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Commitecfab6a

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[PEP 695] Allow Self return types with contravariance (#17786)
Fix variance inference in this fragment from a typing conformance test:```class ClassA[T1, T2, T3](list[T1]): def method1(self, a: T2) -> None: ... def method2(self) -> T3: ...```Previously T2 was incorrectly inferred as invariant due to `list` havingmethods that return `Self`. Be more flexible with return types to allowinferring contravariance for type variables even if there are `Self`return types, in particular.We could probably make this even more lenient, but after thinking aboutthis for a while, I wasn't sure what the most general rule would be, soI decided to just make a tweak to support the likely most common usecase (which is probably actually not that common either).Link to conformance test:https://github.com/python/typing/blob/main/conformance/tests/generics_variance_inference.py#L15C1-L20C12
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3 files changed

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‎mypy/subtypes.py‎

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Original file line numberDiff line numberDiff line change
@@ -2024,6 +2024,16 @@ def infer_variance(info: TypeInfo, i: int) -> bool:
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typ=find_member(member,self_type,self_type)
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iftyp:
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# It's okay for a method in a generic class with a contravariant type
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# variable to return a generic instance of the class, if it doesn't involve
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# variance (i.e. values of type variables are propagated). Our normal rules
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# would disallow this. Replace such return types with 'Any' to allow this.
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#
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# This could probably be more lenient (e.g. allow self type be nested, don't
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# require all type arguments to be identical to self_type), but this will
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# hopefully cover the vast majority of such cases, including Self.
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typ=erase_return_self_types(typ,self_type)
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typ2=expand_type(typ, {tvar.id:object_type})
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ifnotis_subtype(typ,typ2):
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co=False
@@ -2066,3 +2076,20 @@ def infer_class_variances(info: TypeInfo) -> bool:
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ifnotinfer_variance(info,i):
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success=False
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returnsuccess
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deferase_return_self_types(typ:Type,self_type:Instance)->Type:
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"""If a typ is function-like and returns self_type, replace return type with Any."""
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proper_type=get_proper_type(typ)
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ifisinstance(proper_type,CallableType):
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ret=get_proper_type(proper_type.ret_type)
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ifisinstance(ret,Instance)andret==self_type:
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returnproper_type.copy_modified(ret_type=AnyType(TypeOfAny.implementation_artifact))
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elifisinstance(proper_type,Overloaded):
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returnOverloaded(
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[
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cast(CallableType,erase_return_self_types(it,self_type))
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foritinproper_type.items
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]
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)
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returntyp

‎test-data/unit/check-python312.test‎

Lines changed: 57 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -213,7 +213,7 @@ b: Invariant[int]
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if int():
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a = b # E: Incompatible types in assignment (expression has type "Invariant[int]", variable has type "Invariant[object]")
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if int():
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b = a # E: Incompatible types in assignment (expression has type "Invariant[object]", variable has type "Invariant[int]")
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b = a
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c: Covariant[object]
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d: Covariant[int]
@@ -393,6 +393,62 @@ inv3_1: Invariant3[float] = Invariant3[int](1) # E: Incompatible types in assig
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inv3_2: Invariant3[int] = Invariant3[float](1) # E: Incompatible types in assignment (expression has type "Invariant3[float]", variable has type "Invariant3[int]")
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[builtins fixtures/property.pyi]
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[case testPEP695InferVarianceWithInheritedSelf]
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from typing import overload, Self, TypeVar, Generic
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T = TypeVar("T")
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S = TypeVar("S")
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class C(Generic[T]):
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def f(self, x: T) -> Self: ...
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def g(self) -> T: ...
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class D[T1, T2](C[T1]):
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def m(self, x: T2) -> None: ...
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a1: D[int, int] = D[int, object]()
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a2: D[int, object] = D[int, int]() # E: Incompatible types in assignment (expression has type "D[int, int]", variable has type "D[int, object]")
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a3: D[int, int] = D[object, object]() # E: Incompatible types in assignment (expression has type "D[object, object]", variable has type "D[int, int]")
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a4: D[object, int] = D[int, object]() # E: Incompatible types in assignment (expression has type "D[int, object]", variable has type "D[object, int]")
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[case testPEP695InferVarianceWithReturnSelf]
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from typing import Self, overload
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class Cov[T]:
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def f(self) -> Self: ...
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a1: Cov[int] = Cov[float]() # E: Incompatible types in assignment (expression has type "Cov[float]", variable has type "Cov[int]")
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a2: Cov[float] = Cov[int]()
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class Contra[T]:
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def f(self) -> Self: ...
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def g(self, x: T) -> None: ...
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b1: Contra[int] = Contra[float]()
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b2: Contra[float] = Contra[int]() # E: Incompatible types in assignment (expression has type "Contra[int]", variable has type "Contra[float]")
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class Cov2[T]:
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@overload
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def f(self, x): ...
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@overload
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def f(self) -> Self: ...
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def f(self, x=None): ...
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c1: Cov2[int] = Cov2[float]() # E: Incompatible types in assignment (expression has type "Cov2[float]", variable has type "Cov2[int]")
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c2: Cov2[float] = Cov2[int]()
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class Contra2[T]:
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@overload
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def f(self, x): ...
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@overload
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def f(self) -> Self: ...
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def f(self, x=None): ...
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def g(self, x: T) -> None: ...
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d1: Contra2[int] = Contra2[float]()
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d2: Contra2[float] = Contra2[int]() # E: Incompatible types in assignment (expression has type "Contra2[int]", variable has type "Contra2[float]")
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[case testPEP695InheritInvariant]
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class Invariant[T]:
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x: T

‎test-data/unit/pythoneval.test‎

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Original file line numberDiff line numberDiff line change
@@ -2196,3 +2196,23 @@ type K4 = None | B[int]
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type L1 = Never
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type L2 = list[Never]
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[case testPEP695VarianceInferenceSpecialCaseWithTypeshed]
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# flags: --python-version=3.12
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class C1[T1, T2](list[T1]):
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def m(self, a: T2) -> None: ...
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def func1(p: C1[int, object]):
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x: C1[int, int] = p
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class C2[T1, T2, T3](dict[T2, T3]):
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def m(self, a: T1) -> None: ...
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def func2(p: C2[object, int, int]):
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x: C2[int, int, int] = p
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class C3[T1, T2](tuple[T1, ...]):
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def m(self, a: T2) -> None: ...
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def func3(p: C3[int, object]):
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x: C3[int, int] = p

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