot: invalid-method-override
   |         ^^^^^^^^^^^^^^^^^^^ Definition is incompatible with `GrandparentWithReturnType.method`
   |
info: incompatible return types: `str` is not assignable to `int`
info: This violates the Liskov Substitution Principle


error[invalid-method-override]: Invalid override of method `method`
  --> src/stub.pyi:28:9
   |
28 |     def method(self) -> str: ...  # snapshot: invalid-method-override
   |         ------------------- `ParentWithReturnType.method` defined here
29 |
30 | class ChildWithReturnType(ParentWithReturnType):
31 |     # Returns `int` again -- compatible with `GrandparentWithReturnType.method`,
32 |     # but not with `ParentWithReturnType.method`. We report against the immediate parent.
33 |     def method(self) -> int: ...  # snapshot: invalid-method-override
   |         ^^^^^^^^^^^^^^^^^^^ Definition is incompatible with `ParentWithReturnType.method`
   |
info: incompatible return types: `int` is not assignable to `str`
info: This violates the Liskov Substitution Principle


error[invalid-method-override]: Invalid override of method `method`
  --> src/stub.pyi:42:9
   |
42 |     def method(self, x: str) -> None: ...  # snapshot: invalid-method-override
   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Definition is incompatible with `Grandparent.method`
   |
  ::: src/stub.pyi:4:9
   |
 4 |     def method(self, x: int) -> None: ...
   |         ---------------------------- `Grandparent.method` defined here
   |
info: parameter `x` has an incompatible type: `int` is not assignable to `str`
info: This violates the Liskov Substitution Principle

other_stub.pyi:

pyi

class A:
    def get(self, default): ...

class B(A):
    def get(self, default, /): ...  # snapshot: invalid-method-override

get = 56

class C(B):
    # `get` appears in the symbol table of `C`,
    # but that doesn't confuse our diagnostic...
    foo = get

class D(C):
    # compatible with `C.get` and `B.get`, but not with `A.get`.
    # Since `B.get` already violates LSP with `A.get`, we don't report for `D`.
    def get(self, my_default): ...

snapshot

error[invalid-method-override]: Invalid override of method `get`
 --> src/other_stub.pyi:2:9
  |
2 |     def get(self, default): ...
  |         ------------------ `A.get` defined here
3 |
4 | class B(A):
5 |     def get(self, default, /): ...  # snapshot: invalid-method-override
  |         ^^^^^^^^^^^^^^^^^^^^^ Definition is incompatible with `A.get`
  |
info: parameter `default` is positional-only but must also accept keyword arguments
info: This violates the Liskov Substitution Principle

Unannotated overrides of overloaded dunder methods should remain accepted.

pyi

class C(list[int]):
    def __getitem__(self, key): ...

Non-generic methods on generic classes work as expected

toml

[environment]
python-version = "3.12"

pyi

class A[T]:
    def method(self, x: T) -> None: ...

class B[T](A[T]):
    def method(self, x: T) -> None: ...  # fine

class C(A[int]):
    def method(self, x: int) -> None: ...  # fine

class D[T](A[T]):
    def method(self, x: object) -> None: ...  # fine

class E(A[int]):
    def method(self, x: object) -> None: ...  # fine

class F[T](A[T]):
    # `str` is not necessarily a supertype of `T`!
    # error: [invalid-method-override]
    def method(self, x: str) -> None: ...

class G(A[int]):
    def method(self, x: bool) -> None: ...  # error: [invalid-method-override]

Generic methods on non-generic classes work as expected

toml

[environment]
python-version = "3.12"

pyi

from typing import Never, Self

class A:
    def method[T](self, x: T) -> T: ...

class B(A):
    def method[T](self, x: T) -> T: ...  # fine

class C(A):
    def method(self, x: object) -> Never: ...  # fine

class D(A):
    # TODO: we should emit [invalid-method-override] here:
    # `A.method` accepts an argument of any type,
    # but `D.method` only accepts `int`s
    def method(self, x: int) -> int: ...

class A2:
    def method(self, x: int) -> int: ...

class B2(A2):
    # fine: although `B2.method()` will not always return an `int`,
    # an instance of `B2` can be substituted wherever an instance of `A2` is expected,
    # and it *will* always return an `int` if it is passed an `int`
    # (which is all that will be allowed if an instance of `A2` is expected)
    def method[T](self, x: T) -> T: ...

class C2(A2):
    def method[T: int](self, x: T) -> T: ...

class ExplicitReceiverBase:
    def method(self, x: int) -> int: ...

class ExplicitReceiverChild(ExplicitReceiverBase):
    # Binding this method adds `ExplicitReceiverChild <= T`, which is incompatible with the
    # `T = int` specialization needed to implement the base method.
    def method[T](self: T, x: T) -> T: ...  # error: [invalid-method-override]

class D2(A2):
    # The type variable is bound to a type disjoint from `int`,
    # so the method will not accept integers, and therefore this is an invalid override
    def method[T: str](self, x: T) -> T: ...  # error: [invalid-method-override]

class A3:
    def method(self) -> Self: ...

class B3(A3):
    def method(self) -> Self: ...  # fine

class C3(A3):
    # TODO: should this be allowed?
    # Mypy/pyright/pyrefly all allow it,
    # but conceptually it seems similar to `B4.method` below,
    # which mypy/pyrefly agree is a Liskov violation
    # (pyright disagrees as of 20/11/2025: https://github.com/microsoft/pyright/issues/11128)
    # when called on a subclass, `C3.method()` will not return an
    # instance of that subclass
    def method(self) -> C3: ...

class D3(A3):
    def method(self: Self) -> Self: ...  # fine

class E3(A3):
    def method(self: E3) -> Self: ...  # fine

class F3(A3):
    def method(self: A3) -> Self: ...  # fine

class G3(A3):
    def method(self: object) -> Self: ...  # fine

class H3(A3):
    # `A3.method()` can be called on any subtype of `A3`, but `H3.method()` can only be called on
    # objects that are subtypes of `str`.
    def method(self: str) -> Self: ...  # error: [invalid-method-override]

class I3(A3):
    # `I3.method()` cannot be called with any inhabited type.
    def method(self: Never) -> Self: ...  # error: [invalid-method-override]

class A4:
    def method[T: int](self, x: T) -> T: ...

class B4(A4):
    # TODO: we should emit `invalid-method-override` here.
    # `A4.method` promises that if it is passed a `bool`, it will return a `bool`,
    # but this is not necessarily true for `B4.method`: if passed a `bool`,
    # it could return a non-`bool` `int`!
    def method(self, x: int) -> int: ...

Protocol annotations on mixin receivers

A mixin can annotate self with a protocol that the mixin itself does not implement. An override can keep that annotation or omit it:

pyi

from typing import Protocol, overload

class HasValue(Protocol):
    value: int

class Mixin:
    def method(self: HasValue, argument: int) -> None: ...

class SameReceiver(Mixin):
    def method(self: HasValue, argument: int) -> None: ...

class ImplicitReceiver(Mixin):
    def method(self, argument: int) -> None: ...

A subclass that provides the required protocol member can call the annotated method normally:

pyi

class ImplementsProtocol(Mixin):
    value: int
    def method(self: HasValue, argument: int) -> None: ...

receiver: HasValue = ImplementsProtocol()
ImplementsProtocol().method(1)

An override cannot require an unrelated protocol or change the type of an argument accepted by the mixin:

pyi

class HasOtherValue(Protocol):
    other_value: str

class InvalidReceiver(Mixin):
    def method(self: HasOtherValue, argument: int) -> None: ...  # error: [invalid-method-override]

class InvalidArgument(Mixin):
    value: int
    def method(self: HasValue, argument: str) -> None: ...  # snapshot: invalid-method-override

snapshot

error[invalid-method-override]: Invalid override of method `method`
  --> src/mdtest_snippet.pyi:28:9
   |
28 |     def method(self: HasValue, argument: str) -> None: ...  # snapshot: invalid-method-override
   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Definition is incompatible with `Mixin.method`
   |
  ::: src/mdtest_snippet.pyi:7:9
   |
 7 |     def method(self: HasValue, argument: int) -> None: ...
   |         --------------------------------------------- `Mixin.method` defined here
   |
info: parameter `argument` has an incompatible type: `int` is not assignable to `str`
info: This violates the Liskov Substitution Principle

The receiver annotation can also accept more than one protocol:

pyi

class UnionMixin:
    def method(self: HasValue | HasOtherValue, argument: int) -> None: ...

class SameUnionReceiver(UnionMixin):
    def method(self: HasValue | HasOtherValue, argument: int) -> None: ...

Overloaded mixin methods need the same treatment for every receiver-specific overload:

pyi

# TODO: We should emit an `invalid-method-override` diagnostic on the second
# `InvalidOverloadedReceiver.method` overload. Both overload sets need to be
# compared within the `HasValue` receiver domain instead of being filtered
# against the concrete mixin subclass.
class OverloadedMixin:
    @overload
    def method(self: HasValue, argument: int) -> None: ...
    @overload
    def method(self: HasValue, argument: str) -> None: ...

class InvalidOverloadedReceiver(OverloadedMixin):
    @overload
    def method(self: HasValue, argument: int) -> None: ...
    @overload
    def method(self: HasValue, argument: bytes) -> None: ...

The protocol may include the overridden method itself. This must not cause a valid implementation to be rejected while checking the override:

pyi

class Container(Protocol):
    def __contains__(self, key: str) -> bool: ...
    def method(self) -> None: ...

class ContainerMixin:
    def method(self: Container) -> None: ...

class ImplementsContainer(ContainerMixin):
    def __contains__(self, key: str) -> bool: ...
    def method(self: Container) -> None: ...

container: Container = ImplementsContainer()
ImplementsContainer().method()

Generic instance-method receivers

An instance method can relate its receiver and return type using a bounded type variable. An override can express the same relationship with Self:

pyi

from typing import TypeVar
from typing_extensions import Self

InstanceT = TypeVar("InstanceT", bound="InstanceBase")

class InstanceBase:
    def clone(self: InstanceT) -> InstanceT: ...

class InstanceChild(InstanceBase):
    def clone(self: Self) -> Self: ...

Generic methods on generic classes work as expected

toml

[environment]
python-version = "3.12"

pyi

from typing import Never

class A[T]:
    def method[S](self, x: T, y: S) -> S: ...

class B[T](A[T]):
    def method[S](self, x: T, y: S) -> S: ...  # fine

class C(A[int]):
    def method[S](self, x: int, y: S) -> S: ...  # fine

class D[T](A[T]):
    def method[S](self, x: object, y: S) -> S: ...  # fine

class E(A[int]):
    def method[S](self, x: object, y: S) -> S: ...  # fine

class F(A[int]):
    def method(self, x: object, y: object) -> Never: ...  # fine

class A2[T]:
    def method(self, x: T, y: int) -> int: ...

class B2[T](A2[T]):
    def method[S](self, x: T, y: S) -> S: ...  # fine

Fully qualified names are used in diagnostics where appropriate

one.pyi:

pyi

class A:
    def foo(self, x): ...

two.pyi:

pyi

import one

class A(one.A):
    def foo(self, y): ...  # snapshot: invalid-method-override

snapshot

error[invalid-method-override]: Invalid override of method `foo`
 --> src/two.pyi:4:9
  |
4 |     def foo(self, y): ...  # snapshot: invalid-method-override
  |         ^^^^^^^^^^^^ Definition is incompatible with `one.A.foo`
  |
 ::: src/one.pyi:2:9
  |
2 |     def foo(self, x): ...
  |         ------------ `one.A.foo` defined here
  |
info: the parameter named `y` does not match `x` (and can be used as a keyword parameter)
info: This violates the Liskov Substitution Principle

Excluded methods

Certain special constructor methods are excluded from Liskov checks. None of the following classes cause us to emit any errors, therefore:

toml

# This is so that the dataclasses machinery will generate `__replace__` methods for us
# (the synthesized `__replace__` methods should not be reported as invalid overrides!)
[environment]
python-version = "3.13"

pyi

from dataclasses import dataclass, InitVar
from typing_extensions import Self

class Grandparent: ...

class Parent(Grandparent):
    def __new__(cls, x: int) -> Self: ...
    def __init__(self, x: int) -> None: ...

class Child(Parent):
    def __new__(cls, x: str, y: str) -> Self: ...
    def __init__(self, x: str, y: str) -> Self: ...

@dataclass(init=False)
class DataSuper:
    x: InitVar[int]

    def __post_init__(self, x: int) -> None:
        self.x = x

@dataclass(init=False)
class DataSub(DataSuper):
    y: InitVar[str]

    def __post_init__(self, x: int, y: str) -> None:
        self.y = y
        super().__post_init__(x)

Functions assigned in a class body

A function assigned in a class body is bound as a method. It can conflict with a method definition in either base order.

pyi

def returns_str(self) -> str: ...

class Assigned:
    method = returns_str

class Defined:
    def method(self) -> int: ...

class AssignedFirst(Assigned, Defined): ...  # error: [invalid-method-override]
class AssignedSecond(Defined, Assigned): ...  # error: [invalid-method-override]

Edge case: function defined in another module and then assigned in a class body

foo.pyi:

pyi

def x(self, y: str): ...

bar.pyi:

pyi

import foo

class A:
    def x(self, y: int): ...

class B(A):
    x = foo.x  # snapshot: invalid-method-override

class C:
    x = foo.x

class D(C):
    def x(self, y: int): ...  # snapshot: invalid-method-override

snapshot

error[invalid-method-override]: Invalid override of method `x`
 --> src/bar.pyi:4:9
  |
4 |     def x(self, y: int): ...
  |         --------------- `A.x` defined here
5 |
6 | class B(A):
7 |     x = foo.x  # snapshot: invalid-method-override
  |     ^^^^^^^^^ Definition is incompatible with `A.x`
  |
 ::: src/foo.pyi:1:5
  |
1 | def x(self, y: str): ...
  |     --------------- Signature of `B.x`
  |
info: parameter `y` has an incompatible type: `int` is not assignable to `str`
info: This violates the Liskov Substitution Principle


error[invalid-method-override]: Invalid override of method `x`
  --> src/bar.pyi:10:5
   |
10 |     x = foo.x
   |     --------- `C.x` defined here
11 |
12 | class D(C):
13 |     def x(self, y: int): ...  # snapshot: invalid-method-override
   |         ^^^^^^^^^^^^^^^ Definition is incompatible with `C.x`
   |
  ::: src/foo.pyi:1:5
   |
 1 | def x(self, y: str): ...
   |     --------------- Signature of `C.x`
   |
info: parameter `y` has an incompatible type: `str` is not assignable to `int`
info: This violates the Liskov Substitution Principle

Bad override of `eq`

class Bad:
    x: int
    def __eq__(self, other: "Bad") -> bool:  # snapshot: invalid-method-override
        return self.x == other.x

snapshot

error[invalid-method-override]: Invalid override of method `__eq__`
   --> src/mdtest_snippet.py:3:9
    |
  3 |     def __eq__(self, other: "Bad") -> bool:  # snapshot: invalid-method-override
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Definition is incompatible with `object.__eq__`
    |
   ::: stdlib/builtins.pyi:136:9
    |
136 |     def __eq__(self, value: object, /) -> bool: ...
    |         -------------------------------------- `object.__eq__` defined here
    |
info: parameter `value` has an incompatible type: `object` is not assignable to `Bad`
info: This violates the Liskov Substitution Principle
help: It is recommended for `__eq__` to work with arbitrary objects, for example:
help
help:     def __eq__(self, other: object) -> bool:
help:         if not isinstance(other, Bad):
help:             return False
help:         return <logic to compare two `Bad` instances>
help

Class-private names do not override

class X:
    def __get_value(self) -> int:
        return 0

class Y(X):
    def __get_value(self) -> str:
        return "s"

Synthesized methods

NamedTuple classes and dataclasses both have methods generated at runtime that do not have source-code definitions. There are several scenarios to consider here:

A synthesized method on a superclass is overridden by a "normal" (not synthesized) method on a subclass
A "normal" method on a superclass is overridden by a synthesized method on a subclass
A synthesized method on a superclass is overridden by a synthesized method on a subclass

pyi

from dataclasses import dataclass
from typing import NamedTuple

@dataclass(order=True)
class Foo:
    x: int

class Bar(Foo):  # error: [subclass-of-dataclass-with-order]
    def __lt__(self, other: Bar) -> bool: ...  # error: [invalid-method-override]

# Specifying `order=True` on the subclass means that comparison methods are generated for the
# subclass, but cross-class comparisons with the parent still raise `TypeError` at runtime.
# TODO: We should also emit `invalid-method-override` diagnostics for each generated
# comparison method since they have incompatible signatures.
@dataclass(order=True)
class Bar2(Foo):  # error: [subclass-of-dataclass-with-order]
    y: str

# Although this class does not override any methods of `Foo`, the design of the
# `order=True` stdlib dataclasses feature itself violates the Liskov Substitution
# Principle! Instances of `Bar3` cannot be substituted wherever an instance of `Foo` is
# expected, because the generated `__lt__` method on `Foo` raises an error unless the r.h.s.
# and `l.h.s.` have exactly the same `__class__` (it does not permit instances of `Foo` to
# be compared with instances of subclasses of `Foo`).
class Bar3(Foo): ...  # error: [subclass-of-dataclass-with-order]

class Eggs:
    def __lt__(self, other: Eggs) -> bool: ...

# TODO: the generated `Ham.__lt__` method here incompatibly overrides `Eggs.__lt__`.
# We could consider emitting a diagnostic here. As of 2025-11-21, mypy reports a
# diagnostic here but pyright and pyrefly do not.
@dataclass(order=True)
class Ham(Eggs):
    x: int

class Baz(NamedTuple):
    x: int

class Spam(Baz):
    def _asdict(self) -> tuple[int, ...]: ...  # snapshot: invalid-method-override

snapshot

error[invalid-method-override]: Invalid override of method `_asdict`
  --> src/mdtest_snippet.pyi:41:9
   |
41 |     def _asdict(self) -> tuple[int, ...]: ...  # snapshot: invalid-method-override
   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Definition is incompatible with `Baz._asdict`
   |
info: incompatible return types: `tuple[int, ...]` is not assignable to `dict[str, Any]`
info: This violates the Liskov Substitution Principle
info: `Baz._asdict` is a generated method created because `Baz` inherits from `typing.NamedTuple`
  --> src/mdtest_snippet.pyi:37:7
   |
37 | class Baz(NamedTuple):
   |       ^^^^^^^^^^^^^^^ Definition of `Baz`
   |

Staticmethods and classmethods

Methods decorated with @staticmethod or @classmethod are checked in much the same way as other methods.

pyi

class Parent:
    def instance_method(self, x: int) -> int: ...
    @classmethod
    def class_method(cls, x: int) -> int: ...
    @staticmethod
    def static_method(x: int) -> int: ...

class GoodChild1(Parent):
    @classmethod
    def class_method(cls, x: int) -> int: ...
    @staticmethod
    def static_method(x: int) -> int: ...

class GoodChild2(Parent):
    @classmethod
    def class_method(cls, x: object) -> bool: ...
    @staticmethod
    def static_method(x: object) -> bool: ...

When the types are incompatible, we report an error:

pyi

class BadTypesA(Parent):
    @classmethod
    def class_method(cls, x: bool) -> object: ...  # snapshot: invalid-method-override

snapshot

error[invalid-method-override]: Invalid override of method `class_method`
  --> src/mdtest_snippet.pyi:21:9
   |
21 |     def class_method(cls, x: bool) -> object: ...  # snapshot: invalid-method-override
   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Definition is incompatible with `Parent.class_method`
   |
  ::: src/mdtest_snippet.pyi:4:9
   |
 4 |     def class_method(cls, x: int) -> int: ...
   |         -------------------------------- `Parent.class_method` defined here
   |
info: incompatible return types: `object` is not assignable to `int`
info: This violates the Liskov Substitution Principle

pyi

class BadTypesB(Parent):
    @staticmethod
    def static_method(x: bool) -> object: ...  # snapshot: invalid-method-override

snapshot

error[invalid-method-override]: Invalid override of method `static_method`
  --> src/mdtest_snippet.pyi:24:9
   |
24 |     def static_method(x: bool) -> object: ...  # snapshot: invalid-method-override
   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Definition is incompatible with `Parent.static_method`
   |
  ::: src/mdtest_snippet.pyi:6:9
   |
 6 |     def static_method(x: int) -> int: ...
   |         ---------------------------- `Parent.static_method` defined here
   |
info: incompatible return types: `object` is not assignable to `int`
info: This violates the Liskov Substitution Principle

Overwriting an instance method with a staticmethod, or vice versa, is an error:

pyi

class BadChild1A(Parent):
    @staticmethod
    def instance_method(self, x: int) -> int: ...  # snapshot: invalid-method-override

snapshot

error[invalid-method-override]: Invalid override of method `instance_method`
  --> src/mdtest_snippet.pyi:27:9
   |
27 |     def instance_method(self, x: int) -> int: ...  # snapshot: invalid-method-override
   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Definition is incompatible with `Parent.instance_method`
   |
  ::: src/mdtest_snippet.pyi:2:9
   |
 2 |     def instance_method(self, x: int) -> int: ...
   |         ------------------------------------ `Parent.instance_method` defined here
   |
info: `BadChild1A.instance_method` is a staticmethod but `Parent.instance_method` is an instance method
info: This violates the Liskov Substitution Principle

pyi

class BadChild1B(Parent):
    def static_method(x: int) -> int: ...  # snapshot: invalid-method-override

snapshot

error[invalid-method-override]: Invalid override of method `static_method`
  --> src/mdtest_snippet.pyi:29:9
   |
29 |     def static_method(x: int) -> int: ...  # snapshot: invalid-method-override
   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Definition is incompatible with `Parent.static_method`
   |
  ::: src/mdtest_snippet.pyi:6:9
   |
 6 |     def static_method(x: int) -> int: ...
   |         ---------------------------- `Parent.static_method` defined here
   |
info: `BadChild1B.static_method` is an instance method but `Parent.static_method` is a staticmethod
info: This violates the Liskov Substitution Principle

Overwriting a classmethod with an instance method is also an error: Although the method has the same signature as Parent.class_method when accessed on instances, it does not have the same signature as Parent.class_method when accessed on the class object itself:

pyi

class BadChild2A(Parent):
    # TODO: we should emit `invalid-method-override` here.
    def class_method(cls, x: int) -> int: ...

Conversely, overwriting an instance method with a classmethod is also an error: Although the method has the same signature as Parent.class_method when accessed on instances, it does not have the same signature as Parent.class_method when accessed on the class object itself.

Note that whereas BadChild2A.class_method is reported as a Liskov violation by mypy, pyright and pyrefly, pyright is the only one of those three to report a Liskov violation on this method as of 2025-11-23.

pyi

class BadChild2B(Parent):
    # TODO: we should emit `invalid-method-override` here.
    @classmethod
    def instance_method(self, x: int) -> int: ...

Overwriting a classmethod with a staticmethod, or vice versa, is also an error:

pyi

class BadChild3A(Parent):
    @staticmethod
    def class_method(cls, x: int) -> int: ...  # snapshot: invalid-method-override

snapshot

error[invalid-method-override]: Invalid override of method `class_method`
  --> src/mdtest_snippet.pyi:39:9
   |
39 |     def class_method(cls, x: int) -> int: ...  # snapshot: invalid-method-override
   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Definition is incompatible with `Parent.class_method`
   |
  ::: src/mdtest_snippet.pyi:4:9
   |
 4 |     def class_method(cls, x: int) -> int: ...
   |         -------------------------------- `Parent.class_method` defined here
   |
info: `BadChild3A.class_method` is a staticmethod but `Parent.class_method` is a classmethod
info: This violates the Liskov Substitution Principle

pyi

class BadChild3B(Parent):
    @classmethod
    def static_method(x: int) -> int: ...  # snapshot: invalid-method-override

snapshot

error[invalid-method-override]: Invalid override of method `static_method`
  --> src/mdtest_snippet.pyi:42:9
   |
42 |     def static_method(x: int) -> int: ...  # snapshot: invalid-method-override
   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Definition is incompatible with `Parent.static_method`
   |
  ::: src/mdtest_snippet.pyi:6:9
   |
 6 |     def static_method(x: int) -> int: ...
   |         ---------------------------- `Parent.static_method` defined here
   |
info: `BadChild3B.static_method` is a classmethod but `Parent.static_method` is a staticmethod
info: This violates the Liskov Substitution Principle

Explicitly annotated classmethod receivers

An explicitly annotated cls can specialize the class receiver and return type. A subclass can also replace a bounded type variable on the class receiver with Self while preserving compatible arguments:

pyi

from typing import Any, TypeVar
from typing_extensions import Self

class Event:
    @classmethod
    def deserialize(cls: type[Event], data: dict[str, object]) -> Event: ...

class SwapEvent(Event):
    @classmethod
    def deserialize(cls: type[SwapEvent], data: dict[str, object]) -> SwapEvent: ...

class Context:
    @classmethod
    def get(cls: type[Self]) -> Self | None: ...

class SettingsContext(Context):
    @classmethod
    def get(cls) -> SettingsContext | None: ...

ItemT = TypeVar("ItemT", bound="Item")

class Item:
    @classmethod
    def from_component(cls: type[ItemT], component: object) -> ItemT: ...

class DynamicItem(Item):
    @classmethod
    def from_component(cls: type[Self], component: object) -> Self: ...

The override can also specialize a gradual parameter or accept additional keyword arguments:

pyi

ModelT = TypeVar("ModelT", bound="BaseModel")

class BaseModel:
    @classmethod
    def validate(cls: type[ModelT], value: Any) -> Any: ...
    @classmethod
    def strategy(cls: type[ModelT], *, size: int | None = None) -> Any: ...
    @classmethod
    def example(cls: type[ModelT], *, size: int | None = None) -> Any: ...

class FrameModel(BaseModel):
    @classmethod
    def validate(cls: type[Self], value: int) -> Self: ...
    @classmethod
    def strategy(cls: type[Self], **kwargs: Any) -> Any: ...
    @classmethod
    def example(cls: type[Self], **kwargs: Any) -> Self: ...

Narrowing a non-gradual argument accepted by the superclass remains an invalid override for both generic and concrete class receivers:

pyi

class InvalidDynamicItem(Item):
    @classmethod
    # error: [invalid-method-override]
    def from_component(cls: type[Self], component: int) -> Self: ...

class InvalidSwapEvent(Event):
    @classmethod
    # error: [invalid-method-override]
    def deserialize(cls: type[InvalidSwapEvent], data: dict[str, int]) -> InvalidSwapEvent: ...

Overloaded methods with positional-only parameters with defaults

When a base class has an overloaded method where one overload accepts only keyword arguments (**kwargs), and the subclass overrides it with a positional-only parameter that has a default, the override should be valid because callers can still call it without positional arguments.

pyi

from typing import overload

class Base:
    @overload
    def method(self, x: int, /) -> None: ...
    @overload
    def method(self, **kwargs: int) -> None: ...
    def method(self, *args, **kwargs) -> None: ...

class GoodChild(Base):
    # This should be fine: the positional-only parameter has a default,
    # so calls like `obj.method(a=1)` are still valid
    def method(self, x: int = 0, /, **kwargs: int) -> None: ...

class BadChild(Base):
    # `x` has no default, so `obj.method(a=1)` would fail
    def method(self, x: int, /, **kwargs: int) -> None: ...  # error: [invalid-method-override]

Definitely bound members with no reachable definitions(!)

We don't emit a Liskov-violation diagnostic here, but if you're writing code like this, you probably have bigger problems:

from __future__ import annotations

class MaybeEqWhile:
    while ...:
        def __eq__(self, other: MaybeEqWhile) -> bool:
            return True

The Liskov Substitution Principle

The Liskov Substitution Principle

Method return types

Method parameters

ClassVar and instance variables

Direct overrides

Regular class-body assignments

Method definitions

Repeated inherited conflicts

Descriptors

Multiple inheritance

Dataclasses

Protocol implementations

Inherited method conflicts in multiple inheritance

Basic conflicts

Empty bases do not hide conflicts

An earlier compatible method satisfies both contracts

A compatible subclass override satisfies both contracts

An intermediate Any does not hide a conflict

Dynamic bases only hide later conflicts

Generic bases

Methods inherited by a direct base

Properties

Synthesized members

Enum mixins

Instance, static, and class methods

Class methods use the subclass as their receiver

Metaclass descriptors do not replace class method contracts

The implicit object base does not repeat an existing error

Existing violations on generic parents are not repeated

Bases with the same name are distinguished

Inconsistent generic bases do not produce override errors

The entire class hierarchy is checked

Non-generic methods on generic classes work as expected

Generic methods on non-generic classes work as expected

Protocol annotations on mixin receivers

Generic instance-method receivers

Generic methods on generic classes work as expected

Fully qualified names are used in diagnostics where appropriate

Excluded methods

Functions assigned in a class body

Edge case: function defined in another module and then assigned in a class body

Bad override of __eq__

Class-private names do not override

Synthesized methods

Staticmethods and classmethods

Explicitly annotated classmethod receivers

Overloaded methods with positional-only parameters with defaults

Definitely bound members with no reachable definitions(!)

`ClassVar` and instance variables

An intermediate `Any` does not hide a conflict

The implicit `object` base does not repeat an existing error

Bad override of `eq`