"enum" --- Support for enumerations
***********************************

3.4 新版功能.

**Source code:** Lib/enum.py

======================================================================

An enumeration is a set of symbolic names (members) bound to unique,
constant values.  Within an enumeration, the members can be compared
by identity, and the enumeration itself can be iterated over.


模块内容
========

This module defines four enumeration classes that can be used to
define unique sets of names and values: "Enum", "IntEnum", "Flag", and
"IntFlag".  It also defines one decorator, "unique()", and one helper,
"auto".

class enum.Enum

   Base class for creating enumerated constants.  See section
   Functional API for an alternate construction syntax.

class enum.IntEnum

   Base class for creating enumerated constants that are also
   subclasses of "int".

class enum.IntFlag

   Base class for creating enumerated constants that can be combined
   using the bitwise operators without losing their "IntFlag"
   membership. "IntFlag" members are also subclasses of "int".

class enum.Flag

   Base class for creating enumerated constants that can be combined
   using the bitwise operations without losing their "Flag"
   membership.

enum.unique()

   Enum class decorator that ensures only one name is bound to any one
   value.

class enum.auto

   Instances are replaced with an appropriate value for Enum members.

3.6 新版功能: "Flag", "IntFlag", "auto"


Creating an Enum
================

Enumerations are created using the "class" syntax, which makes them
easy to read and write.  An alternative creation method is described
in Functional API.  To define an enumeration, subclass "Enum" as
follows:

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

注解: Enum member valuesMember values can be anything: "int", "str",
  etc.. If the exact value is unimportant you may use "auto" instances
  and an appropriate value will be chosen for you.  Care must be taken
  if you mix "auto" with other values.

注解: Nomenclature

  * The class "Color" is an *enumeration* (or *enum*)

  * The attributes "Color.RED", "Color.GREEN", etc., are
    *enumeration members* (or *enum members*) and are functionally
    constants.

  * The enum members have *names* and *values* (the name of
    "Color.RED" is "RED", the value of "Color.BLUE" is "3", etc.)

注解: Even though we use the "class" syntax to create Enums, Enums
  are not normal Python classes.  See How are Enums different? for
  more details.

Enumeration members have human readable string representations:

   >>> print(Color.RED)
   Color.RED

...while their "repr" has more information:

   >>> print(repr(Color.RED))
   <Color.RED: 1>

The *type* of an enumeration member is the enumeration it belongs to:

   >>> type(Color.RED)
   <enum 'Color'>
   >>> isinstance(Color.GREEN, Color)
   True
   >>>

Enum members also have a property that contains just their item name:

   >>> print(Color.RED.name)
   RED

Enumerations support iteration, in definition order:

   >>> class Shake(Enum):
   ...     VANILLA = 7
   ...     CHOCOLATE = 4
   ...     COOKIES = 9
   ...     MINT = 3
   ...
   >>> for shake in Shake:
   ...     print(shake)
   ...
   Shake.VANILLA
   Shake.CHOCOLATE
   Shake.COOKIES
   Shake.MINT

Enumeration members are hashable, so they can be used in dictionaries
and sets:

   >>> apples = {}
   >>> apples[Color.RED] = 'red delicious'
   >>> apples[Color.GREEN] = 'granny smith'
   >>> apples == {Color.RED: 'red delicious', Color.GREEN: 'granny smith'}
   True


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: Attempted to reuse key: 'SQUARE'

However, two enum members are allowed to have the same value.  Given
two members A and B with the same value (and A defined first), B is an
alias to A.  By-value lookup of the value of A and B will return A.
By-name lookup of B will also return 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>

注解: 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, the following decorator can
be used to ensure each value is used only once in the enumeration:

@enum.unique

A "class" decorator specifically for enumerations.  It searches an
enumeration's "__members__" gathering any aliases it finds; if any are
found "ValueError" is raised with the details:

   >>> 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()
   ...
   >>> list(Color)
   [<Color.RED: 1>, <Color.BLUE: 2>, <Color.GREEN: 3>]

The values are chosen by "_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()
   ...
   >>> list(Ordinal)
   [<Ordinal.NORTH: 'NORTH'>, <Ordinal.SOUTH: 'SOUTH'>, <Ordinal.EAST: 'EAST'>, <Ordinal.WEST: 'WEST'>]

注解: The goal of the default "_generate_next_value_()" methods is
  to provide the next "int" in sequence with the last "int" provided,
  but the way it does this is an implementation detail and may change.


Iteration
=========

Iterating over the members of an enum does not provide the aliases:

   >>> list(Shape)
   [<Shape.SQUARE: 2>, <Shape.DIAMOND: 1>, <Shape.CIRCLE: 3>]

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']


比较
====

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


Allowed members and attributes of enumerations
==============================================

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__()"
then whatever value(s) were given to the enum member will be passed
into those methods.  See Planet for an example.


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: Cannot extend enumerations

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.

注解: 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.


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>
   >>> Animal.ANT.value
   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 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__)

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)

值:
   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}

模块:
   name of module where new Enum class can be found.

qualname:
   where in module new Enum class can be found.

类型:
   type to mix in to new Enum class.

start:
   number to start counting at if only names are passed in.

在 3.5 版更改: 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]


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.  However, as the name implies, "IntFlag" members
also subclass "int" and can be used wherever an "int" is used.  Any
operation on an "IntFlag" member besides the bit-wise operations will
lose the "IntFlag" membership.

3.6 新版功能.

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.-8: -8>

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

Because "IntFlag" members are also subclasses of "int" they can be
combined with them:

   >>> Perm.X | 8
   <Perm.8|X: 9>


标志
----

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.

3.6 新版功能.

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: 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: 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

注解: 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.


Others
------

While "IntEnum" is part of the "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 "StrEnum" that mixes in "str" instead of "int".

Some rules:

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

2. 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 data type such as "int"
   or "str".

3. 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.

4. %-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.

5. Formatted string literals, "str.format()", and "format()" will
   use the mixed-in type's "__format__()".  If the "Enum" class's
   "str()" or "repr()" is desired, use the *!s* or *!r* format codes.


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


Interesting examples
====================

While "Enum", "IntEnum", "IntFlag", and "Flag" 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.

Whichever method you choose, you should provide a "repr()" that also
hides the (unimportant) value:

   >>> class NoValue(Enum):
   ...     def __repr__(self):
   ...         return '<%s.%s>' % (self.__class__.__name__, self.name)
   ...


Using "auto"
~~~~~~~~~~~~

Using "auto" would look like:

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


Using "object"
~~~~~~~~~~~~~~

Using "object" would look like:

   >>> class Color(NoValue):
   ...     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(NoValue):
   ...     RED = 'stop'
   ...     GREEN = 'go'
   ...     BLUE = 'too fast!'
   ...
   >>> Color.GREEN
   <Color.GREEN>
   >>> Color.GREEN.value
   'go'


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

Using an auto-numbering "__new__()" would look like:

   >>> class AutoNumber(NoValue):
   ...     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

注解: 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.


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 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'

注解: 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.


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)>]


How are Enums different?
========================

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


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

The "EnumMeta" 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*.  "EnumMeta"
is responsible for ensuring that various other methods on the final
"Enum" class are correct (such as "__new__()", "__getnewargs__()",
"__str__()" and "__repr__()").


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

The most interesting thing about Enum members is that they are
singletons. "EnumMeta" 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.


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 / 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 "list()" or a "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
  "auto" to get an appropriate value for an enum member; may be
  overridden

3.6 新版功能: "_missing_", "_order_", "_generate_next_value_"

3.7 新版功能: "_ignore_"

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_

注解: In Python 2 code the "_order_" attribute is necessary as
  definition order is lost before it can be recorded.


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

"Enum" members are instances of their "Enum" class, and are normally
accessed as "EnumClass.member".  Under certain circumstances they can
also be accessed as "EnumClass.member.member", but you should never do
this as that lookup may fail or, worse, return something besides the
"Enum" member you are looking for (this is another good reason to use
all-uppercase names for members):

   >>> class FieldTypes(Enum):
   ...     name = 0
   ...     value = 1
   ...     size = 2
   ...
   >>> FieldTypes.value.size
   <FieldTypes.size: 2>
   >>> FieldTypes.size.value
   2

在 3.5 版更改.


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

"Enum" members 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)

"Enum" classes always evaluate as "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 "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__', 'name', 'surface_gravity', 'value']


Combining members of "Flag"
~~~~~~~~~~~~~~~~~~~~~~~~~~~

If a combination of Flag members is not named, the "repr()" will
include all named flags and all named combinations of flags that are
in the value:

   >>> 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.CYAN|MAGENTA|BLUE|YELLOW|GREEN|RED: 7>
