Add support for __init_subclass__ (#2654)

* add tests for init_subclass

* enforce that init_subclass must return None

* infer the first argument to __init_subclass__ correctly

Author: JelleZijlstra

mypy/checker.py

@@ -550,10 +550,10 @@ def check_func_def(self, defn: FuncItem, typ: CallableType, name: str) -> None:
 
                 if fdef:
                     # Check if __init__ has an invalid, non-None return type.
-                    if (fdef.info and fdef.name() == '__init__' and
+                    if (fdef.info and fdef.name() in ('__init__', '__init_subclass__') and
                             not isinstance(typ.ret_type, (Void, NoneTyp)) and
                             not self.dynamic_funcs[-1]):
-                        self.fail(messages.INIT_MUST_HAVE_NONE_RETURN_TYPE,
+                        self.fail(messages.MUST_HAVE_NONE_RETURN_TYPE.format(fdef.name()),
                                   item.type)
 
                     show_untyped = not self.is_typeshed_stub or self.options.warn_incomplete_stub
@@ -618,7 +618,7 @@ def is_implicit_any(t: Type) -> bool:
                     if (isinstance(defn, FuncDef) and ref_type is not None and i == 0
                             and not defn.is_static
                             and typ.arg_kinds[0] not in [nodes.ARG_STAR, nodes.ARG_STAR2]):
-                        if defn.is_class or defn.name() == '__new__':
+                        if defn.is_class or defn.name() in ('__new__', '__init_subclass__'):
                             ref_type = mypy.types.TypeType(ref_type)
                         erased = erase_to_bound(arg_type)
                         if not is_subtype_ignoring_tvars(ref_type, erased):

mypy/messages.py

@@ -44,7 +44,7 @@
 INCOMPATIBLE_TYPES_IN_YIELD = 'Incompatible types in yield'
 INCOMPATIBLE_TYPES_IN_YIELD_FROM = 'Incompatible types in "yield from"'
 INCOMPATIBLE_TYPES_IN_STR_INTERPOLATION = 'Incompatible types in string interpolation'
-INIT_MUST_HAVE_NONE_RETURN_TYPE = 'The return type of "__init__" must be None'
+MUST_HAVE_NONE_RETURN_TYPE = 'The return type of "{}" must be None'
 TUPLE_INDEX_MUST_BE_AN_INT_LITERAL = 'Tuple index must be an integer literal'
 TUPLE_SLICE_MUST_BE_AN_INT_LITERAL = 'Tuple slice must be an integer literal'
 TUPLE_INDEX_OUT_OF_RANGE = 'Tuple index out of range'

mypy/semanal.py

@@ -335,7 +335,7 @@ def prepare_method_signature(self, func: FuncDef) -> None:
             elif isinstance(functype, CallableType):
                 self_type = functype.arg_types[0]
                 if isinstance(self_type, AnyType):
-                    if func.is_class or func.name() == '__new__':
+                    if func.is_class or func.name() in ('__new__', '__init_subclass__'):
                         leading_type = self.class_type(self.type)
                     else:
                         leading_type = fill_typevars(self.type)

mypy/sharedparse.py

@@ -36,6 +36,7 @@
     "__imod__",
     "__imul__",
     "__init__",
+    "__init_subclass__",
     "__int__",
     "__invert__",
     "__ior__",
@@ -84,6 +85,7 @@
 
 MAGIC_METHODS_ALLOWING_KWARGS = {
     "__init__",
+    "__init_subclass__",
     "__new__",
 }
 

test-data/unit/check-classes.test

@@ -382,6 +382,20 @@ class A:
 [out]
 main:3: error: The return type of "__init__" must be None
 
+[case testInitSubclassWithReturnValueType]
+import typing
+class A:
+    def __init_subclass__(cls) -> 'A': pass
+[out]
+main:3: error: The return type of "__init_subclass__" must be None
+
+[case testInitSubclassWithImplicitReturnValueType]
+import typing
+class A:
+    def __init_subclass__(cls, x: int=1): pass
+[out]
+main:3: error: The return type of "__init_subclass__" must be None
+
 [case testGlobalFunctionInitWithReturnType]
 import typing
 a = __init__() # type: A
@@ -1205,7 +1219,7 @@ class D:
     def __get__(self, inst: Any, own: str) -> Any: pass
 class A:
     f = D()
-A().f  # E: Argument 2 to "__get__" of "D" has incompatible type Type[A]; expected "str" 
+A().f  # E: Argument 2 to "__get__" of "D" has incompatible type Type[A]; expected "str"
 
 [case testDescriptorGetSetDifferentTypes]
 from typing import Any

test-data/unit/check-selftype.test

@@ -108,9 +108,9 @@ reveal_type(B.new())  # E: Revealed type is '__main__.B*'
 
 [case testSelfTypeOverride]
 from typing import TypeVar, cast
- 
+
 T = TypeVar('T', bound='A', covariant=True)
- 
+
 class A:
     def copy(self: T) -> T: pass
 
@@ -124,15 +124,15 @@ class C(A):
 reveal_type(C().copy)  # E: Revealed type is 'def () -> __main__.C*'
 reveal_type(C().copy())  # E: Revealed type is '__main__.C*'
 reveal_type(cast(A, C()).copy)  # E: Revealed type is 'def () -> __main__.A*'
-reveal_type(cast(A, C()).copy())  # E: Revealed type is '__main__.A*' 
+reveal_type(cast(A, C()).copy())  # E: Revealed type is '__main__.A*'
 
 [builtins fixtures/bool.pyi]
 
 [case testSelfTypeSuper]
 from typing import TypeVar, cast
- 
+
 T = TypeVar('T', bound='A', covariant=True)
- 
+
 class A:
     def copy(self: T) -> T: pass
 
@@ -147,7 +147,7 @@ class B(A):
 
 [case testSelfTypeRecursiveBinding]
 from typing import TypeVar, Callable, Type
- 
+
 T = TypeVar('T', bound='A', covariant=True)
 class A:
     # TODO: This is potentially unsafe, as we use T in an argument type
@@ -173,7 +173,7 @@ reveal_type(B.new)  # E: Revealed type is 'def (factory: def (__main__.B*) -> __
 
 [case testSelfTypeBound]
 from typing import TypeVar, Callable, cast
- 
+
 TA = TypeVar('TA', bound='A', covariant=True)
 
 class A:
@@ -200,9 +200,9 @@ class B(A):
 -- # TODO: fail for this
 -- [case testSelfTypeBare]
 -- from typing import TypeVar, Type
--- 
+--
 -- T = TypeVar('T', bound='E')
--- 
+--
 -- class E:
 --     def copy(self: T, other: T) -> T: pass
 
@@ -262,7 +262,7 @@ class B:
     @classmethod
     def cfoo(cls: Type[Q]) -> Q:
         return cls()
-    
+
 class C:
     def foo(self: C) -> C: return self
 
@@ -272,7 +272,7 @@ class C:
 
 class D:
     def foo(self: str) -> str:  # E: The erased type of self 'builtins.str' is not a supertype of its class '__main__.D'
-        return self    
+        return self
 
     @staticmethod
     def bar(self: str) -> str:
@@ -302,28 +302,43 @@ class C:
 [case testSelfTypeNew]
 from typing import TypeVar, Type
 
-T = TypeVar('T', bound=A)    
-class A: 
+T = TypeVar('T', bound=A)
+class A:
     def __new__(cls: Type[T]) -> T:
         return cls()
 
+    def __init_subclass__(cls: Type[T]) -> None:
+        pass
+
 class B:
     def __new__(cls: Type[T]) -> T:  # E: The erased type of self 'Type[__main__.A]' is not a supertype of its class 'Type[__main__.B]'
         return cls()
 
-class C: 
+    def __init_subclass__(cls: Type[T]) -> None:  # E: The erased type of self 'Type[__main__.A]' is not a supertype of its class 'Type[__main__.B]'
+        pass
+
+class C:
     def __new__(cls: Type[C]) -> C:
         return cls()
 
+    def __init_subclass__(cls: Type[C]) -> None:
+        pass
+
 class D:
     def __new__(cls: D) -> D:  # E: The erased type of self '__main__.D' is not a supertype of its class 'Type[__main__.D]'
         return cls
 
+    def __init_subclass__(cls: D) -> None:  # E: The erased type of self '__main__.D' is not a supertype of its class 'Type[__main__.D]'
+        pass
+
 class E:
     def __new__(cls) -> E:
         reveal_type(cls)  # E: Revealed type is 'def () -> __main__.E'
         return cls()
 
+    def __init_subclass__(cls) -> None:
+        reveal_type(cls)  # E: Revealed type is 'def () -> __main__.E'
+
 [case testSelfTypeProperty]
 from typing import TypeVar