Added draft chapter to typing spec for enumerations.

This is the same set of changes as PR #1591. The CI script that enforces CLA signing was confused, so it didn't permit merging the original PR.

docs/spec/enums.rst

@@ -0,0 +1,363 @@
+Enumerations
+============
+
+The ``enum.Enum`` class behaves differently from other Python classes in several
+ways that require special-case handling in type checkers. This section discusses
+the Enum behaviors that should be supported by type checkers and others which
+may be supported optionally. It is recommended that library and type stub
+authors avoid using optional behaviors because these may not be supported
+by some type checkers.
+
+
+Enum Definition
+---------------
+
+Enum classes can be defined using a "class syntax" or a "function syntax".
+The function syntax offers several ways to specify enum members: names passed
+as individual arguments, a list or tuple of names, a string of
+comma-delimited or space-delimited names, a list or tuple of tuples that contain
+name/value pairs, and a dictionary of name/value items.
+
+Type checkers should support the class syntax, but the function syntax (in
+its various forms) is optional::
+
+    class Color1(Enum): # Supported
+        RED = 1
+        GREEN = 2
+        BLUE = 3
+
+    Color2 = Enum('Color2', 'RED', 'GREEN', 'BLUE')  # Optional
+    Color3 = Enum('Color3', ['RED', 'GREEN', 'BLUE'])  # Optional
+    Color4 = Enum('Color4', ('RED', 'GREEN', 'BLUE'))  # Optional
+    Color5 = Enum('Color5', 'RED, GREEN, BLUE')  # Optional
+    Color6 = Enum('Color6', 'RED GREEN BLUE')  # Optional
+    Color7 = Enum('Color7', [('RED': 1), ('GREEN': 2), ('BLUE': 3)])  # Optional
+    Color8 = Enum('Color8', (('RED': 1), ('GREEN': 2), ('BLUE': 3)))  # Optional
+    Color9 = Enum('Color9', {'RED': 1, 'GREEN': 2, 'BLUE': 3})  # Optional
+
+Enum classes can also be defined using a subclass of ``enum.Enum`` or any class
+that uses ``enum.EnumType`` (or a subclass thereof) as a metaclass. Note that
+``enum.EnumType`` was named ``enum.EnumMeta`` prior to Python 3.11. Type
+checkers should treat such classes as enums::
+
+    class CustomEnum1(Enum):
+        pass
+
+    class Color7(CustomEnum1):  # Supported
+        RED = 1
+        GREEN = 2
+        BLUE = 3
+
+    class CustomEnumType(EnumType):
+        pass
+
+    class CustomEnum2(metaclass=CustomEnumType):
+        pass
+
+    class Color8(CustomEnum2):  # Supported
+        RED = 1
+        GREEN = 2
+        BLUE = 3
+
+
+Enum Behaviors
+--------------
+
+Enum classes are iterable and indexable, and they can be called with a value
+to look up the enum member with that value. Type checkers should support these
+behaviors::
+
+    class Color(Enum):
+        RED = 1
+        GREEN = 2
+        BLUE = 3
+
+    for color in Color:
+        reveal_type(color)  # Revealed type is 'Color'
+
+    reveal_type(Color["RED"])  # Revealed type is 'Literal[Color.RED]' (or 'Color')
+    reveal_type(Color(3))  # Revealed type is 'Literal[Color.BLUE]' (or 'Color')
+
+Unlike most Python classes, Calling an enum class does not invoke its constructor.
+Instead, the call performs a value-based lookup of an enum member.
+
+An Enum class with one or more defined members cannot be subclassed. They are
+implicitly "final". Type checkers should enforce this::
+
+    class EnumWithNoMembers(Enum):
+        pass
+
+    class Shape(EnumWithNoMembers):  # OK (because no members are defined)
+        SQUARE = 1
+        CIRCLE = 2
+
+    class ExtendedShape(Shape):  # Type checker error: Shape is implicitly final
+        TRIANGLE = 3
+
+
+Defining Members
+----------------
+
+When using the "class syntax", enum classes can define both members and
+other (non-member) attributes. The ``EnumType`` metaclass applies a set
+of rules to distinguish between members and non-members. Type checkers
+should honor the most common of these rules. The lesser-used rules are
+optional. Some of these rules may be impossible to evaluate and enforce
+statically in cases where dynamic values are used.
+
+* If an attribute is defined in the class body with a type annotation but
+  with no assigned value, a type checker should assume this is a non-member
+  attribute::
+
+    class Pet(Enum):
+        genus: str  # Non-member attribute
+        species: str  # Non-member attribute
+
+        CAT = 1  # Member attribute
+        DOG = 2  # Member attribute
+
+  Within a type stub, members can be defined using the actual runtime values,
+  or a placeholder of ``...`` can be used::
+
+    class Pet(Enum):
+        genus: str  # Non-member attribute
+        species: str  # Non-member attribute
+
+        CAT = ...  # Member attribute
+        DOG = ...  # Member attribute
+
+* Members defined within an enum class should not include explicit type
+  annotations. Type checkers should infer a literal type for all members.
+  A type checker should report an error if a type annotation is used
+  for an enum member because this type will be incorrect and misleading
+  to readers of the code::
+
+    class Pet(Enum):
+        CAT = 1  # OK
+        DOG: int = 2  # Type checker error
+
+* Methods, callables, descriptors (including properties), and nested classes
+  that are defined in the class are not treated as enum members by the
+  ``EnumType`` metaclass and should likewise not be treated as enum members by
+  a type checker::
+
+    def identity(x): return x
+
+    class Pet(Enum):
+        CAT = 1  # Member attribute
+        DOG = 2  # Member attribute
+
+        converter = lambda x: str(x)  # Non-member attribute
+        transform = identity  # Non-member attribute
+
+        @property
+        def species(self) -> str:  # Non-member property
+            return "mammal"
+
+        def speak(self) -> None:  # Non-member method
+            print("meow" if self is Pet.CAT else "woof")
+
+        class Nested: ... # Non-member nested class
+
+* An attribute that is assigned the value of another member of the same enum
+  is not a member itself. Instead, it is an alias for the first member::
+
+    class TrafficLight(Enum):
+        RED = 1
+        GREEN = 2
+        YELLOW = 3
+
+        AMBER = YELLOW  # Alias for YELLOW
+
+    reveal_type(TrafficLight.AMBER)  # Revealed type is Literal[TrafficLight.YELLOW]
+
+* If using Python 3.11 or newer, the ``enum.member`` and ``enum.nonmember``
+  classes can be used to unambiguously distinguish members from non-members.
+  Type checkers should support these classes::
+
+    class Example(Enum):
+        a = member(1)  # Member attribute
+        b = nonmember(2)  # Non-member attribute
+
+        @member
+        def c(self) -> None:  # Member method
+            pass
+
+    reveal_type(Example.a)  # Revealed type is Literal[Example.a]
+    reveal_type(Example.b)  # Revealed type is int or Literal[2]
+    reveal_type(Example.c)  # Revealed type is Literal[Example.c]
+
+* An attribute with a private name (beginning with, but not ending in, a double
+  underscore) is treated as a non-member.
+
+    class Example(Enum):
+        A = 1  # Member attribute
+        __B = 2  # Non-member attribute
+
+    reveal_type(Example.A)  # Revealed type is Literal[Example.A]
+    reveal_type(Example.__B)  # Type Error: Private name is mangled
+
+* An enum class can define a class symbol named ``_ignore_``. This can be a list
+  of names or a string containing a space-delimited list of names that are
+  deleted from the enum class at runtime. Type checkers may support this
+  mechanism::
+
+    class Pet(Enum):
+        _ignore_ = "DOG FISH"
+        CAT = 1  # Member attribute
+        DOG = 2  # temporary variable, will be removed from the final enum class
+        FISH = 3  # temporary variable, will be removed from the final enum class
+
+
+Member Names
+------------
+
+All enum member objects have an attribute ``_name_`` that contains the member's
+name. They also have a property ``name`` that returns the same name. Type
+checkers may infer a literal type for the name of a member::
+
+    class Color(Enum):
+        RED = 1
+        GREEN = 2
+        BLUE = 3
+
+    reveal_type(Color.RED._name_)  # Revealed type is Literal["RED"] (or str)
+    reveal_type(Color.RED.name)  # Revealed type is Literal["RED"] (or str)
+
+    def func1(red_or_blue: Literal[Color.RED, Color.BLUE]):
+        reveal_type(red_or_blue.name)  # Revealed type is Literal["RED", "BLUE"] (or str)
+
+    def func2(any_color: Color):
+        reveal_type(any_color.name)  # Revealed type is Literal["RED", "BLUE", "GREEN"] (or str)
+
+
+Member Values
+-------------
+
+All enum member objects have an attribute ``_value_`` that contains the member's
+value. They also have a property ``value`` that returns the same value. Type
+checkers may infer the type of a member's value::
+
+    class Color(Enum):
+        RED = 1
+        GREEN = 2
+        BLUE = 3
+
+    reveal_type(Color.RED._value_)  # Revealed type is Literal[1] (or int or object or Any)
+    reveal_type(Color.RED.value)  # Revealed type is Literal[1] (or int or object or Any)
+
+    def func1(red_or_blue: Literal[Color.RED, Color.BLUE]):
+        reveal_type(red_or_blue.value)  # Revealed type is Literal[1, 2] (or int or object or Any)
+
+    def func2(any_color: Color):
+        reveal_type(any_color.value)  # Revealed type is Literal[1, 2, 3] (or int or object or Any)
+
+
+The value of ``_value_`` can be assigned in a constructor method. This technique
+is sometimes used to initialize both the member value and non-member attributes.
+If the value assigned in the class body is a tuple, the unpacked tuple value is
+passed to the constructor. Type checkers may validate consistency between assigned
+tuple values and the constructor signature::
+
+    class Planet(Enum):
+        def __init__(self, value: int, mass: float, radius: float):
+            self._value_ = value
+            self.mass = mass
+            self.radius = radius
+
+        MERCURY = (1, 3.303e+23, 2.4397e6)
+        VENUS = (2, 4.869e+24, 6.0518e6)
+        EARTH = (3, 5.976e+24, 6.37814e6)
+        MARS = (6.421e+23, 3.3972e6)  # Type checker error (optional)
+        JUPITER = 5  # Type checker error (optional)
+
+    reveal_type(Planet.MERCURY.value)  # Revealed type is Literal[1] (or int or object or Any)
+
+
+The class ``enum.auto`` and method ``_generate_next_value_`` can be used within
+an enum class to automatically generate values for enum members. Type checkers
+may support these to infer literal types for member values::
+
+    class Color(Enum):
+        RED = auto()
+        GREEN = auto()
+        BLUE = auto()
+
+    reveal_type(Color.RED.value)  # Revealed type is Literal[1] (or int or object or Any)
+
+
+If an enum class provides an explicit type annotation for ``_value_``, type
+checkers should enforce this declared type when values are assigned to
+``_value_``::
+
+    class Color(Enum):
+        _value_: int
+        RED = 1 # OK
+        GREEN = "green"  # Type error
+
+    class Planet(Enum):
+        _value_: str
+
+        def __init__(self, value: int, mass: float, radius: float):
+            self._value_ = value # Type error
+
+        MERCURY = (1, 3.303e+23, 2.4397e6)
+
+If the literal values for enum members are not supplied, as they sometimes
+are not within a type stub file, a type checker can use the type of the
+``_value_`` attribute::
+
+    class ColumnType(Enum):
+        _value_: int
+        DORIC = ...
+        IONIC = ...
+        CORINTHIAN = ...
+
+    reveal_type(ColumnType.DORIC.value)  # Revealed type is int (or object or Any)
+
+
+Enum Literal Expansion
+----------------------
+
+From the perspective of the type system, most enum classes are equivalent
+to the union of the literal members within that enum. (This rule
+does not apply to classes that derive from ``enum.Flag`` because these enums
+allow flags to be combined in arbitrary ways.) Because of the equivalency
+between an enum class and the union of literal members within that enum, the
+two types may be used interchangeably. Type checkers may therefore expand
+an enum type (that does not derive from ``enum.Flag``) into a union of
+literal values during type narrowing and exhaustion detection::
+
+    class Color(Enum):
+        RED = 1
+        GREEN = 2
+        BLUE = 3
+
+    def print_color1(c: Color):
+        if c is Color.RED or c is Color.BLUE:
+            print("red or blue")
+        else:
+            reveal_type(c)  # Revealed type is Literal[Color.GREEN]
+
+    def print_color2(c: Color):
+        match c:
+            case Color.RED | Color.BLUE:
+                print("red or blue")
+            case Color.GREEN:
+                print("green")
+            case _:
+                reveal_type(c)  # Revealed type is Never
+
+
+Likewise, a type checker should treat a complete union of all literal members
+as compatible with the enum type::
+
+    class Answer(Enum):
+        Yes = 1
+        No = 2
+
+    def func(val: object) -> Answer:
+        if val is not Answer.Yes and val is not Answer.No:
+            raise ValueError("Invalid value")
+        reveal_type(val)  # Revealed type is Answer (or Literal[Answer.Yes, Answer.No])
+        return val  # OK

docs/spec/index.rst

@@ -24,6 +24,7 @@ Specification for the Python type system
    typeddict
    tuples
    namedtuples
+   enums
    narrowing
    directives
    distributing