Enum HOWTO
**********

An "Enum" is a set of symbolic names bound to unique values.  They are
similar to global variables, but they offer a more useful "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

Or perhaps the RGB primary colors:

   >>> from enum import Enum
   >>> class Color(Enum):
   ...     RED = 1
   ...     GREEN = 2
   ...     BLUE = 3

As you can see, creating an "Enum" is as simple as writing a class
that inherits from "Enum" itself.

備註:

  Case of Enum MembersBecause Enums are used to represent constants,
  and to help avoid issues with name clashes between mixin-class
  methods/attributes and enum names, we strongly 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: 3>

As you can see, the "repr()" of a member shows the enum name, the
member name, and the value.  The "str()" of a member shows only the
enum name and member name:

   >>> print(Weekday.THURSDAY)
   Weekday.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 "name":

   >>> print(Weekday.TUESDAY.name)
   TUESDAY

Likewise, they have an attribute for their "value":

   >>> Weekday.WEDNESDAY.value
   3

Unlike many languages that treat enumerations solely as name/value
pairs, Python Enums can have behavior added.  For example,
"datetime.date" has two methods for returning the weekday: "weekday()"
and "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 "Weekday" enum to extract the day from the
"date" instance and return the matching enum member:

   @classmethod
   def from_date(cls, date):
       return cls(date.isoweekday())

The complete "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: 2>

Of course, if you're reading this on some other day, you'll see that
day instead.

This "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 "list" -- we could use a
different type of "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 "Flag", and the values
are all powers of 2.

Just like the original "Weekday" enum above, we can have a single
selection:

   >>> first_week_day = Weekday.MONDAY
   >>> first_week_day
   <Weekday.MONDAY: 1>

But "Flag" also allows us to combine several members into a single
variable:

   >>> weekend = Weekday.SATURDAY | Weekday.SUNDAY
   >>> weekend
   <Weekday.SATURDAY|SUNDAY: 96>

You can even iterate over a "Flag" variable:

   >>> for day in weekend:
   ...     print(day)
   Weekday.SATURDAY
   Weekday.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 "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()
   ...     WEEKEND = SATURDAY | SUNDAY


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: 1>
   >>> Color(3)
   <Color.BLUE: 3>

If you want to access enum members by *name*, use item access:

   >>> Color['RED']
   <Color.RED: 1>
   >>> Color['GREEN']
   <Color.GREEN: 2>

If you have an enum member and need its "name" or "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: 2>
   >>> Shape.ALIAS_FOR_SQUARE
   <Shape.SQUARE: 2>
   >>> Shape(2)
   <Shape.SQUARE: 2>

備註:

  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 "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 "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 "_generate_next_value_()", which can be
overridden:

   >>> class AutoName(Enum):
   ...     @staticmethod
   ...     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 Ordinal]
   ['NORTH', 'SOUTH', 'EAST', 'WEST']

備註:

  The "_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: 2>, <Shape.DIAMOND: 1>, <Shape.CIRCLE: 3>]
   >>> list(Weekday)
   [<Weekday.MONDAY: 1>, <Weekday.TUESDAY: 2>, <Weekday.WEDNESDAY: 4>, <Weekday.THURSDAY: 8>, <Weekday.FRIDAY: 16>, <Weekday.SATURDAY: 32>, <Weekday.SUNDAY: 64>]

Note that the aliases "Shape.ALIAS_FOR_SQUARE" and "Weekday.WEEKEND"
aren't shown.

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: 2>)
   ('DIAMOND', <Shape.DIAMOND: 1>)
   ('CIRCLE', <Shape.CIRCLE: 3>)
   ('ALIAS_FOR_SQUARE', <Shape.SQUARE: 2>)

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']

備註:

  Aliases for flags include values with multiple flags set, such as
  "3", and no flags set, i.e. "0".


比較
====

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, "IntEnum" was explicitly designed to behave differently,
see below):

   >>> Color.BLUE == 2
   False

警告:

  It is possible to reload modules -- if a reloaded module contains
  enums, they will be recreated, and the new members may not compare
  identical/equal to the original members.


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: 3>
   >>> 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 ("__str__()",
"__add__()", etc.), descriptors (methods are also descriptors), and
variable names listed in "_ignore_".

Note:  if your enumeration defines "__new__()" and/or "__init__()",
any value(s) given to the enum member will be passed into those
methods. See Planet for an example.

備註:

  The "__new__()" method, if defined, is used during creation of the
  Enum members; it is then replaced by Enum's "__new__()" which is
  used after class creation for lookup of existing members.  See When
  to use __new__() vs. __init__() for more details.


Restricted Enum subclassing
===========================

A new "Enum" class must have one base enum class, up to one concrete
data type, and as many "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: <enum 'MoreColor'> cannot extend <enum 'Color'>

但這是允許的：

   >>> 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.)


Dataclass support
=================

When inheriting from a "dataclass", the "__repr__()" omits the
inherited class' name.  For example:

   >>> from dataclasses import dataclass, field
   >>> @dataclass
   ... class CreatureDataMixin:
   ...     size: str
   ...     legs: int
   ...     tail: bool = field(repr=False, default=True)
   ...
   >>> class Creature(CreatureDataMixin, Enum):
   ...     BEETLE = 'small', 6
   ...     DOG = 'medium', 4
   ...
   >>> Creature.DOG
   <Creature.DOG: size='medium', legs=4>

Use the "dataclass()" argument "repr=False" to use the standard
"repr()".

在 3.12 版的變更: Only the dataclass fields are shown in the value
area, not the dataclass' name.

備註:

  Adding "dataclass()" decorator to "Enum" and its subclasses is not
  supported. It will not raise any errors, but it will produce very
  strange results at runtime, such as members being equal to each
  other:

     >>> @dataclass               # don't do this: it does not make any sense
     ... class Color(Enum):
     ...    RED = 1
     ...    BLUE = 2
     ...
     >>> Color.RED is Color.BLUE
     False
     >>> Color.RED == Color.BLUE  # problem is here: they should not be equal
     True


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.

備註:

  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 "__reduce_ex__()" in the enumeration class.  The default
method is by-value, but enums with complicated values may want to use
by-name:

   >>> import enum
   >>> class MyEnum(enum.Enum):
   ...     __reduce_ex__ = enum.pickle_by_enum_name

備註:

  Using by-name for flags is not recommended, as unnamed aliases will
  not unpickle.


Functional API
==============

The "Enum" class is callable, providing the following functional API:

   >>> Animal = Enum('Animal', 'ANT BEE CAT DOG')
   >>> Animal
   <enum 'Animal'>
   >>> Animal.ANT
   <Animal.ANT: 1>
   >>> list(Animal)
   [<Animal.ANT: 1>, <Animal.BEE: 2>, <Animal.CAT: 3>, <Animal.DOG: 4>]

The semantics of this API resemble "namedtuple". The first argument of
the call to "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 "Enum" is
returned.  In other words, the above assignment to "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 a 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__)

警告:

  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
"__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.

在 3.5 版的變更: The *start* parameter was added.


Derived Enumerations
====================


IntEnum
-------

The first variation of "Enum" that is provided is also a subclass of
"int".  Members of an "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 "Enum" enumerations:

   >>> class Shape(IntEnum):
   ...     CIRCLE = 1
   ...     SQUARE = 2
   ...
   >>> class Color(Enum):
   ...     RED = 1
   ...     GREEN = 2
   ...
   >>> Shape.CIRCLE == Color.RED
   False

"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 "Enum" that is provided is also a subclass of
"str".  Members of a "StrEnum" can be compared to strings; by
extension, string enumerations of different types can also be compared
to each other.

在 3.11 版被加入.


IntFlag
-------

The next variation of "Enum" provided, "IntFlag", is also based on
"int".  The difference being "IntFlag" members can be combined using
the bitwise operators (&, |, ^, ~) and the result is still an
"IntFlag" member, if possible.  Like "IntEnum", "IntFlag" members are
also integers and can be used wherever an "int" is used.

備註:

  Any operation on an "IntFlag" member besides the bit-wise operations
  will lose the "IntFlag" membership.Bit-wise operations that result
  in invalid "IntFlag" values will lose the "IntFlag" membership.  See
  "FlagBoundary" for details.

在 3.6 版被加入.

在 3.11 版的變更.

Sample "IntFlag" class:

   >>> from enum import IntFlag
   >>> class Perm(IntFlag):
   ...     R = 4
   ...     W = 2
   ...     X = 1
   ...
   >>> Perm.R | Perm.W
   <Perm.R|W: 6>
   >>> 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: 7>
   >>> ~Perm.RWX
   <Perm: 0>
   >>> Perm(7)
   <Perm.RWX: 7>

備註:

  Named combinations are considered aliases.  Aliases do not show up
  during iteration, but can be returned from by-value lookups.

在 3.11 版的變更.

Another important difference between "IntFlag" and "Enum" is that if
no flags are set (the value is 0), its boolean evaluation is "False":

   >>> Perm.R & Perm.X
   <Perm: 0>
   >>> bool(Perm.R & Perm.X)
   False

Because "IntFlag" members are also subclasses of "int" they can be
combined with them (but may lose "IntFlag" membership:

   >>> Perm.X | 4
   <Perm.R|X: 5>

   >>> Perm.X + 8
   9

備註:

  The negation operator, "~", always returns an "IntFlag" member with
  a positive value:

     >>> (~Perm.X).value == (Perm.R|Perm.W).value == 6
     True

"IntFlag" members can also be iterated over:

   >>> list(RW)
   [<Perm.R: 4>, <Perm.W: 2>]

在 3.11 版被加入.


Flag
----

The last variation is "Flag".  Like "IntFlag", "Flag" members can be
combined using the bitwise operators (&, |, ^, ~).  Unlike "IntFlag",
they cannot be combined with, nor compared against, any other "Flag"
enumeration, nor "int".  While it is possible to specify the values
directly it is recommended to use "auto" as the value and let "Flag"
select an appropriate value.

在 3.6 版被加入.

Like "IntFlag", if a combination of "Flag" members results in no flags
being set, the boolean evaluation is "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 will not:

   >>> class Color(Flag):
   ...     RED = auto()
   ...     BLUE = auto()
   ...     GREEN = auto()
   ...     WHITE = RED | BLUE | GREEN
   ...
   >>> Color.WHITE
   <Color.WHITE: 7>

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: 0>
   >>> bool(Color.BLACK)
   False

"Flag" members can also be iterated over:

   >>> purple = Color.RED | Color.BLUE
   >>> list(purple)
   [<Color.RED: 1>, <Color.BLUE: 2>]

在 3.11 版被加入.

備註:

  For the majority of new code, "Enum" and "Flag" are strongly
  recommended, since "IntEnum" and "IntFlag" break some semantic
  promises of an enumeration (by being comparable to integers, and
  thus by transitivity to other unrelated enumerations).  "IntEnum"
  and "IntFlag" should be used only in cases where "Enum" and "Flag"
  will not do; for example, when integer constants are replaced with
  enumerations, or for interoperability with other systems.


其他
----

While "IntEnum" is part of the "enum" module, it would be very simple
to implement independently:

   class IntEnum(int, ReprEnum):   # or Enum instead of ReprEnum
       pass

This demonstrates how similar derived enumerations can be defined; for
example a "FloatEnum" that mixes in "float" instead of "int".

一些規則：

1. When subclassing "Enum", mix-in types must appear before the "Enum"
   class itself in the sequence of bases, as in the "IntEnum" example
   above.

2. Mix-in types must be subclassable. For example, "bool" and "range"
   are not subclassable and will throw an error during Enum creation
   if used as the mix-in type.

3. While "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. "int" above.  This restriction does not apply to mix-ins which
   only add methods and don't specify another type.

4. When another data type is mixed in, the "value" attribute is *not
   the same* as the enum member itself, although it is equivalent and
   will compare equal.

5. A "data type" is a mixin that defines "__new__()", or a "dataclass"

6. %-style formatting:  "%s" and "%r" call the "Enum" class's
   "__str__()" and "__repr__()" respectively; other codes (such as
   "%i" or "%h" for IntEnum) treat the enum member as its mixed-in
   type.

7. Formatted string literals, "str.format()", and "format()" will use
   the enum's "__str__()" method.

備註:

  Because "IntEnum", "IntFlag", and "StrEnum" are designed to be drop-
  in replacements for existing constants, their "__str__()" method has
  been reset to their data types' "__str__()" method.


When to use "__new__()" vs. "__init__()"
========================================

"__new__()" must be used whenever you want to customize the actual
value of the "Enum" member.  Any other modifications may go in either
"__new__()" or "__init__()", with "__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'])
   Coordinate.PY

   >>> print(Coordinate(3))
   Coordinate.VY

警告:

  *Do not* call "super().__new__()", as the lookup-only "__new__" is
  the one that is found; instead, use the data type directly.


Finer Points
------------


Supported "__dunder__" names
~~~~~~~~~~~~~~~~~~~~~~~~~~~~

"__members__" is a read-only ordered mapping of "member_name":"member"
items.  It is only available on the class.

"__new__()", if specified, must create and return the enum members; it
is also a very good idea to set the member's "_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 in "__new__"

* "_missing_()" -- a lookup function used when a value is not found;
  may be overridden

* "_ignore_" -- a list of names, either as a "list" or a "str", that
  will not be transformed into members, and will be removed from the
  final class

* "_generate_next_value_()" -- used to get an appropriate value for an
  enum member; may be overridden

* "_add_alias_()" -- adds a new name as an alias to an existing
  member.

* "_add_value_alias_()" -- adds a new value as an alias to an existing
  member.  See MultiValueEnum for an example.

  備註:

    For standard "Enum" classes the next value chosen is the highest
    value seen incremented by one.For "Flag" classes the next value
    chosen will be the next highest power-of-two.

  在 3.13 版的變更: Prior versions would use the last seen value
  instead of the highest value.

在 3.6 版被加入: "_missing_"、"_order_"、"_generate_next_value_"

在 3.7 版被加入: "_ignore_"

在 3.13 版被加入: "_add_alias_", "_add_value_alias_"

To help keep Python 2 / Python 3 code in sync an "_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']

備註:

  In Python 2 code the "_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.

在 3.11 版的變更.


"Enum" member type
~~~~~~~~~~~~~~~~~~

Enum members are instances of their enum class, and are normally
accessed as "EnumClass.member".  In certain situations, such as
writing custom enum behavior, being able to access one member directly
from another is useful, and is supported; however, in order to avoid
name clashes between member names and attributes/methods from mixed-in
classes, upper-case names are strongly recommended.

在 3.5 版的變更.


Creating members that are mixed with other data types
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

When subclassing other data types, such as "int" or "str", with an
"Enum", all values after the "=" are passed to that data type's
constructor.  For example:

   >>> class MyEnum(IntEnum):      # help(int) -> int(x, base=10) -> integer
   ...     example = '11', 16      # so x='11' and base=16
   ...
   >>> MyEnum.example.value        # and hex(11) is...
   17


Boolean value of "Enum" classes and members
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Enum classes that are mixed with non-"Enum" types (such as "int",
"str", etc.) are evaluated according to the mixed-in type's rules;
otherwise, all members evaluate as "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 "Enum" classes always evaluate as "True".


"Enum" classes with methods
~~~~~~~~~~~~~~~~~~~~~~~~~~~

If you give your enum subclass extra methods, like the Planet class
below, those methods will show up in a "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 "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: 3>
   >>> Color(7)      # not named combination
   <Color.RED|GREEN|BLUE: 7>


"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: 1>, <Color.GREEN: 2>, <Color.BLUE: 4>]

* negating a flag or flag set returns a new flag/flag set with the
  corresponding positive integer value:

     >>> Color.BLUE
     <Color.BLUE: 4>

     >>> ~Color.BLUE
     <Color.RED|GREEN: 3>

* names of pseudo-flags are constructed from their members' names:

     >>> (Color.RED | Color.GREEN).name
     'RED|GREEN'

     >>> class Perm(IntFlag):
     ...     R = 4
     ...     W = 2
     ...     X = 1
     ...
     >>> (Perm.R & Perm.W).name is None  # effectively Perm(0)
     True

* multi-bit flags, aka aliases, can be returned from operations:

     >>> Color.RED | Color.BLUE
     <Color.PURPLE: 5>

     >>> Color(7)  # 或 Color(-1)
     <Color.WHITE: 7>

     >>> Color(0)
     <Color.BLACK: 0>

* membership / containment checking: zero-valued flags are always
  considered to be contained:

     >>> Color.BLACK in Color.WHITE
     True

  otherwise, only if all bits of one flag are in the other flag will
  True be returned:

     >>> Color.PURPLE in Color.WHITE
     True

     >>> Color.GREEN in Color.PURPLE
     False

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).


How are Enums and Flags different?
==================================

Enums have a custom metaclass that affects many aspects of both
derived "Enum" classes and their instances (members).


Enum Classes
------------

The "EnumType" metaclass is responsible for providing the
"__contains__()", "__dir__()", "__iter__()" and other methods that
allow one to do things with an "Enum" class that fail on a typical
class, such as "list(Color)" or "some_enum_var in Color".  "EnumType"
is responsible for ensuring that various other methods on the final
"Enum" class are correct (such as "__new__()", "__getnewargs__()",
"__str__()" and "__repr__()").


Flag Classes
------------

Flags have an expanded view of aliasing: to be canonical, the value of
a flag needs to be a power-of-two value, and not a duplicate name.
So, in addition to the "Enum" definition of alias, a flag with no
value (a.k.a. "0") or with more than one power-of-two value (e.g. "3")
is considered an alias.


Enum Members (aka instances)
----------------------------

The most interesting thing about enum members is that they are
singletons. "EnumType" creates them all while it is creating the enum
class itself, and then puts a custom "__new__()" in place to ensure
that no new ones are ever instantiated by returning only the existing
member instances.


Flag Members
------------

Flag members can be iterated over just like the "Flag" class, and only
the canonical members will be returned.  For example:

   >>> list(Color)
   [<Color.RED: 1>, <Color.GREEN: 2>, <Color.BLUE: 4>]

(Note that "BLACK", "PURPLE", and "WHITE" do not show up.)

Inverting a flag member returns the corresponding positive value,
rather than a negative value --- for example:

   >>> ~Color.RED
   <Color.GREEN|BLUE: 6>

Flag members have a length corresponding to the number of power-of-two
values they contain.  For example:

   >>> len(Color.PURPLE)
   2


Enum Cookbook
=============

While "Enum", "IntEnum", "StrEnum", "Flag", and "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 "auto" for the value

* use instances of "object" as the value

* use a descriptive string as the value

* use a tuple as the value and a custom "__new__()" to replace the
  tuple with an "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.


使用 "auto"
~~~~~~~~~~~

Using "auto" would look like:

   >>> class Color(Enum):
   ...     RED = auto()
   ...     BLUE = auto()
   ...     GREEN = auto()
   ...
   >>> Color.GREEN
   <Color.GREEN: 3>


使用 "object"
~~~~~~~~~~~~~

Using "object" would look like:

   >>> class Color(Enum):
   ...     RED = object()
   ...     GREEN = object()
   ...     BLUE = object()
   ...
   >>> Color.GREEN                         
   <Color.GREEN: <object object at 0x...>>

This is also a good example of why you might want to write your own
"__repr__()":

   >>> class Color(Enum):
   ...     RED = object()
   ...     GREEN = object()
   ...     BLUE = object()
   ...     def __repr__(self):
   ...         return "<%s.%s>" % (self.__class__.__name__, self._name_)
   ...
   >>> 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: 'go'>


Using a custom "__new__()"
~~~~~~~~~~~~~~~~~~~~~~~~~~

Using an auto-numbering "__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: 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: 2>
   >>> Swatch.SEA_GREEN.pantone
   '1246'
   >>> Swatch.BLEACHED_CORAL.pantone
   'unknown'

備註:

  The "__new__()" method, if defined, is used during creation of the
  Enum members; it is then replaced by Enum's "__new__()" which is
  used after class creation for lookup of existing members.

警告:

  *Do not* call "super().__new__()", as the lookup-only "__new__" is
  the one that is found; instead, use the data type directly -- e.g.:

     obj = int.__new__(cls, value)


OrderedEnum
-----------

An ordered enumeration that is not based on "IntEnum" and so maintains
the normal "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 value 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'

備註:

  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 "unique()" decorator can be used
  instead.


MultiValueEnum
--------------

Supports having more than one value per member:

   >>> class MultiValueEnum(Enum):
   ...     def __new__(cls, value, *values):
   ...         self = object.__new__(cls)
   ...         self._value_ = value
   ...         for v in values:
   ...             self._add_value_alias_(v)
   ...         return self
   ...
   >>> class DType(MultiValueEnum):
   ...     float32 = 'f', 8
   ...     double64 = 'd', 9
   ...
   >>> DType('f')
   <DType.float32: 'f'>
   >>> DType(9)
   <DType.double64: 'd'>


Planet
------

If "__new__()" or "__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


TimePeriod
----------

An example to show the "_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: datetime.timedelta(0)>, <Period.day_1: datetime.timedelta(days=1)>]
   >>> list(Period)[-2:]
   [<Period.day_365: datetime.timedelta(days=365)>, <Period.day_366: datetime.timedelta(days=366)>]


Subclassing EnumType
====================

While most enum needs can be met by customizing "Enum" subclasses,
either with class decorators or custom functions, "EnumType" can be
subclassed to provide a different Enum experience.
