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

Author:
   Ethan Furman <ethan at stoneleaf dot us>


Basic Enum Tutorial
===================

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

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

Note:

  Case of Enum MembersBecause 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 "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

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

But "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 "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 "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()


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 "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
   >>> 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 "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):
   ...     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']

Note:

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

Note:  if your enumeration defines "__new__()" and/or "__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 "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: 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 "__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
   >>> Animal.ANT.value
   1
   >>> list(Animal)
   [Animal.ANT, Animal.BEE, Animal.CAT, Animal.DOG]

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 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
"__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.

Changed in version 3.5: 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.  "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 "StrEnum", "Directions.NORTH" would have been the "tuple"
"('north',)".

New in version 3.10.


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.  However, as the name implies,
"IntFlag" members also subclass "int" and can be used wherever an
"int" is used.

Note:

  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.

New in version 3.6.

Changed in version 3.10.

Sample "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.

Changed in version 3.10.

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|Perm.X

   >>> Perm.X | 8
   9

Note:

  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, Perm.W]

New in version 3.10.


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.

New in version 3.6.

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 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

"Flag" members can also be iterated over:

   >>> purple = Color.RED | Color.BLUE
   >>> list(purple)
   [Color.RED, Color.BLUE]

New in version 3.10.

Note:

  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 type.

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__()" unless "__str__()" or
   "__format__()" is overridden in the subclass, in which case the
   overridden methods or "Enum" methods will be used. Use the !s and
   !r format codes to force usage of the "Enum" class's "__str__()"
   and "__repr__()" methods.


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'])
   PY

   >>> print(Coordinate(3))
   VY


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

Note:

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

New in version 3.6: "_missing_", "_order_", "_generate_next_value_"

New in version 3.7: "_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_:
   ['RED', 'BLUE', 'GREEN']
   ['RED', 'GREEN', 'BLUE']

Note:

  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.

Changed in version 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" "Enum" will return to not allowing it,
while in "3.10" and "3.11" it will raise a "DeprecationWarning":

   >>> class FieldTypes(Enum):
   ...     name = 0
   ...     value = 1
   ...     size = 2
   ...
   >>> FieldTypes.value.size       
   DeprecationWarning: accessing one member from another is not supported,
     and will be disabled in 3.12
   <FieldTypes.size: 2>

Changed in version 3.5.

Changed in version 3.10.


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):
   ...     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-"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
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__', '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
   >>> Color(7)      # not named combination
   Color.RED|Color.GREEN|Color.BLUE


"StrEnum" and "str.__str__()"
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

An important difference between "StrEnum" and other Enums is the
"__str__()" method; because "StrEnum" members are strings, some parts
of Python will read the string data directly, while others will call
"str()". To make those two operations have the same result,
"StrEnum.__str__()" will be the same as "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).


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 "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__()").


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.

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.


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

Using "auto" would look like:

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


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

Using "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 "__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>
   >>> 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 "__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'

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 "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, Period.day_1]
   >>> list(Period)[-2:]
   [Period.day_365, Period.day_366]


Conforming input to Flag
------------------------

Creating a "Flag" enum that is more resilient out-of-bounds results to
mathematical operations, you can use the "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


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.
