Make Type[T] comparisons work (#1806)

This commit introduces a workaround to fix the bug discussed in #1787

Previously, code where you compared two types (eg `int == int` or `int != int`) would cause mypy to incorrectly report a "too few arguments" error.

Author: Michael0x2a

mypy/checkexpr.py

@@ -286,8 +286,8 @@ def check_call(self, callee: Type, args: List[Node],
                     callee)
         elif isinstance(callee, Instance):
             call_function = analyze_member_access('__call__', callee, context,
-                                         False, False, self.named_type, self.not_ready_callback,
-                                         self.msg)
+                                         False, False, False, self.named_type,
+                                         self.not_ready_callback, self.msg)
             return self.check_call(call_function, args, arg_kinds, context, arg_names,
                                    callable_node, arg_messages)
         elif isinstance(callee, TypeVarType):
@@ -861,7 +861,7 @@ def analyze_ordinary_member_access(self, e: MemberExpr,
         else:
             # This is a reference to a non-module attribute.
             return analyze_member_access(e.name, self.accept(e.expr), e,
-                                         is_lvalue, False,
+                                         is_lvalue, False, False,
                                          self.named_type, self.not_ready_callback, self.msg)
 
     def analyze_external_member_access(self, member: str, base_type: Type,
@@ -870,7 +870,7 @@ def analyze_external_member_access(self, member: str, base_type: Type,
         refer to private definitions. Return the result type.
         """
         # TODO remove; no private definitions in mypy
-        return analyze_member_access(member, base_type, context, False, False,
+        return analyze_member_access(member, base_type, context, False, False, False,
                                      self.named_type, self.not_ready_callback, self.msg)
 
     def visit_int_expr(self, e: IntExpr) -> Type:
@@ -1008,7 +1008,7 @@ def check_op_local(self, method: str, base_type: Type, arg: Node,
 
         Return tuple (result type, inferred operator method type).
         """
-        method_type = analyze_member_access(method, base_type, context, False, False,
+        method_type = analyze_member_access(method, base_type, context, False, False, True,
                                             self.named_type, self.not_ready_callback, local_errors)
         return self.check_call(method_type, [arg], [nodes.ARG_POS],
                                context, arg_messages=local_errors)
@@ -1434,7 +1434,7 @@ def analyze_super(self, e: SuperExpr, is_lvalue: bool) -> Type:
                     if not self.chk.typing_mode_full():
                         return AnyType()
                     return analyze_member_access(e.name, self_type(e.info), e,
-                                                 is_lvalue, True,
+                                                 is_lvalue, True, False,
                                                  self.named_type, self.not_ready_callback,
                                                  self.msg, base)
         else:

mypy/checkmember.py

@@ -8,7 +8,8 @@
     DeletedType, NoneTyp, TypeType
 )
 from mypy.nodes import TypeInfo, FuncBase, Var, FuncDef, SymbolNode, Context
-from mypy.nodes import ARG_POS, ARG_STAR, ARG_STAR2, function_type, Decorator, OverloadedFuncDef
+from mypy.nodes import ARG_POS, ARG_STAR, ARG_STAR2, OpExpr, ComparisonExpr
+from mypy.nodes import function_type, Decorator, OverloadedFuncDef
 from mypy.messages import MessageBuilder
 from mypy.maptype import map_instance_to_supertype
 from mypy.expandtype import expand_type_by_instance
@@ -23,6 +24,7 @@ def analyze_member_access(name: str,
                           node: Context,
                           is_lvalue: bool,
                           is_super: bool,
+                          is_operator: bool,
                           builtin_type: Callable[[str], Instance],
                           not_ready_callback: Callable[[str, Context], None],
                           msg: MessageBuilder,
@@ -79,45 +81,59 @@ def analyze_member_access(name: str,
     elif isinstance(typ, NoneTyp):
         # The only attribute NoneType has are those it inherits from object
         return analyze_member_access(name, builtin_type('builtins.object'), node, is_lvalue,
-                                     is_super, builtin_type, not_ready_callback, msg,
+                                     is_super, is_operator, builtin_type, not_ready_callback, msg,
                                      report_type=report_type)
     elif isinstance(typ, UnionType):
         # The base object has dynamic type.
         msg.disable_type_names += 1
-        results = [analyze_member_access(name, subtype, node, is_lvalue,
-                                         is_super, builtin_type, not_ready_callback, msg)
+        results = [analyze_member_access(name, subtype, node, is_lvalue, is_super,
+                                         is_operator, builtin_type, not_ready_callback, msg)
                    for subtype in typ.items]
         msg.disable_type_names -= 1
         return UnionType.make_simplified_union(results)
     elif isinstance(typ, TupleType):
         # Actually look up from the fallback instance type.
-        return analyze_member_access(name, typ.fallback, node, is_lvalue,
-                                     is_super, builtin_type, not_ready_callback, msg)
+        return analyze_member_access(name, typ.fallback, node, is_lvalue, is_super,
+                                     is_operator, builtin_type, not_ready_callback, msg)
     elif isinstance(typ, FunctionLike) and typ.is_type_obj():
         # Class attribute.
         # TODO super?
         ret_type = typ.items()[0].ret_type
         if isinstance(ret_type, TupleType):
             ret_type = ret_type.fallback
         if isinstance(ret_type, Instance):
-            result = analyze_class_attribute_access(ret_type, name, node, is_lvalue,
-                                                    builtin_type, not_ready_callback, msg)
-            if result:
-                return result
+            if not is_operator:
+                # When Python sees an operator (eg `3 == 4`), it automatically translates that
+                # into something like `int.__eq__(3, 4)` instead of `(3).__eq__(4)` as an
+                # optimation.
+                #
+                # While it normally it doesn't matter which of the two versions are used, it
+                # does cause inconsistencies when working with classes. For example, translating
+                # `int == int` to `int.__eq__(int)` would not work since `int.__eq__` is meant to
+                # compare two int _instances_. What we really want is `type(int).__eq__`, which
+                # is meant to compare two types or classes.
+                #
+                # This check makes sure that when we encounter an operator, we skip looking up
+                # the corresponding method in the current instance to avoid this edge case.
+                # See https://github.com/python/mypy/pull/1787 for more info.
+                result = analyze_class_attribute_access(ret_type, name, node, is_lvalue,
+                                                        builtin_type, not_ready_callback, msg)
+                if result:
+                    return result
             # Look up from the 'type' type.
             return analyze_member_access(name, typ.fallback, node, is_lvalue, is_super,
-                                         builtin_type, not_ready_callback, msg,
+                                         is_operator, builtin_type, not_ready_callback, msg,
                                          report_type=report_type)
         else:
             assert False, 'Unexpected type {}'.format(repr(ret_type))
     elif isinstance(typ, FunctionLike):
         # Look up from the 'function' type.
         return analyze_member_access(name, typ.fallback, node, is_lvalue, is_super,
-                                     builtin_type, not_ready_callback, msg,
+                                     is_operator, builtin_type, not_ready_callback, msg,
                                      report_type=report_type)
     elif isinstance(typ, TypeVarType):
         return analyze_member_access(name, typ.upper_bound, node, is_lvalue, is_super,
-                                     builtin_type, not_ready_callback, msg,
+                                     is_operator, builtin_type, not_ready_callback, msg,
                                      report_type=report_type)
     elif isinstance(typ, DeletedType):
         msg.deleted_as_rvalue(typ, node)
@@ -130,14 +146,15 @@ def analyze_member_access(name: str,
         elif isinstance(typ.item, TypeVarType):
             if isinstance(typ.item.upper_bound, Instance):
                 item = typ.item.upper_bound
-        if item:
+        if item and not is_operator:
+            # See comment above for why operators are skipped
             result = analyze_class_attribute_access(item, name, node, is_lvalue,
                                                     builtin_type, not_ready_callback, msg)
             if result:
                 return result
         fallback = builtin_type('builtins.type')
         return analyze_member_access(name, fallback, node, is_lvalue, is_super,
-                                     builtin_type, not_ready_callback, msg,
+                                     is_operator, builtin_type, not_ready_callback, msg,
                                      report_type=report_type)
     return msg.has_no_attr(report_type, name, node)
 

test-data/unit/check-classes.test

@@ -2006,3 +2006,44 @@ reveal_type(User)  # E: Revealed type is 'builtins.type'
 [builtins fixtures/args.py]
 [out]
 
+[case testTypeTypeComparisonWorks]
+class User: pass
+
+User == User
+User == type(User())
+type(User()) == User
+type(User()) == type(User())
+
+User != User
+User != type(User())
+type(User()) != User
+type(User()) != type(User())
+
+int == int
+int == type(3)
+type(3) == int
+type(3) == type(3)
+
+int != int
+int != type(3)
+type(3) != int
+type(3) != type(3)
+
+User is User
+User is type(User)
+type(User) is User
+type(User) is type(User)
+
+int is int
+int is type(3)
+type(3) is int
+type(3) is type(3)
+
+int.__eq__(int)
+int.__eq__(3, 4)
+[builtins fixtures/args.py]
+[out]
+main:33: error: Too few arguments for "__eq__" of "int"
+main:33: error: Unsupported operand types for == ("int" and "int")
+
+

test-data/unit/fixtures/args.py

@@ -8,17 +8,21 @@
 
 class object:
     def __init__(self) -> None: pass
+    def __eq__(self, o: object) -> bool: pass
+    def __ne__(self, o: object) -> bool: pass
 
 class type:
     @overload
     def __init__(self, o: object) -> None: pass
     @overload
     def __init__(self, name: str, bases: Tuple[type, ...], dict: Dict[str, Any]) -> None: pass
+    def __call__(self, *args: Any, **kwargs: Any) -> Any: pass
 
 class tuple(Iterable[Tco], Generic[Tco]): pass
 class dict(Generic[T, S]): pass
 
-class int: pass
+class int:
+    def __eq__(self, o: object) -> bool: pass
 class str: pass
 class bool: pass
 class function: pass