8.13. "enum" — Support for enumerations
***************************************

New in version 3.4.

**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.


8.13.1. Module Contents
=======================

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

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".

enum.unique()

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


8.13.2. 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
   ...

Note: Nomenclature

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

  * The attributes "Color.red", "Color.green", etc., are
    *enumeration members* (or *enum members*).

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

Note: 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


8.13.3. 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


8.13.4. 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>

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.


8.13.5. 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


8.13.6. 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 an ordered dictionary mapping
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']


8.13.7. 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: unorderable types: Color() < 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


8.13.8. 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.) and descriptors (methods are also descriptors).

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.


8.13.9. Restricted subclassing of enumerations
==============================================

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.)


8.13.10. 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.


8.13.11. 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__)

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.


8.13.12. Derived Enumerations
=============================


8.13.12.1. IntEnum
------------------

A variation of "Enum" is provided which 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]

For the vast majority of code, "Enum" is strongly recommended, since
"IntEnum" breaks some semantic promises of an enumeration (by being
comparable to integers, and thus by transitivity to other unrelated
enumerations).  It should be used only in special cases where there’s
no other choice; for example, when integer constants are replaced with
enumerations and backwards compatibility is required with code that
still expects integers.


8.13.12.2. 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. "str.format()" (or "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.


8.13.13. Interesting examples
=============================

While "Enum" and "IntEnum" 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.


8.13.13.1. AutoNumber
---------------------

Avoids having to specify the value for each enumeration member:

   >>> 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.value == 2
   True

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.


8.13.13.2. 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


8.13.13.3. 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.


8.13.13.4. 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


8.13.14. How are Enums different?
=================================

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


8.13.14.1. 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_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__()").


8.13.14.2. 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.


8.13.14.3. Finer Points
-----------------------

"Enum" members are instances of an "Enum" class, and even though they
are accessible as *EnumClass.member*, they should not be accessed
directly from the member as that lookup may fail or, worse, return
something besides the "Enum" member you looking for:

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

Changed in version 3.5.

The "__members__" attribute is only available on the class.

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']

The "__new__()" method will only be used for the creation of the
"Enum" members – after that it is replaced.  Any custom "__new__()"
method must create the object and set the "_value_" attribute
appropriately.

If you wish to change how "Enum" members are looked up you should
either write a helper function or a "classmethod()" for the "Enum"
subclass.
