is_singleton_type: move business logic to type definitions (#13165)

This greatly reduces complexity as each type can determine for
themselves if they are representing a singleton.

`get_enum_values` has to be moved as well, as it is used by
`get_singleton_type`.

Relates-to: #13117

Co-authored-by: László Vaskó <[email protected]>

mypy/typeops.py

@@ -719,11 +719,6 @@ def is_literal_type_like(t: Optional[Type]) -> bool:
         return False
 
 
-def get_enum_values(typ: Instance) -> List[str]:
-    """Return the list of values for an Enum."""
-    return [name for name, sym in typ.type.names.items() if isinstance(sym.node, Var)]
-
-
 def is_singleton_type(typ: Type) -> bool:
     """Returns 'true' if this type is a "singleton type" -- if there exists
     exactly only one runtime value associated with this type.
@@ -741,16 +736,7 @@ def is_singleton_type(typ: Type) -> bool:
     constructing two distinct instances of 100001.
     """
     typ = get_proper_type(typ)
-    # TODO:
-    # Also make this return True if the type corresponds to NotImplemented?
-    return (
-            isinstance(typ, NoneType)
-            or (isinstance(typ, LiteralType)
-                and (typ.is_enum_literal() or isinstance(typ.value, bool)))
-            or (isinstance(typ, Instance) and (
-                typ.type.is_enum and len(get_enum_values(typ)) == 1
-                or typ.type.fullname == 'builtins.ellipsis'))
-    )
+    return typ.is_singleton_type()
 
 
 def try_expanding_sum_type_to_union(typ: Type, target_fullname: str) -> ProperType:
@@ -827,7 +813,7 @@ def try_contracting_literals_in_union(types: Sequence[Type]) -> List[ProperType]
             fullname = typ.fallback.type.fullname
             if typ.fallback.type.is_enum or isinstance(typ.value, bool):
                 if fullname not in sum_types:
-                    sum_types[fullname] = (set(get_enum_values(typ.fallback))
+                    sum_types[fullname] = (set(typ.fallback.get_enum_values())
                                            if typ.fallback.type.is_enum
                                            else {True, False},
                                            [])
@@ -855,7 +841,7 @@ def coerce_to_literal(typ: Type) -> Type:
         if typ.last_known_value:
             return typ.last_known_value
         elif typ.type.is_enum:
-            enum_values = get_enum_values(typ)
+            enum_values = typ.get_enum_values()
             if len(enum_values) == 1:
                 return LiteralType(value=enum_values[0], fallback=typ)
     return original_type

mypy/types.py

@@ -240,6 +240,9 @@ def serialize(self) -> Union[JsonDict, str]:
     def deserialize(cls, data: JsonDict) -> 'Type':
         raise NotImplementedError(f'Cannot deserialize {cls.__name__} instance')
 
+    def is_singleton_type(self) -> bool:
+        return False
+
 
 class TypeAliasType(Type):
     """A type alias to another type.
@@ -1027,6 +1030,9 @@ def deserialize(cls, data: JsonDict) -> 'NoneType':
         assert data['.class'] == 'NoneType'
         return NoneType()
 
+    def is_singleton_type(self) -> bool:
+        return True
+
 
 # NoneType used to be called NoneTyp so to avoid needlessly breaking
 # external plugins we keep that alias here.
@@ -1231,6 +1237,21 @@ def copy_modified(self, *,
     def has_readable_member(self, name: str) -> bool:
         return self.type.has_readable_member(name)
 
+    def is_singleton_type(self) -> bool:
+        # TODO:
+        # Also make this return True if the type corresponds to NotImplemented?
+        return (
+            self.type.is_enum and len(self.get_enum_values()) == 1
+            or self.type.fullname == 'builtins.ellipsis'
+        )
+
+    def get_enum_values(self) -> List[str]:
+        """Return the list of values for an Enum."""
+        return [
+            name for name, sym in self.type.names.items()
+            if isinstance(sym.node, mypy.nodes.Var)
+        ]
+
 
 class FunctionLike(ProperType):
     """Abstract base class for function types."""
@@ -2268,6 +2289,9 @@ def deserialize(cls, data: JsonDict) -> 'LiteralType':
             fallback=Instance.deserialize(data['fallback']),
         )
 
+    def is_singleton_type(self) -> bool:
+        return self.is_enum_literal() or isinstance(self.value, bool)
+
 
 class StarType(ProperType):
     """The star type *type_parameter.