bpo-40066: Enum: modify `repr()` and `str()` (GH-22392)

* Enum: streamline repr() and str(); improve docs

- repr() is now ``enum_class.member_name``
- stdlib global enums are ``module_name.member_name``
- str() is now ``member_name``
- add HOW-TO section for ``Enum``
- change main documentation to be an API reference

2 files changed, 1417 insertions, 0 deletions

diff --git a/Doc/howto/enum.rst b/Doc/howto/enum.rst
new file mode 100644
index 0000000..9ece93e
--- /dev/null
+++ b/Doc/howto/enum.rst
@@ -0,0 +1,1416 @@
+==========
+Enum HOWTO
+==========
+
+:Author: Ethan Furman <ethan at stoneleaf dot us>
+
+.. _enum-basic-tutorial:
+
+.. currentmodule:: enum
+
+Basic Enum Tutorial
+-------------------
+
+An :class:`Enum` is a set of symbolic names bound to unique values.  They are
+similar to global variables, but they offer a more useful :func:`repr()`,
+grouping, type-safety, and a few other features.
+
+They are most useful when you have a variable that can take one of a limited
+selection of values.  For example, the days of the week::
+
+    >>> from enum import Enum
+    >>> class Weekday(Enum):
+    ...     MONDAY = 1
+    ...     TUESDAY = 2
+    ...     WEDNESDAY = 3
+    ...     THURSDAY = 4
+    ...     FRIDAY = 5
+    ...     SATURDAY = 6
+    ...     SUNDAY = 7
+
+As you can see, creating an :class:`Enum` is as simple as writing a class that
+inherits from :class:`Enum` itself.
+
+.. note:: Case of Enum Members
+
+    Because Enums are used to represent constants we recommend using
+    UPPER_CASE names for members, and will be using that style in our examples.
+
+Depending on the nature of the enum a member's value may or may not be
+important, but either way that value can be used to get the corresponding
+member::
+
+    >>> Weekday(3)
+    Weekday.WEDNESDAY
+
+As you can see, the ``repr()`` of a member shows the enum name and the
+member name.  The ``str()`` on a member shows only its name::
+
+    >>> print(Weekday.THURSDAY)
+    THURSDAY
+
+The *type* of an enumeration member is the enum it belongs to::
+
+    >>> type(Weekday.MONDAY)
+    <enum 'Weekday'>
+    >>> isinstance(Weekday.FRIDAY, Weekday)
+    True
+
+Enum members have an attribute that contains just their :attr:`name`::
+
+    >>> print(Weekday.TUESDAY.name)
+    TUESDAY
+
+Likewise, they have an attribute for their :attr:`value`::
+
+
+    >>> Weekday.WEDNESDAY.value
+    3
+
+Unlike many languages that treat enumerations solely as name/value pairs,
+Python Enums can have behavior added.  For example, :class:`datetime.date`
+has two methods for returning the weekday: :meth:`weekday` and :meth:`isoweekday`.
+The difference is that one of them counts from 0-6 and the other from 1-7.
+Rather than keep track of that ourselves we can add a method to the :class:`Weekday`
+enum to extract the day from the :class:`date` instance and return the matching
+enum member::
+
+        @classmethod
+        def from_date(cls, date):
+            return cls(date.isoweekday())
+
+The complete :class:`Weekday` enum now looks like this::
+
+    >>> class Weekday(Enum):
+    ...     MONDAY = 1
+    ...     TUESDAY = 2
+    ...     WEDNESDAY = 3
+    ...     THURSDAY = 4
+    ...     FRIDAY = 5
+    ...     SATURDAY = 6
+    ...     SUNDAY = 7
+    ...     #
+    ...     @classmethod
+    ...     def from_date(cls, date):
+    ...         return cls(date.isoweekday())
+
+Now we can find out what today is!  Observe::
+
+    >>> from datetime import date
+    >>> Weekday.from_date(date.today())
+    Weekday.TUESDAY
+
+Of course, if you're reading this on some other day, you'll see that day instead.
+
+This :class:`Weekday` enum is great if our variable only needs one day, but
+what if we need several?  Maybe we're writing a function to plot chores during
+a week, and don't want to use a :class:`list` -- we could use a different type
+of :class:`Enum`::
+
+    >>> from enum import Flag
+    >>> class Weekday(Flag):
+    ...     MONDAY = 1
+    ...     TUESDAY = 2
+    ...     WEDNESDAY = 4
+    ...     THURSDAY = 8
+    ...     FRIDAY = 16
+    ...     SATURDAY = 32
+    ...     SUNDAY = 64
+
+We've changed two things: we're inherited from :class:`Flag`, and the values are
+all powers of 2.
+
+Just like the original :class:`Weekday` enum above, we can have a single selection::
+
+    >>> first_week_day = Weekday.MONDAY
+    >>> first_week_day
+    Weekday.MONDAY
+
+But :class:`Flag` also allows us to combine several members into a single
+variable::
+
+    >>> weekend = Weekday.SATURDAY | Weekday.SUNDAY
+    >>> weekend
+    Weekday.SATURDAY|Weekday.SUNDAY
+
+You can even iterate over a :class:`Flag` variable::
+
+    >>> for day in weekend:
+    ...     print(day)
+    SATURDAY
+    SUNDAY
+
+Okay, let's get some chores set up::
+
+    >>> chores_for_ethan = {
+    ...     'feed the cat': Weekday.MONDAY | Weekday.WEDNESDAY | Weekday.FRIDAY,
+    ...     'do the dishes': Weekday.TUESDAY | Weekday.THURSDAY,
+    ...     'answer SO questions': Weekday.SATURDAY,
+    ...     }
+
+And a function to display the chores for a given day::
+
+    >>> def show_chores(chores, day):
+    ...     for chore, days in chores.items():
+    ...         if day in days:
+    ...             print(chore)
+    >>> show_chores(chores_for_ethan, Weekday.SATURDAY)
+    answer SO questions
+
+In cases where the actual values of the members do not matter, you can save
+yourself some work and use :func:`auto()` for the values::
+
+    >>> from enum import auto
+    >>> class Weekday(Flag):
+    ...     MONDAY = auto()
+    ...     TUESDAY = auto()
+    ...     WEDNESDAY = auto()
+    ...     THURSDAY = auto()
+    ...     FRIDAY = auto()
+    ...     SATURDAY = auto()
+    ...     SUNDAY = auto()
+
+
+.. _enum-advanced-tutorial:
+
+Programmatic access to enumeration members and their attributes
+---------------------------------------------------------------
+
+Sometimes it's useful to access members in enumerations programmatically (i.e.
+situations where ``Color.RED`` won't do because the exact color is not known
+at program-writing time).  ``Enum`` allows such access::
+
+    >>> Color(1)
+    Color.RED
+    >>> Color(3)
+    Color.BLUE
+
+If you want to access enum members by *name*, use item access::
+
+    >>> Color['RED']
+    Color.RED
+    >>> Color['GREEN']
+    Color.GREEN
+
+If you have an enum member and need its :attr:`name` or :attr:`value`::
+
+    >>> member = Color.RED
+    >>> member.name
+    'RED'
+    >>> member.value
+    1
+
+
+Duplicating enum members and values
+-----------------------------------
+
+Having two enum members with the same name is invalid::
+
+    >>> class Shape(Enum):
+    ...     SQUARE = 2
+    ...     SQUARE = 3
+    ...
+    Traceback (most recent call last):
+    ...
+    TypeError: 'SQUARE' already defined as: 2
+
+However, an enum member can have other names associated with it.  Given two
+entries ``A`` and ``B`` with the same value (and ``A`` defined first), ``B``
+is an alias for the member ``A``.  By-value lookup of the value of ``A`` will
+return the member ``A``.  By-name lookup of ``A`` will return the member ``A``.
+By-name lookup of ``B`` will also return the member ``A``::
+
+    >>> class Shape(Enum):
+    ...     SQUARE = 2
+    ...     DIAMOND = 1
+    ...     CIRCLE = 3
+    ...     ALIAS_FOR_SQUARE = 2
+    ...
+    >>> Shape.SQUARE
+    Shape.SQUARE
+    >>> Shape.ALIAS_FOR_SQUARE
+    Shape.SQUARE
+    >>> Shape(2)
+    Shape.SQUARE
+
+.. note::
+
+    Attempting to create a member with the same name as an already
+    defined attribute (another member, a method, etc.) or attempting to create
+    an attribute with the same name as a member is not allowed.
+
+
+Ensuring unique enumeration values
+----------------------------------
+
+By default, enumerations allow multiple names as aliases for the same value.
+When this behavior isn't desired, you can use the :func:`unique` decorator::
+
+    >>> from enum import Enum, unique
+    >>> @unique
+    ... class Mistake(Enum):
+    ...     ONE = 1
+    ...     TWO = 2
+    ...     THREE = 3
+    ...     FOUR = 3
+    ...
+    Traceback (most recent call last):
+    ...
+    ValueError: duplicate values found in <enum 'Mistake'>: FOUR -> THREE
+
+
+Using automatic values
+----------------------
+
+If the exact value is unimportant you can use :class:`auto`::
+
+    >>> from enum import Enum, auto
+    >>> class Color(Enum):
+    ...     RED = auto()
+    ...     BLUE = auto()
+    ...     GREEN = auto()
+    ...
+    >>> [member.value for member in Color]
+    [1, 2, 3]
+
+The values are chosen by :func:`_generate_next_value_`, which can be
+overridden::
+
+    >>> class AutoName(Enum):
+    ...     def _generate_next_value_(name, start, count, last_values):
+    ...         return name
+    ...
+    >>> class Ordinal(AutoName):
+    ...     NORTH = auto()
+    ...     SOUTH = auto()
+    ...     EAST = auto()
+    ...     WEST = auto()
+    ...
+    >>> [member.value for member in Color]
+    ['NORTH', 'SOUTH', 'EAST', 'WEST']
+
+.. note::
+
+    The :meth:`_generate_next_value_` method must be defined before any members.
+
+Iteration
+---------
+
+Iterating over the members of an enum does not provide the aliases::
+
+    >>> list(Shape)
+    [Shape.SQUARE, Shape.DIAMOND, Shape.CIRCLE]
+
+The special attribute ``__members__`` is a read-only ordered mapping of names
+to members.  It includes all names defined in the enumeration, including the
+aliases::
+
+    >>> for name, member in Shape.__members__.items():
+    ...     name, member
+    ...
+    ('SQUARE', Shape.SQUARE)
+    ('DIAMOND', Shape.DIAMOND)
+    ('CIRCLE', Shape.CIRCLE)
+    ('ALIAS_FOR_SQUARE', Shape.SQUARE)
+
+The ``__members__`` attribute can be used for detailed programmatic access to
+the enumeration members.  For example, finding all the aliases::
+
+    >>> [name for name, member in Shape.__members__.items() if member.name != name]
+    ['ALIAS_FOR_SQUARE']
+
+
+Comparisons
+-----------
+
+Enumeration members are compared by identity::
+
+    >>> Color.RED is Color.RED
+    True
+    >>> Color.RED is Color.BLUE
+    False
+    >>> Color.RED is not Color.BLUE
+    True
+
+Ordered comparisons between enumeration values are *not* supported.  Enum
+members are not integers (but see `IntEnum`_ below)::
+
+    >>> Color.RED < Color.BLUE
+    Traceback (most recent call last):
+      File "<stdin>", line 1, in <module>
+    TypeError: '<' not supported between instances of 'Color' and 'Color'
+
+Equality comparisons are defined though::
+
+    >>> Color.BLUE == Color.RED
+    False
+    >>> Color.BLUE != Color.RED
+    True
+    >>> Color.BLUE == Color.BLUE
+    True
+
+Comparisons against non-enumeration values will always compare not equal
+(again, :class:`IntEnum` was explicitly designed to behave differently, see
+below)::
+
+    >>> Color.BLUE == 2
+    False
+
+
+Allowed members and attributes of enumerations
+----------------------------------------------
+
+Most of the examples above use integers for enumeration values.  Using integers is
+short and handy (and provided by default by the `Functional API`_), but not
+strictly enforced.  In the vast majority of use-cases, one doesn't care what
+the actual value of an enumeration is.  But if the value *is* important,
+enumerations can have arbitrary values.
+
+Enumerations are Python classes, and can have methods and special methods as
+usual.  If we have this enumeration::
+
+    >>> class Mood(Enum):
+    ...     FUNKY = 1
+    ...     HAPPY = 3
+    ...
+    ...     def describe(self):
+    ...         # self is the member here
+    ...         return self.name, self.value
+    ...
+    ...     def __str__(self):
+    ...         return 'my custom str! {0}'.format(self.value)
+    ...
+    ...     @classmethod
+    ...     def favorite_mood(cls):
+    ...         # cls here is the enumeration
+    ...         return cls.HAPPY
+    ...
+
+Then::
+
+    >>> Mood.favorite_mood()
+    Mood.HAPPY
+    >>> Mood.HAPPY.describe()
+    ('HAPPY', 3)
+    >>> str(Mood.FUNKY)
+    'my custom str! 1'
+
+The rules for what is allowed are as follows: names that start and end with
+a single underscore are reserved by enum and cannot be used; all other
+attributes defined within an enumeration will become members of this
+enumeration, with the exception of special methods (:meth:`__str__`,
+:meth:`__add__`, etc.), descriptors (methods are also descriptors), and
+variable names listed in :attr:`_ignore_`.
+
+Note:  if your enumeration defines :meth:`__new__` and/or :meth:`__init__` then
+any value(s) given to the enum member will be passed into those methods.
+See `Planet`_ for an example.
+
+
+Restricted Enum subclassing
+---------------------------
+
+A new :class:`Enum` class must have one base enum class, up to one concrete
+data type, and as many :class:`object`-based mixin classes as needed.  The
+order of these base classes is::
+
+    class EnumName([mix-in, ...,] [data-type,] base-enum):
+        pass
+
+Also, subclassing an enumeration is allowed only if the enumeration does not define
+any members.  So this is forbidden::
+
+    >>> class MoreColor(Color):
+    ...     PINK = 17
+    ...
+    Traceback (most recent call last):
+    ...
+    TypeError: MoreColor: cannot extend enumeration 'Color'
+
+But this is allowed::
+
+    >>> class Foo(Enum):
+    ...     def some_behavior(self):
+    ...         pass
+    ...
+    >>> class Bar(Foo):
+    ...     HAPPY = 1
+    ...     SAD = 2
+    ...
+
+Allowing subclassing of enums that define members would lead to a violation of
+some important invariants of types and instances.  On the other hand, it makes
+sense to allow sharing some common behavior between a group of enumerations.
+(See `OrderedEnum`_ for an example.)
+
+
+Pickling
+--------
+
+Enumerations can be pickled and unpickled::
+
+    >>> from test.test_enum import Fruit
+    >>> from pickle import dumps, loads
+    >>> Fruit.TOMATO is loads(dumps(Fruit.TOMATO))
+    True
+
+The usual restrictions for pickling apply: picklable enums must be defined in
+the top level of a module, since unpickling requires them to be importable
+from that module.
+
+.. note::
+
+    With pickle protocol version 4 it is possible to easily pickle enums
+    nested in other classes.
+
+It is possible to modify how enum members are pickled/unpickled by defining
+:meth:`__reduce_ex__` in the enumeration class.
+
+
+Functional API
+--------------
+
+The :class:`Enum` class is callable, providing the following functional API::
+
+    >>> Animal = Enum('Animal', 'ANT BEE CAT DOG')
+    >>> Animal
+    <enum 'Animal'>
+    >>> Animal.ANT
+    Animal.ANT
+    >>> Animal.ANT.value
+    1
+    >>> list(Animal)
+    [Animal.ANT, Animal.BEE, Animal.CAT, Animal.DOG]
+
+The semantics of this API resemble :class:`~collections.namedtuple`. The first
+argument of the call to :class:`Enum` is the name of the enumeration.
+
+The second argument is the *source* of enumeration member names.  It can be a
+whitespace-separated string of names, a sequence of names, a sequence of
+2-tuples with key/value pairs, or a mapping (e.g. dictionary) of names to
+values.  The last two options enable assigning arbitrary values to
+enumerations; the others auto-assign increasing integers starting with 1 (use
+the ``start`` parameter to specify a different starting value).  A
+new class derived from :class:`Enum` is returned.  In other words, the above
+assignment to :class:`Animal` is equivalent to::
+
+    >>> class Animal(Enum):
+    ...     ANT = 1
+    ...     BEE = 2
+    ...     CAT = 3
+    ...     DOG = 4
+    ...
+
+The reason for defaulting to ``1`` as the starting number and not ``0`` is
+that ``0`` is ``False`` in a boolean sense, but by default enum members all
+evaluate to ``True``.
+
+Pickling enums created with the functional API can be tricky as frame stack
+implementation details are used to try and figure out which module the
+enumeration is being created in (e.g. it will fail if you use a utility
+function in separate module, and also may not work on IronPython or Jython).
+The solution is to specify the module name explicitly as follows::
+
+    >>> Animal = Enum('Animal', 'ANT BEE CAT DOG', module=__name__)
+
+.. warning::
+
+    If ``module`` is not supplied, and Enum cannot determine what it is,
+    the new Enum members will not be unpicklable; to keep errors closer to
+    the source, pickling will be disabled.
+
+The new pickle protocol 4 also, in some circumstances, relies on
+:attr:`~definition.__qualname__` being set to the location where pickle will be able
+to find the class.  For example, if the class was made available in class
+SomeData in the global scope::
+
+    >>> Animal = Enum('Animal', 'ANT BEE CAT DOG', qualname='SomeData.Animal')
+
+The complete signature is::
+
+    Enum(
+        value='NewEnumName',
+        names=<...>,
+        *,
+        module='...',
+        qualname='...',
+        type=<mixed-in class>,
+        start=1,
+        )
+
+:value: What the new enum class will record as its name.
+
+:names: The enum members.  This can be a whitespace or comma separated string
+  (values will start at 1 unless otherwise specified)::
+
+    'RED GREEN BLUE' | 'RED,GREEN,BLUE' | 'RED, GREEN, BLUE'
+
+  or an iterator of names::
+
+    ['RED', 'GREEN', 'BLUE']
+
+  or an iterator of (name, value) pairs::
+
+    [('CYAN', 4), ('MAGENTA', 5), ('YELLOW', 6)]
+
+  or a mapping::
+
+    {'CHARTREUSE': 7, 'SEA_GREEN': 11, 'ROSEMARY': 42}
+
+:module: name of module where new enum class can be found.
+
+:qualname: where in module new enum class can be found.
+
+:type: type to mix in to new enum class.
+
+:start: number to start counting at if only names are passed in.
+
+.. versionchanged:: 3.5
+   The *start* parameter was added.
+
+
+Derived Enumerations
+--------------------
+
+IntEnum
+^^^^^^^
+
+The first variation of :class:`Enum` that is provided is also a subclass of
+:class:`int`.  Members of an :class:`IntEnum` can be compared to integers;
+by extension, integer enumerations of different types can also be compared
+to each other::
+
+    >>> from enum import IntEnum
+    >>> class Shape(IntEnum):
+    ...     CIRCLE = 1
+    ...     SQUARE = 2
+    ...
+    >>> class Request(IntEnum):
+    ...     POST = 1
+    ...     GET = 2
+    ...
+    >>> Shape == 1
+    False
+    >>> Shape.CIRCLE == 1
+    True
+    >>> Shape.CIRCLE == Request.POST
+    True
+
+However, they still can't be compared to standard :class:`Enum` enumerations::
+
+    >>> class Shape(IntEnum):
+    ...     CIRCLE = 1
+    ...     SQUARE = 2
+    ...
+    >>> class Color(Enum):
+    ...     RED = 1
+    ...     GREEN = 2
+    ...
+    >>> Shape.CIRCLE == Color.RED
+    False
+
+:class:`IntEnum` values behave like integers in other ways you'd expect::
+
+    >>> int(Shape.CIRCLE)
+    1
+    >>> ['a', 'b', 'c'][Shape.CIRCLE]
+    'b'
+    >>> [i for i in range(Shape.SQUARE)]
+    [0, 1]
+
+
+StrEnum
+^^^^^^^
+
+The second variation of :class:`Enum` that is provided is also a subclass of
+:class:`str`.  Members of a :class:`StrEnum` can be compared to strings;
+by extension, string enumerations of different types can also be compared
+to each other.  :class:`StrEnum` exists to help avoid the problem of getting
+an incorrect member::
+
+    >>> from enum import StrEnum
+    >>> class Directions(StrEnum):
+    ...     NORTH = 'north',    # notice the trailing comma
+    ...     SOUTH = 'south'
+
+Before :class:`StrEnum`, ``Directions.NORTH`` would have been the :class:`tuple`
+``('north',)``.
+
+.. versionadded:: 3.10
+
+
+IntFlag
+^^^^^^^
+
+The next variation of :class:`Enum` provided, :class:`IntFlag`, is also based
+on :class:`int`.  The difference being :class:`IntFlag` members can be combined
+using the bitwise operators (&, \|, ^, ~) and the result is still an
+:class:`IntFlag` member, if possible.  However, as the name implies, :class:`IntFlag`
+members also subclass :class:`int` and can be used wherever an :class:`int` is
+used.
+
+.. note::
+
+    Any operation on an :class:`IntFlag` member besides the bit-wise operations will
+    lose the :class:`IntFlag` membership.
+
+    Bit-wise operations that result in invalid :class:`IntFlag` values will lose the
+    :class:`IntFlag` membership.  See :class:`FlagBoundary` for
+    details.
+
+.. versionadded:: 3.6
+.. versionchanged:: 3.10
+
+Sample :class:`IntFlag` class::
+
+    >>> from enum import IntFlag
+    >>> class Perm(IntFlag):
+    ...     R = 4
+    ...     W = 2
+    ...     X = 1
+    ...
+    >>> Perm.R | Perm.W
+    Perm.R|Perm.W
+    >>> Perm.R + Perm.W
+    6
+    >>> RW = Perm.R | Perm.W
+    >>> Perm.R in RW
+    True
+
+It is also possible to name the combinations::
+
+    >>> class Perm(IntFlag):
+    ...     R = 4
+    ...     W = 2
+    ...     X = 1
+    ...     RWX = 7
+    >>> Perm.RWX
+    Perm.RWX
+    >>> ~Perm.RWX
+    Perm(0)
+    >>> Perm(7)
+    Perm.RWX
+
+.. note::
+
+    Named combinations are considered aliases.  Aliases do not show up during
+    iteration, but can be returned from by-value lookups.
+
+.. versionchanged:: 3.10
+
+Another important difference between :class:`IntFlag` and :class:`Enum` is that
+if no flags are set (the value is 0), its boolean evaluation is :data:`False`::
+
+    >>> Perm.R & Perm.X
+    Perm(0)
+    >>> bool(Perm.R & Perm.X)
+    False
+
+Because :class:`IntFlag` members are also subclasses of :class:`int` they can
+be combined with them (but may lose :class:`IntFlag` membership::
+
+    >>> Perm.X | 4
+    Perm.R|Perm.X
+
+    >>> Perm.X | 8
+    9
+
+.. note::
+
+    The negation operator, ``~``, always returns an :class:`IntFlag` member with a
+    positive value::
+
+        >>> (~Perm.X).value == (Perm.R|Perm.W).value == 6
+        True
+
+:class:`IntFlag` members can also be iterated over::
+
+    >>> list(RW)
+    [Perm.R, Perm.W]
+
+.. versionadded:: 3.10
+
+
+Flag
+^^^^
+
+The last variation is :class:`Flag`.  Like :class:`IntFlag`, :class:`Flag`
+members can be combined using the bitwise operators (&, \|, ^, ~).  Unlike
+:class:`IntFlag`, they cannot be combined with, nor compared against, any
+other :class:`Flag` enumeration, nor :class:`int`.  While it is possible to
+specify the values directly it is recommended to use :class:`auto` as the
+value and let :class:`Flag` select an appropriate value.
+
+.. versionadded:: 3.6
+
+Like :class:`IntFlag`, if a combination of :class:`Flag` members results in no
+flags being set, the boolean evaluation is :data:`False`::
+
+    >>> from enum import Flag, auto
+    >>> class Color(Flag):
+    ...     RED = auto()
+    ...     BLUE = auto()
+    ...     GREEN = auto()
+    ...
+    >>> Color.RED & Color.GREEN
+    Color(0)
+    >>> bool(Color.RED & Color.GREEN)
+    False
+
+Individual flags should have values that are powers of two (1, 2, 4, 8, ...),
+while combinations of flags won't::
+
+    >>> class Color(Flag):
+    ...     RED = auto()
+    ...     BLUE = auto()
+    ...     GREEN = auto()
+    ...     WHITE = RED | BLUE | GREEN
+    ...
+    >>> Color.WHITE
+    Color.WHITE
+
+Giving a name to the "no flags set" condition does not change its boolean
+value::
+
+    >>> class Color(Flag):
+    ...     BLACK = 0
+    ...     RED = auto()
+    ...     BLUE = auto()
+    ...     GREEN = auto()
+    ...
+    >>> Color.BLACK
+    Color.BLACK
+    >>> bool(Color.BLACK)
+    False
+
+:class:`Flag` members can also be iterated over::
+
+    >>> purple = Color.RED | Color.BLUE
+    >>> list(purple)
+    [Color.RED, Color.BLUE]
+
+.. versionadded:: 3.10
+
+.. note::
+
+    For the majority of new code, :class:`Enum` and :class:`Flag` are strongly
+    recommended, since :class:`IntEnum` and :class:`IntFlag` break some
+    semantic promises of an enumeration (by being comparable to integers, and
+    thus by transitivity to other unrelated enumerations).  :class:`IntEnum`
+    and :class:`IntFlag` should be used only in cases where :class:`Enum` and
+    :class:`Flag` will not do; for example, when integer constants are replaced
+    with enumerations, or for interoperability with other systems.
+
+
+Others
+^^^^^^
+
+While :class:`IntEnum` is part of the :mod:`enum` module, it would be very
+simple to implement independently::
+
+    class IntEnum(int, Enum):
+        pass
+
+This demonstrates how similar derived enumerations can be defined; for example
+a :class:`StrEnum` that mixes in :class:`str` instead of :class:`int`.
+
+Some rules:
+
+1. When subclassing :class:`Enum`, mix-in types must appear before
+   :class:`Enum` itself in the sequence of bases, as in the :class:`IntEnum`
+   example above.
+2. While :class:`Enum` can have members of any type, once you mix in an
+   additional type, all the members must have values of that type, e.g.
+   :class:`int` above.  This restriction does not apply to mix-ins which only
+   add methods and don't specify another type.
+3. When another data type is mixed in, the :attr:`value` attribute is *not the
+   same* as the enum member itself, although it is equivalent and will compare
+   equal.
+4. %-style formatting:  `%s` and `%r` call the :class:`Enum` class's
+   :meth:`__str__` and :meth:`__repr__` respectively; other codes (such as
+   `%i` or `%h` for IntEnum) treat the enum member as its mixed-in type.
+5. :ref:`Formatted string literals <f-strings>`, :meth:`str.format`,
+   and :func:`format` will use the mixed-in type's :meth:`__format__`
+   unless :meth:`__str__` or :meth:`__format__` is overridden in the subclass,
+   in which case the overridden methods or :class:`Enum` methods will be used.
+   Use the !s and !r format codes to force usage of the :class:`Enum` class's
+   :meth:`__str__` and :meth:`__repr__` methods.
+
+When to use :meth:`__new__` vs. :meth:`__init__`
+------------------------------------------------
+
+:meth:`__new__` must be used whenever you want to customize the actual value of
+the :class:`Enum` member.  Any other modifications may go in either
+:meth:`__new__` or :meth:`__init__`, with :meth:`__init__` being preferred.
+
+For example, if you want to pass several items to the constructor, but only
+want one of them to be the value::
+
+    >>> class Coordinate(bytes, Enum):
+    ...     """
+    ...     Coordinate with binary codes that can be indexed by the int code.
+    ...     """
+    ...     def __new__(cls, value, label, unit):
+    ...         obj = bytes.__new__(cls, [value])
+    ...         obj._value_ = value
+    ...         obj.label = label
+    ...         obj.unit = unit
+    ...         return obj
+    ...     PX = (0, 'P.X', 'km')
+    ...     PY = (1, 'P.Y', 'km')
+    ...     VX = (2, 'V.X', 'km/s')
+    ...     VY = (3, 'V.Y', 'km/s')
+    ...
+
+    >>> print(Coordinate['PY'])
+    PY
+
+    >>> print(Coordinate(3))
+    VY
+
+
+Finer Points
+^^^^^^^^^^^^
+
+Supported ``__dunder__`` names
+""""""""""""""""""""""""""""""
+
+:attr:`__members__` is a read-only ordered mapping of ``member_name``:``member``
+items.  It is only available on the class.
+
+:meth:`__new__`, if specified, must create and return the enum members; it is
+also a very good idea to set the member's :attr:`_value_` appropriately.  Once
+all the members are created it is no longer used.
+
+
+Supported ``_sunder_`` names
+""""""""""""""""""""""""""""
+
+- ``_name_`` -- name of the member
+- ``_value_`` -- value of the member; can be set / modified in ``__new__``
+
+- ``_missing_`` -- a lookup function used when a value is not found; may be
+  overridden
+- ``_ignore_`` -- a list of names, either as a :class:`list` or a :class:`str`,
+  that will not be transformed into members, and will be removed from the final
+  class
+- ``_order_`` -- used in Python 2/3 code to ensure member order is consistent
+  (class attribute, removed during class creation)
+- ``_generate_next_value_`` -- used by the `Functional API`_ and by
+  :class:`auto` to get an appropriate value for an enum member; may be
+  overridden
+
+.. note::
+
+    For standard :class:`Enum` classes the next value chosen is the last value seen
+    incremented by one.
+
+    For :class:`Flag` classes the next value chosen will be the next highest
+    power-of-two, regardless of the last value seen.
+
+.. versionadded:: 3.6 ``_missing_``, ``_order_``, ``_generate_next_value_``
+.. versionadded:: 3.7 ``_ignore_``
+
+To help keep Python 2 / Python 3 code in sync an :attr:`_order_` attribute can
+be provided.  It will be checked against the actual order of the enumeration
+and raise an error if the two do not match::
+
+    >>> class Color(Enum):
+    ...     _order_ = 'RED GREEN BLUE'
+    ...     RED = 1
+    ...     BLUE = 3
+    ...     GREEN = 2
+    ...
+    Traceback (most recent call last):
+    ...
+    TypeError: member order does not match _order_:
+    ['RED', 'BLUE', 'GREEN']
+    ['RED', 'GREEN', 'BLUE']
+
+.. note::
+
+    In Python 2 code the :attr:`_order_` attribute is necessary as definition
+    order is lost before it can be recorded.
+
+
+_Private__names
+"""""""""""""""
+
+Private names are not converted to enum members, but remain normal attributes.
+
+.. versionchanged:: 3.10
+
+
+``Enum`` member type
+""""""""""""""""""""
+
+Enum members are instances of their enum class, and are normally accessed as
+``EnumClass.member``.  In Python versions ``3.5`` to ``3.9`` you could access
+members from other members -- this practice was discouraged, and in ``3.12``
+:class:`Enum` will return to not allowing it, while in ``3.10`` and ``3.11``
+it will raise a :exc:`DeprecationWarning`::
+
+    >>> class FieldTypes(Enum):
+    ...     name = 0
+    ...     value = 1
+    ...     size = 2
+    ...
+    >>> FieldTypes.value.size       # doctest: +SKIP
+    DeprecationWarning: accessing one member from another is not supported,
+      and will be disabled in 3.12
+    <FieldTypes.size: 2>
+
+.. versionchanged:: 3.5
+.. versionchanged:: 3.10
+
+
+Creating members that are mixed with other data types
+"""""""""""""""""""""""""""""""""""""""""""""""""""""
+
+When subclassing other data types, such as :class:`int` or :class:`str`, with
+an :class:`Enum`, all values after the `=` are passed to that data type's
+constructor.  For example::
+
+    >>> class MyEnum(IntEnum):
+    ...     example = '11', 16      # '11' will be interpreted as a hexadecimal
+    ...                             # number
+    >>> MyEnum.example.value
+    17
+
+
+Boolean value of ``Enum`` classes and members
+"""""""""""""""""""""""""""""""""""""""""""""
+
+Enum classes that are mixed with non-:class:`Enum` types (such as
+:class:`int`, :class:`str`, etc.) are evaluated according to the mixed-in
+type's rules; otherwise, all members evaluate as :data:`True`.  To make your
+own enum's boolean evaluation depend on the member's value add the following to
+your class::
+
+    def __bool__(self):
+        return bool(self.value)
+
+Plain :class:`Enum` classes always evaluate as :data:`True`.
+
+
+``Enum`` classes with methods
+"""""""""""""""""""""""""""""
+
+If you give your enum subclass extra methods, like the `Planet`_
+class above, those methods will show up in a :func:`dir` of the member,
+but not of the class::
+
+    >>> dir(Planet)
+    ['EARTH', 'JUPITER', 'MARS', 'MERCURY', 'NEPTUNE', 'SATURN', 'URANUS', 'VENUS', '__class__', '__doc__', '__members__', '__module__']
+    >>> dir(Planet.EARTH)
+    ['__class__', '__doc__', '__module__', 'mass', 'name', 'radius', 'surface_gravity', 'value']
+
+
+Combining members of ``Flag``
+"""""""""""""""""""""""""""""
+
+Iterating over a combination of :class:`Flag` members will only return the members that
+are comprised of a single bit::
+
+    >>> class Color(Flag):
+    ...     RED = auto()
+    ...     GREEN = auto()
+    ...     BLUE = auto()
+    ...     MAGENTA = RED | BLUE
+    ...     YELLOW = RED | GREEN
+    ...     CYAN = GREEN | BLUE
+    ...
+    >>> Color(3)  # named combination
+    Color.YELLOW
+    >>> Color(7)      # not named combination
+    Color.RED|Color.GREEN|Color.BLUE
+
+``StrEnum`` and :meth:`str.__str__`
+"""""""""""""""""""""""""""""""""""
+
+An important difference between :class:`StrEnum` and other Enums is the
+:meth:`__str__` method; because :class:`StrEnum` members are strings, some
+parts of Python will read the string data directly, while others will call
+:meth:`str()`. To make those two operations have the same result,
+:meth:`StrEnum.__str__` will be the same as :meth:`str.__str__` so that
+``str(StrEnum.member) == StrEnum.member`` is true.
+
+``Flag`` and ``IntFlag`` minutia
+""""""""""""""""""""""""""""""""
+
+Using the following snippet for our examples::
+
+    >>> class Color(IntFlag):
+    ...     BLACK = 0
+    ...     RED = 1
+    ...     GREEN = 2
+    ...     BLUE = 4
+    ...     PURPLE = RED | BLUE
+    ...     WHITE = RED | GREEN | BLUE
+    ...
+
+the following are true:
+
+- single-bit flags are canonical
+- multi-bit and zero-bit flags are aliases
+- only canonical flags are returned during iteration::
+
+    >>> list(Color.WHITE)
+    [Color.RED, Color.GREEN, Color.BLUE]
+
+- negating a flag or flag set returns a new flag/flag set with the
+  corresponding positive integer value::
+
+    >>> Color.BLUE
+    Color.BLUE
+
+    >>> ~Color.BLUE
+    Color.RED|Color.GREEN
+
+- names of pseudo-flags are constructed from their members' names::
+
+    >>> (Color.RED | Color.GREEN).name
+    'RED|GREEN'
+
+- multi-bit flags, aka aliases, can be returned from operations::
+
+    >>> Color.RED | Color.BLUE
+    Color.PURPLE
+
+    >>> Color(7)  # or Color(-1)
+    Color.WHITE
+
+    >>> Color(0)
+    Color.BLACK
+
+- membership / containment checking has changed slightly -- zero valued flags
+  are never considered to be contained::
+
+    >>> Color.BLACK in Color.WHITE
+    False
+
+  otherwise, if all bits of one flag are in the other flag, True is returned::
+
+    >>> Color.PURPLE in Color.WHITE
+    True
+
+There is a new boundary mechanism that controls how out-of-range / invalid
+bits are handled: ``STRICT``, ``CONFORM``, ``EJECT``, and ``KEEP``:
+
+  * STRICT --> raises an exception when presented with invalid values
+  * CONFORM --> discards any invalid bits
+  * EJECT --> lose Flag status and become a normal int with the given value
+  * KEEP --> keep the extra bits
+           - keeps Flag status and extra bits
+           - extra bits do not show up in iteration
+           - extra bits do show up in repr() and str()
+
+The default for Flag is ``STRICT``, the default for ``IntFlag`` is ``EJECT``,
+and the default for ``_convert_`` is ``KEEP`` (see ``ssl.Options`` for an
+example of when ``KEEP`` is needed).
+
+
+.. _enum-class-differences:
+
+How are Enums different?
+------------------------
+
+Enums have a custom metaclass that affects many aspects of both derived :class:`Enum`
+classes and their instances (members).
+
+
+Enum Classes
+^^^^^^^^^^^^
+
+The :class:`EnumType` metaclass is responsible for providing the
+:meth:`__contains__`, :meth:`__dir__`, :meth:`__iter__` and other methods that
+allow one to do things with an :class:`Enum` class that fail on a typical
+class, such as `list(Color)` or `some_enum_var in Color`.  :class:`EnumType` is
+responsible for ensuring that various other methods on the final :class:`Enum`
+class are correct (such as :meth:`__new__`, :meth:`__getnewargs__`,
+:meth:`__str__` and :meth:`__repr__`).
+
+
+Enum Members (aka instances)
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+The most interesting thing about enum members is that they are singletons.
+:class:`EnumType` creates them all while it is creating the enum class itself,
+and then puts a custom :meth:`__new__` in place to ensure that no new ones are
+ever instantiated by returning only the existing member instances.
+
+
+.. _enum-cookbook:
+
+
+While :class:`Enum`, :class:`IntEnum`, :class:`StrEnum`, :class:`Flag`, and
+:class:`IntFlag` are expected to cover the majority of use-cases, they cannot
+cover them all.  Here are recipes for some different types of enumerations
+that can be used directly, or as examples for creating one's own.
+
+
+Omitting values
+^^^^^^^^^^^^^^^
+
+In many use-cases one doesn't care what the actual value of an enumeration
+is. There are several ways to define this type of simple enumeration:
+
+- use instances of :class:`auto` for the value
+- use instances of :class:`object` as the value
+- use a descriptive string as the value
+- use a tuple as the value and a custom :meth:`__new__` to replace the
+  tuple with an :class:`int` value
+
+Using any of these methods signifies to the user that these values are not
+important, and also enables one to add, remove, or reorder members without
+having to renumber the remaining members.
+
+
+Using :class:`auto`
+"""""""""""""""""""
+
+Using :class:`auto` would look like::
+
+    >>> class Color(Enum):
+    ...     RED = auto()
+    ...     BLUE = auto()
+    ...     GREEN = auto()
+    ...
+    >>> Color.GREEN
+    <Color.GREEN>
+
+
+Using :class:`object`
+"""""""""""""""""""""
+
+Using :class:`object` would look like::
+
+    >>> class Color(Enum):
+    ...     RED = object()
+    ...     GREEN = object()
+    ...     BLUE = object()
+    ...
+    >>> Color.GREEN
+    <Color.GREEN>
+
+
+Using a descriptive string
+""""""""""""""""""""""""""
+
+Using a string as the value would look like::
+
+    >>> class Color(Enum):
+    ...     RED = 'stop'
+    ...     GREEN = 'go'
+    ...     BLUE = 'too fast!'
+    ...
+    >>> Color.GREEN
+    <Color.GREEN>
+    >>> Color.GREEN.value
+    'go'
+
+
+Using a custom :meth:`__new__`
+""""""""""""""""""""""""""""""
+
+Using an auto-numbering :meth:`__new__` would look like::
+
+    >>> class AutoNumber(Enum):
+    ...     def __new__(cls):
+    ...         value = len(cls.__members__) + 1
+    ...         obj = object.__new__(cls)
+    ...         obj._value_ = value
+    ...         return obj
+    ...
+    >>> class Color(AutoNumber):
+    ...     RED = ()
+    ...     GREEN = ()
+    ...     BLUE = ()
+    ...
+    >>> Color.GREEN
+    <Color.GREEN>
+    >>> Color.GREEN.value
+    2
+
+To make a more general purpose ``AutoNumber``, add ``*args`` to the signature::
+
+    >>> class AutoNumber(Enum):
+    ...     def __new__(cls, *args):      # this is the only change from above
+    ...         value = len(cls.__members__) + 1
+    ...         obj = object.__new__(cls)
+    ...         obj._value_ = value
+    ...         return obj
+    ...
+
+Then when you inherit from ``AutoNumber`` you can write your own ``__init__``
+to handle any extra arguments::
+
+    >>> class Swatch(AutoNumber):
+    ...     def __init__(self, pantone='unknown'):
+    ...         self.pantone = pantone
+    ...     AUBURN = '3497'
+    ...     SEA_GREEN = '1246'
+    ...     BLEACHED_CORAL = () # New color, no Pantone code yet!
+    ...
+    >>> Swatch.SEA_GREEN
+    <Swatch.SEA_GREEN>
+    >>> Swatch.SEA_GREEN.pantone
+    '1246'
+    >>> Swatch.BLEACHED_CORAL.pantone
+    'unknown'
+
+.. note::
+
+    The :meth:`__new__` method, if defined, is used during creation of the Enum
+    members; it is then replaced by Enum's :meth:`__new__` which is used after
+    class creation for lookup of existing members.
+
+
+OrderedEnum
+^^^^^^^^^^^
+
+An ordered enumeration that is not based on :class:`IntEnum` and so maintains
+the normal :class:`Enum` invariants (such as not being comparable to other
+enumerations)::
+
+    >>> class OrderedEnum(Enum):
+    ...     def __ge__(self, other):
+    ...         if self.__class__ is other.__class__:
+    ...             return self.value >= other.value
+    ...         return NotImplemented
+    ...     def __gt__(self, other):
+    ...         if self.__class__ is other.__class__:
+    ...             return self.value > other.value
+    ...         return NotImplemented
+    ...     def __le__(self, other):
+    ...         if self.__class__ is other.__class__:
+    ...             return self.value <= other.value
+    ...         return NotImplemented
+    ...     def __lt__(self, other):
+    ...         if self.__class__ is other.__class__:
+    ...             return self.value < other.value
+    ...         return NotImplemented
+    ...
+    >>> class Grade(OrderedEnum):
+    ...     A = 5
+    ...     B = 4
+    ...     C = 3
+    ...     D = 2
+    ...     F = 1
+    ...
+    >>> Grade.C < Grade.A
+    True
+
+
+DuplicateFreeEnum
+^^^^^^^^^^^^^^^^^
+
+Raises an error if a duplicate member name is found instead of creating an
+alias::
+
+    >>> class DuplicateFreeEnum(Enum):
+    ...     def __init__(self, *args):
+    ...         cls = self.__class__
+    ...         if any(self.value == e.value for e in cls):
+    ...             a = self.name
+    ...             e = cls(self.value).name
+    ...             raise ValueError(
+    ...                 "aliases not allowed in DuplicateFreeEnum:  %r --> %r"
+    ...                 % (a, e))
+    ...
+    >>> class Color(DuplicateFreeEnum):
+    ...     RED = 1
+    ...     GREEN = 2
+    ...     BLUE = 3
+    ...     GRENE = 2
+    ...
+    Traceback (most recent call last):
+    ...
+    ValueError: aliases not allowed in DuplicateFreeEnum:  'GRENE' --> 'GREEN'
+
+.. note::
+
+    This is a useful example for subclassing Enum to add or change other
+    behaviors as well as disallowing aliases.  If the only desired change is
+    disallowing aliases, the :func:`unique` decorator can be used instead.
+
+
+Planet
+^^^^^^
+
+If :meth:`__new__` or :meth:`__init__` is defined the value of the enum member
+will be passed to those methods::
+
+    >>> class Planet(Enum):
+    ...     MERCURY = (3.303e+23, 2.4397e6)
+    ...     VENUS   = (4.869e+24, 6.0518e6)
+    ...     EARTH   = (5.976e+24, 6.37814e6)
+    ...     MARS    = (6.421e+23, 3.3972e6)
+    ...     JUPITER = (1.9e+27,   7.1492e7)
+    ...     SATURN  = (5.688e+26, 6.0268e7)
+    ...     URANUS  = (8.686e+25, 2.5559e7)
+    ...     NEPTUNE = (1.024e+26, 2.4746e7)
+    ...     def __init__(self, mass, radius):
+    ...         self.mass = mass       # in kilograms
+    ...         self.radius = radius   # in meters
+    ...     @property
+    ...     def surface_gravity(self):
+    ...         # universal gravitational constant  (m3 kg-1 s-2)
+    ...         G = 6.67300E-11
+    ...         return G * self.mass / (self.radius * self.radius)
+    ...
+    >>> Planet.EARTH.value
+    (5.976e+24, 6378140.0)
+    >>> Planet.EARTH.surface_gravity
+    9.802652743337129
+
+.. _enum-time-period:
+
+TimePeriod
+^^^^^^^^^^
+
+An example to show the :attr:`_ignore_` attribute in use::
+
+    >>> from datetime import timedelta
+    >>> class Period(timedelta, Enum):
+    ...     "different lengths of time"
+    ...     _ignore_ = 'Period i'
+    ...     Period = vars()
+    ...     for i in range(367):
+    ...         Period['day_%d' % i] = i
+    ...
+    >>> list(Period)[:2]
+    [Period.day_0, Period.day_1]
+    >>> list(Period)[-2:]
+    [Period.day_365, Period.day_366]
+
+
+Conforming input to Flag
+^^^^^^^^^^^^^^^^^^^^^^^^
+
+Creating a :class:`Flag` enum that is more resilient out-of-bounds results to
+mathematical operations, you can use the :attr:`FlagBoundary.CONFORM` setting::
+
+    >>> from enum import Flag, CONFORM, auto
+    >>> class Weekday(Flag, boundary=CONFORM):
+    ...     MONDAY = auto()
+    ...     TUESDAY = auto()
+    ...     WEDNESDAY = auto()
+    ...     THURSDAY = auto()
+    ...     FRIDAY = auto()
+    ...     SATURDAY = auto()
+    ...     SUNDAY = auto()
+    >>> today = Weekday.TUESDAY
+    >>> Weekday(today + 22)  # what day is three weeks from tomorrow?
+    >>> Weekday.WEDNESDAY
+
+
+.. _enumtype-examples:
+
+Subclassing EnumType
+--------------------
+
+While most enum needs can be met by customizing :class:`Enum` subclasses,
+either with class decorators or custom functions, :class:`EnumType` can be
+subclassed to provide a different Enum experience.
+
diff --git a/Doc/howto/index.rst b/Doc/howto/index.rst
index 593341c..e0dacd2 100644
--- a/Doc/howto/index.rst
+++ b/Doc/howto/index.rst
@@ -17,6 +17,7 @@ Currently, the HOWTOs are:
    cporting.rst
    curses.rst
    descriptor.rst
+   enum.rst
    functional.rst
    logging.rst
    logging-cookbook.rst