Minor updates to protocol semantics (#3996)

This updates protocol semantic according to the discussion in python/typing#464:

    None should be considered a subtype of an empty protocol
    method = None rule should apply only to callable protocol members
    issublcass() is prohibited for protocols with non-method members.

Fixes #3906
Fixes #3938
Fixes #3939

mypy/checkexpr.py

@@ -36,7 +36,7 @@
 from mypy import join
 from mypy.meet import narrow_declared_type
 from mypy.maptype import map_instance_to_supertype
-from mypy.subtypes import is_subtype, is_equivalent, find_member
+from mypy.subtypes import is_subtype, is_equivalent, find_member, non_method_protocol_members
 from mypy import applytype
 from mypy import erasetype
 from mypy.checkmember import analyze_member_access, type_object_type, bind_self
@@ -264,22 +264,37 @@ def visit_call_expr(self, e: CallExpr, allow_none_return: bool = False) -> Type:
                         callee_type = self.apply_method_signature_hook(
                             e, callee_type, object_type, signature_hook)
         ret_type = self.check_call_expr_with_callee_type(callee_type, e, fullname, object_type)
-        if (isinstance(e.callee, RefExpr) and len(e.args) == 2 and
-                e.callee.fullname in ('builtins.isinstance', 'builtins.issubclass')):
-            for expr in mypy.checker.flatten(e.args[1]):
-                tp = self.chk.type_map[expr]
-                if (isinstance(tp, CallableType) and tp.is_type_obj() and
-                        tp.type_object().is_protocol and
-                        not tp.type_object().runtime_protocol):
-                    self.chk.fail('Only @runtime protocols can be used with'
-                                  ' instance and class checks', e)
+        if isinstance(e.callee, RefExpr) and len(e.args) == 2:
+            if e.callee.fullname in ('builtins.isinstance', 'builtins.issubclass'):
+                self.check_runtime_protocol_test(e)
+            if e.callee.fullname == 'builtins.issubclass':
+                self.check_protocol_issubclass(e)
         if isinstance(ret_type, UninhabitedType):
             self.chk.binder.unreachable()
         if not allow_none_return and isinstance(ret_type, NoneTyp):
             self.chk.msg.does_not_return_value(callee_type, e)
             return AnyType(TypeOfAny.from_error)
         return ret_type
 
+    def check_runtime_protocol_test(self, e: CallExpr) -> None:
+        for expr in mypy.checker.flatten(e.args[1]):
+            tp = self.chk.type_map[expr]
+            if (isinstance(tp, CallableType) and tp.is_type_obj() and
+                    tp.type_object().is_protocol and
+                    not tp.type_object().runtime_protocol):
+                self.chk.fail('Only @runtime protocols can be used with'
+                              ' instance and class checks', e)
+
+    def check_protocol_issubclass(self, e: CallExpr) -> None:
+        for expr in mypy.checker.flatten(e.args[1]):
+            tp = self.chk.type_map[expr]
+            if (isinstance(tp, CallableType) and tp.is_type_obj() and
+                    tp.type_object().is_protocol):
+                attr_members = non_method_protocol_members(tp.type_object())
+                if attr_members:
+                    self.chk.msg.report_non_method_protocol(tp.type_object(),
+                                                            attr_members, e)
+
     def check_typeddict_call(self, callee: TypedDictType,
                              arg_kinds: List[int],
                              arg_names: Sequence[Optional[str]],

mypy/meet.py

@@ -44,6 +44,8 @@ def narrow_declared_type(declared: Type, narrowed: Type) -> Type:
         return narrowed
     elif isinstance(declared, (Instance, TupleType)):
         return meet_types(declared, narrowed)
+    elif isinstance(declared, TypeType) and isinstance(narrowed, TypeType):
+        return TypeType.make_normalized(narrow_declared_type(declared.item, narrowed.item))
     return narrowed
 
 

mypy/messages.py

@@ -1056,6 +1056,15 @@ def concrete_only_call(self, typ: Type, context: Context) -> None:
         self.fail("Only concrete class can be given where {} is expected"
                   .format(self.format(typ)), context)
 
+    def report_non_method_protocol(self, tp: TypeInfo, members: List[str],
+                                   context: Context) -> None:
+        self.fail("Only protocols that don't have non-method members can be"
+                  " used with issubclass()", context)
+        if len(members) < 3:
+            attrs = ', '.join(members)
+            self.note('Protocol "{}" has non-method member(s): {}'
+                      .format(tp.name(), attrs), context)
+
     def note_call(self, subtype: Type, call: Type, context: Context) -> None:
         self.note('"{}.__call__" has type {}'.format(self.format_bare(subtype),
                                                      self.format(call, verbosity=1)), context)

mypy/subtypes.py

@@ -123,7 +123,9 @@ def visit_any(self, left: AnyType) -> bool:
     def visit_none_type(self, left: NoneTyp) -> bool:
         if experiments.STRICT_OPTIONAL:
             return (isinstance(self.right, NoneTyp) or
-                    is_named_instance(self.right, 'builtins.object'))
+                    is_named_instance(self.right, 'builtins.object') or
+                    isinstance(self.right, Instance) and self.right.type.is_protocol and
+                    not self.right.type.protocol_members)
         else:
             return True
 
@@ -386,7 +388,7 @@ def f(self) -> A: ...
                 is_compat = is_proper_subtype(subtype, supertype)
             if not is_compat:
                 return False
-            if isinstance(subtype, NoneTyp) and member.startswith('__') and member.endswith('__'):
+            if isinstance(subtype, NoneTyp) and isinstance(supertype, CallableType):
                 # We want __hash__ = None idiom to work even without --strict-optional
                 return False
             subflags = get_member_flags(member, left.type)
@@ -516,6 +518,21 @@ def find_node_type(node: Union[Var, FuncBase], itype: Instance, subtype: Type) -
     return typ
 
 
+def non_method_protocol_members(tp: TypeInfo) -> List[str]:
+    """Find all non-callable members of a protocol."""
+
+    assert tp.is_protocol
+    result = []  # type: List[str]
+    anytype = AnyType(TypeOfAny.special_form)
+    instance = Instance(tp, [anytype] * len(tp.defn.type_vars))
+
+    for member in tp.protocol_members:
+        typ = find_member(member, instance, instance)
+        if not isinstance(typ, CallableType):
+            result.append(member)
+    return result
+
+
 def is_callable_subtype(left: CallableType, right: CallableType,
                         ignore_return: bool = False,
                         ignore_pos_arg_names: bool = False,

test-data/unit/check-protocols.test

@@ -2118,3 +2118,64 @@ main:10: note:         def other(self, *args: Any, hint: Optional[str] = ..., **
 main:10: note:     Got:
 main:10: note:         def other(self) -> int
 
+[case testObjectAllowedInProtocolBases]
+from typing import Protocol
+class P(Protocol, object):
+    pass
+[out]
+
+[case testNoneSubtypeOfEmptyProtocol]
+from typing import Protocol
+class P(Protocol):
+    pass
+
+x: P = None
+[out]
+
+[case testNoneSubtypeOfAllProtocolsWithoutStrictOptional]
+from typing import Protocol
+class P(Protocol):
+    attr: int
+    def meth(self, arg: str) -> str:
+        pass
+
+x: P = None
+[out]
+
+[case testNoneSubtypeOfEmptyProtocolStrict]
+# flags: --strict-optional
+from typing import Protocol
+class P(Protocol):
+    pass
+x: P = None
+
+class PBad(Protocol):
+    x: int
+y: PBad = None  # E: Incompatible types in assignment (expression has type "None", variable has type "PBad")
+[out]
+
+[case testOnlyMethodProtocolUsableWithIsSubclass]
+from typing import Protocol, runtime, Union, Type
+@runtime
+class P(Protocol):
+    def meth(self) -> int:
+        pass
+@runtime
+class PBad(Protocol):
+    x: str
+
+class C:
+    x: str
+    def meth(self) -> int:
+        pass
+class E: pass
+
+cls: Type[Union[C, E]]
+issubclass(cls, PBad)  # E: Only protocols that don't have non-method members can be used with issubclass() \
+                       # N: Protocol "PBad" has non-method member(s): x
+if issubclass(cls, P):
+    reveal_type(cls)  # E: Revealed type is 'Type[__main__.C]'
+else:
+    reveal_type(cls)  # E: Revealed type is 'Type[__main__.E]'
+[builtins fixtures/isinstance.pyi]
+[out]