Support for singleton types in unions with Enum (#7693)

This PR adds supports for singleton types in unions using `Enum`s as described
in [PEP 484][0] (without using `Final`).

As suggested by @Michael0x2a in [the corresponding issue][1], adding another
case to the `is_singleton_type` and `coerce_to_literal`functions allows mypy
to recognize an `Enum` with 1 value as a singleton type.

Fixes #7279

  [0]: https://www.python.org/dev/peps/pep-0484/#support-for-singleton-types-in-unions
  [1]: #7279 (comment)

Author: ilai-deutel

mypy/checker.py

@@ -4743,6 +4743,11 @@ def is_private(node_name: str) -> bool:
     return node_name.startswith('__') and not node_name.endswith('__')
 
 
+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.
@@ -4751,7 +4756,8 @@ def is_singleton_type(typ: Type) -> bool:
     'is_singleton_type(t)' returns True if and only if the expression 'a is b' is
     always true.
 
-    Currently, this returns True when given NoneTypes and enum LiteralTypes.
+    Currently, this returns True when given NoneTypes, enum LiteralTypes and
+    enum types with a single value.
 
     Note that other kinds of LiteralTypes cannot count as singleton types. For
     example, suppose we do 'a = 100000 + 1' and 'b = 100001'. It is not guaranteed
@@ -4761,7 +4767,10 @@ def is_singleton_type(typ: Type) -> bool:
     typ = get_proper_type(typ)
     # TODO: Also make this return True if the type is a bool LiteralType.
     # Also make this return True if the type corresponds to ... (ellipsis) or NotImplemented?
-    return isinstance(typ, NoneType) or (isinstance(typ, LiteralType) and typ.is_enum_literal())
+    return (
+        isinstance(typ, NoneType) or (isinstance(typ, LiteralType) and typ.is_enum_literal())
+        or (isinstance(typ, Instance) and typ.type.is_enum and len(get_enum_values(typ)) == 1)
+    )
 
 
 def try_expanding_enum_to_union(typ: Type, target_fullname: str) -> ProperType:
@@ -4808,17 +4817,21 @@ class Status(Enum):
 
 
 def coerce_to_literal(typ: Type) -> ProperType:
-    """Recursively converts any Instances that have a last_known_value into the
-    corresponding LiteralType.
+    """Recursively converts any Instances that have a last_known_value or are
+    instances of enum types with a single value into the corresponding LiteralType.
     """
     typ = get_proper_type(typ)
     if isinstance(typ, UnionType):
         new_items = [coerce_to_literal(item) for item in typ.items]
         return make_simplified_union(new_items)
-    elif isinstance(typ, Instance) and typ.last_known_value:
-        return typ.last_known_value
-    else:
-        return typ
+    elif isinstance(typ, Instance):
+        if typ.last_known_value:
+            return typ.last_known_value
+        elif typ.type.is_enum:
+            enum_values = get_enum_values(typ)
+            if len(enum_values) == 1:
+                return LiteralType(value=enum_values[0], fallback=typ)
+    return typ
 
 
 def has_bool_item(typ: ProperType) -> bool:

test-data/unit/check-enum.test

@@ -808,7 +808,7 @@ else:
 
 [builtins fixtures/bool.pyi]
 
-[case testEnumReachabilityPEP484Example1]
+[case testEnumReachabilityPEP484ExampleWithFinal]
 # flags: --strict-optional
 from typing import Union
 from typing_extensions import Final
@@ -833,7 +833,7 @@ def func(x: Union[int, None, Empty] = _empty) -> int:
         return x + 2
 [builtins fixtures/primitives.pyi]
 
-[case testEnumReachabilityPEP484Example2]
+[case testEnumReachabilityPEP484ExampleWithMultipleValues]
 from typing import Union
 from enum import Enum
 
@@ -852,5 +852,59 @@ def process(response: Union[str, Reason] = '') -> str:
         # response can be only str, all other possible values exhausted
         reveal_type(response)  # N: Revealed type is 'builtins.str'
         return 'PROCESSED: ' + response
+[builtins fixtures/primitives.pyi]
+
+
+[case testEnumReachabilityPEP484ExampleSingleton]
+# flags: --strict-optional
+from typing import Union
+from typing_extensions import Final
+from enum import Enum
+
+class Empty(Enum):
+    token = 0
+_empty = Empty.token
+
+def func(x: Union[int, None, Empty] = _empty) -> int:
+    boom = x + 42       # E: Unsupported left operand type for + ("None") \
+                        # E: Unsupported left operand type for + ("Empty") \
+                        # N: Left operand is of type "Union[int, None, Empty]"
+    if x is _empty:
+        reveal_type(x)  # N: Revealed type is 'Literal[__main__.Empty.token]'
+        return 0
+    elif x is None:
+        reveal_type(x)  # N: Revealed type is 'None'
+        return 1
+    else:  # At this point typechecker knows that x can only have type int
+        reveal_type(x)  # N: Revealed type is 'builtins.int'
+        return x + 2
+[builtins fixtures/primitives.pyi]
+
+[case testEnumReachabilityPEP484ExampleSingletonWithMethod]
+# flags: --strict-optional
+from typing import Union
+from typing_extensions import Final
+from enum import Enum
 
+class Empty(Enum):
+    token = lambda x: x
+
+    def f(self) -> int:
+        return 1
+
+_empty = Empty.token
+
+def func(x: Union[int, None, Empty] = _empty) -> int:
+    boom = x + 42       # E: Unsupported left operand type for + ("None") \
+                        # E: Unsupported left operand type for + ("Empty") \
+                        # N: Left operand is of type "Union[int, None, Empty]"
+    if x is _empty:
+        reveal_type(x)  # N: Revealed type is 'Literal[__main__.Empty.token]'
+        return 0
+    elif x is None:
+        reveal_type(x)  # N: Revealed type is 'None'
+        return 1
+    else:  # At this point typechecker knows that x can only have type int
+        reveal_type(x)  # N: Revealed type is 'builtins.int'
+        return x + 2
 [builtins fixtures/primitives.pyi]