Enum HOWTO¶
An Enum
is a set of symbolic names bound to unique values. They are
similar to global variables, but they offer a more useful repr()
,
grouping, type-safety, and a few other features.
They are most useful when you have a variable that can take one of a limited selection of values. For example, the days of the week:
>>> from enum import Enum
>>> class Weekday(Enum):
... MONDAY = 1
... TUESDAY = 2
... WEDNESDAY = 3
... THURSDAY = 4
... FRIDAY = 5
... SATURDAY = 6
... SUNDAY = 7
Or perhaps the RGB primary colors:
>>> from enum import Enum
>>> class Color(Enum):
... RED = 1
... GREEN = 2
... BLUE = 3
As you can see, creating an Enum
is as simple as writing a class that
inherits from Enum
itself.
Note
Case of Enum Members
Because Enums are used to represent constants we recommend using UPPER_CASE names for members, and will be using that style in our examples.
Depending on the nature of the enum a member’s value may or may not be important, but either way that value can be used to get the corresponding member:
>>> Weekday(3)
<Weekday.WEDNESDAY: 3>
As you can see, the repr()
of a member shows the enum name, the member name,
and the value. The str()
of a member shows only the enum name and member
name:
>>> print(Weekday.THURSDAY)
Weekday.THURSDAY
The type of an enumeration member is the enum it belongs to:
>>> type(Weekday.MONDAY)
<enum 'Weekday'>
>>> isinstance(Weekday.FRIDAY, Weekday)
True
Enum members have an attribute that contains just their name
:
>>> print(Weekday.TUESDAY.name)
TUESDAY
Likewise, they have an attribute for their value
:
>>> Weekday.WEDNESDAY.value
3
Unlike many languages that treat enumerations solely as name/value pairs,
Python Enums can have behavior added. For example, datetime.date
has two methods for returning the weekday: weekday()
and isoweekday()
.
The difference is that one of them counts from 0-6 and the other from 1-7.
Rather than keep track of that ourselves we can add a method to the Weekday
enum to extract the day from the date
instance and return the matching
enum member:
@classmethod
def from_date(cls, date):
return cls(date.isoweekday())
The complete Weekday
enum now looks like this:
>>> class Weekday(Enum):
... MONDAY = 1
... TUESDAY = 2
... WEDNESDAY = 3
... THURSDAY = 4
... FRIDAY = 5
... SATURDAY = 6
... SUNDAY = 7
... #
... @classmethod
... def from_date(cls, date):
... return cls(date.isoweekday())
Now we can find out what today is! Observe:
>>> from datetime import date
>>> Weekday.from_date(date.today())
<Weekday.TUESDAY: 2>
Of course, if you’re reading this on some other day, you’ll see that day instead.
This Weekday
enum is great if our variable only needs one day, but
what if we need several? Maybe we’re writing a function to plot chores during
a week, and don’t want to use a list
– we could use a different type
of Enum
:
>>> from enum import Flag
>>> class Weekday(Flag):
... MONDAY = 1
... TUESDAY = 2
... WEDNESDAY = 4
... THURSDAY = 8
... FRIDAY = 16
... SATURDAY = 32
... SUNDAY = 64
We’ve changed two things: we’re inherited from Flag
, and the values are
all powers of 2.
Just like the original Weekday
enum above, we can have a single selection:
>>> first_week_day = Weekday.MONDAY
>>> first_week_day
<Weekday.MONDAY: 1>
But Flag
also allows us to combine several members into a single
variable:
>>> weekend = Weekday.SATURDAY | Weekday.SUNDAY
>>> weekend
<Weekday.SATURDAY|SUNDAY: 96>
You can even iterate over a Flag
variable:
>>> for day in weekend:
... print(day)
Weekday.SATURDAY
Weekday.SUNDAY
Okay, let’s get some chores set up:
>>> chores_for_ethan = {
... 'feed the cat': Weekday.MONDAY | Weekday.WEDNESDAY | Weekday.FRIDAY,
... 'do the dishes': Weekday.TUESDAY | Weekday.THURSDAY,
... 'answer SO questions': Weekday.SATURDAY,
... }
And a function to display the chores for a given day:
>>> def show_chores(chores, day):
... for chore, days in chores.items():
... if day in days:
... print(chore)
>>> show_chores(chores_for_ethan, Weekday.SATURDAY)
answer SO questions
In cases where the actual values of the members do not matter, you can save
yourself some work and use auto()
for the values:
>>> from enum import auto
>>> class Weekday(Flag):
... MONDAY = auto()
... TUESDAY = auto()
... WEDNESDAY = auto()
... THURSDAY = auto()
... FRIDAY = auto()
... SATURDAY = auto()
... SUNDAY = auto()
... WEEKEND = SATURDAY | SUNDAY
Programmatic access to enumeration members and their attributes¶
Sometimes it’s useful to access members in enumerations programmatically (i.e.
situations where Color.RED
won’t do because the exact color is not known
at program-writing time). Enum
allows such access:
>>> Color(1)
<Color.RED: 1>
>>> Color(3)
<Color.BLUE: 3>
If you want to access enum members by name, use item access:
>>> Color['RED']
<Color.RED: 1>
>>> Color['GREEN']
<Color.GREEN: 2>
If you have an enum member and need its name
or value
:
>>> member = Color.RED
>>> member.name
'RED'
>>> member.value
1
Duplicating enum members and values¶
Having two enum members with the same name is invalid:
>>> class Shape(Enum):
... SQUARE = 2
... SQUARE = 3
...
Traceback (most recent call last):
...
TypeError: 'SQUARE' already defined as 2
However, an enum member can have other names associated with it. Given two
entries A
and B
with the same value (and A
defined first), B
is an alias for the member A
. By-value lookup of the value of A
will
return the member A
. By-name lookup of A
will return the member A
.
By-name lookup of B
will also return the member A
:
>>> class Shape(Enum):
... SQUARE = 2
... DIAMOND = 1
... CIRCLE = 3
... ALIAS_FOR_SQUARE = 2
...
>>> Shape.SQUARE
<Shape.SQUARE: 2>
>>> Shape.ALIAS_FOR_SQUARE
<Shape.SQUARE: 2>
>>> Shape(2)
<Shape.SQUARE: 2>
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: 2>, <Shape.DIAMOND: 1>, <Shape.CIRCLE: 3>]
>>> list(Weekday)
[<Weekday.MONDAY: 1>, <Weekday.TUESDAY: 2>, <Weekday.WEDNESDAY: 4>, <Weekday.THURSDAY: 8>, <Weekday.FRIDAY: 16>, <Weekday.SATURDAY: 32>, <Weekday.SUNDAY: 64>]
Note that the aliases Shape.ALIAS_FOR_SQUARE
and Weekday.WEEKEND
aren’t shown.
The special attribute __members__
is a read-only ordered mapping of names
to members. It includes all names defined in the enumeration, including the
aliases:
>>> for name, member in Shape.__members__.items():
... name, member
...
('SQUARE', <Shape.SQUARE: 2>)
('DIAMOND', <Shape.DIAMOND: 1>)
('CIRCLE', <Shape.CIRCLE: 3>)
('ALIAS_FOR_SQUARE', <Shape.SQUARE: 2>)
The __members__
attribute can be used for detailed programmatic access to
the enumeration members. For example, finding all the aliases:
>>> [name for name, member in Shape.__members__.items() if member.name != name]
['ALIAS_FOR_SQUARE']
Note
Aliases for flags include values with multiple flags set, such as 3
,
and no flags set, i.e. 0
.
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: 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
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: <enum 'MoreColor'> cannot extend <enum '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: 1>
>>> list(Animal)
[<Animal.ANT: 1>, <Animal.BEE: 2>, <Animal.CAT: 3>, <Animal.DOG: 4>]
The semantics of this API resemble namedtuple
. The first
argument of the call to Enum
is the name of the enumeration.
The second argument is the source of enumeration member names. It can be a
whitespace-separated string of names, a sequence of names, a sequence of
2-tuples with key/value pairs, or a mapping (e.g. dictionary) of names to
values. The last two options enable assigning arbitrary values to
enumerations; the others auto-assign increasing integers starting with 1 (use
the start
parameter to specify a different starting value). A
new class derived from Enum
is returned. In other words, the above
assignment to Animal
is equivalent to:
>>> class Animal(Enum):
... ANT = 1
... BEE = 2
... CAT = 3
... DOG = 4
...
The reason for defaulting to 1
as the starting number and not 0
is
that 0
is False
in a boolean sense, but by default enum members all
evaluate to True
.
Pickling enums created with the functional API can be tricky as frame stack implementation details are used to try and figure out which module the enumeration is being created in (e.g. it will fail if you use a utility function in a separate module, and also may not work on IronPython or Jython). The solution is to specify the module name explicitly as follows:
>>> Animal = Enum('Animal', 'ANT BEE CAT DOG', module=__name__)
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.
New in version 3.11.
IntFlag¶
The next variation of Enum
provided, IntFlag
, is also based
on int
. The difference being IntFlag
members can be combined
using the bitwise operators (&, |, ^, ~) and the result is still an
IntFlag
member, if possible. Like IntEnum
, IntFlag
members are also integers and can be used wherever an int
is used.
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 3.11.
Sample IntFlag
class:
>>> from enum import IntFlag
>>> class Perm(IntFlag):
... R = 4
... W = 2
... X = 1
...
>>> Perm.R | Perm.W
<Perm.R|W: 6>
>>> Perm.R + Perm.W
6
>>> RW = Perm.R | Perm.W
>>> Perm.R in RW
True
It is also possible to name the combinations:
>>> class Perm(IntFlag):
... R = 4
... W = 2
... X = 1
... RWX = 7
>>> Perm.RWX
<Perm.RWX: 7>
>>> ~Perm.RWX
<Perm: 0>
>>> Perm(7)
<Perm.RWX: 7>
Note
Named combinations are considered aliases. Aliases do not show up during iteration, but can be returned from by-value lookups.
Changed in version 3.11.
Another important difference between IntFlag
and Enum
is that
if no flags are set (the value is 0), its boolean evaluation is False
:
>>> Perm.R & Perm.X
<Perm: 0>
>>> bool(Perm.R & Perm.X)
False
Because IntFlag
members are also subclasses of int
they can
be combined with them (but may lose IntFlag
membership:
>>> Perm.X | 4
<Perm.R|X: 5>
>>> Perm.X | 8
9
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: 4>, <Perm.W: 2>]
New in version 3.11.
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 will not:
>>> class Color(Flag):
... RED = auto()
... BLUE = auto()
... GREEN = auto()
... WHITE = RED | BLUE | GREEN
...
>>> Color.WHITE
<Color.WHITE: 7>
Giving a name to the “no flags set” condition does not change its boolean value:
>>> class Color(Flag):
... BLACK = 0
... RED = auto()
... BLUE = auto()
... GREEN = auto()
...
>>> Color.BLACK
<Color.BLACK: 0>
>>> bool(Color.BLACK)
False
Flag
members can also be iterated over:
>>> purple = Color.RED | Color.BLUE
>>> list(purple)
[<Color.RED: 1>, <Color.BLUE: 2>]
New in version 3.11.
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 FloatEnum
that mixes in float
instead of int
.
Some rules:
When subclassing
Enum
, mix-in types must appear beforeEnum
itself in the sequence of bases, as in theIntEnum
example above.Mix-in types must be subclassable. For example,
bool
andrange
are not subclassable and will throw an error during Enum creation if used as the mix-in type.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.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.%-style formatting:
%s
and%r
call theEnum
class’s__str__()
and__repr__()
respectively; other codes (such as%i
or%h
for IntEnum) treat the enum member as its mixed-in type.Formatted string literals,
str.format()
, andformat()
will use the enum’s__str__()
method.
When to use __new__()
vs. __init__()
¶
__new__()
must be used whenever you want to customize the actual value of
the Enum
member. Any other modifications may go in either
__new__()
or __init__()
, with __init__()
being preferred.
For example, if you want to pass several items to the constructor, but only want one of them to be the value:
>>> class Coordinate(bytes, Enum):
... """
... Coordinate with binary codes that can be indexed by the int code.
... """
... def __new__(cls, value, label, unit):
... obj = bytes.__new__(cls, [value])
... obj._value_ = value
... obj.label = label
... obj.unit = unit
... return obj
... PX = (0, 'P.X', 'km')
... PY = (1, 'P.Y', 'km')
... VX = (2, 'V.X', 'km/s')
... VY = (3, 'V.Y', 'km/s')
...
>>> print(Coordinate['PY'])
Coordinate.PY
>>> print(Coordinate(3))
Coordinate.VY
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 alist
or astr
, 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 byauto
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 3.11.
Enum
member type¶
Enum members are instances of their enum class, and are normally accessed as
EnumClass.member
. In Python versions 3.5
to 3.10
you could access
members from other members – this practice was discouraged, and in 3.11
Enum
returns to not allowing it:
>>> class FieldTypes(Enum):
... name = 0
... value = 1
... size = 2
...
>>> FieldTypes.value.size
Traceback (most recent call last):
...
AttributeError: <enum 'FieldTypes'> member has no attribute 'size'
Changed in version 3.5.
Changed in version 3.11.
Creating members that are mixed with other data types¶
When subclassing other data types, such as int
or str
, with
an Enum
, all values after the =
are passed to that data type’s
constructor. For example:
>>> class MyEnum(IntEnum): # help(int) -> int(x, base=10) -> integer
... example = '11', 16 # so x='11' and base=16
...
>>> MyEnum.example.value # and hex(11) is...
17
Boolean value of Enum
classes and members¶
Enum classes that are mixed with non-Enum
types (such as
int
, str
, etc.) are evaluated according to the mixed-in
type’s rules; otherwise, all members evaluate as True
. To make your
own enum’s boolean evaluation depend on the member’s value add the following to
your class:
def __bool__(self):
return bool(self.value)
Enum
classes with methods¶
If you give your enum subclass extra methods, like the Planet
class below, those methods will show up in a dir()
of the member,
but not of the class:
>>> dir(Planet)
['EARTH', 'JUPITER', 'MARS', 'MERCURY', 'NEPTUNE', 'SATURN', 'URANUS', 'VENUS', '__class__', '__doc__', '__members__', '__module__']
>>> dir(Planet.EARTH)
['__class__', '__doc__', '__module__', 'mass', 'name', 'radius', 'surface_gravity', 'value']
Combining members of Flag
¶
Iterating over a combination of Flag
members will only return the members that
are comprised of a single bit:
>>> class Color(Flag):
... RED = auto()
... GREEN = auto()
... BLUE = auto()
... MAGENTA = RED | BLUE
... YELLOW = RED | GREEN
... CYAN = GREEN | BLUE
...
>>> Color(3) # named combination
<Color.YELLOW: 3>
>>> Color(7) # not named combination
<Color.RED|GREEN|BLUE: 7>
Flag
and IntFlag
minutia¶
Using the following snippet for our examples:
>>> class Color(IntFlag):
... BLACK = 0
... RED = 1
... GREEN = 2
... BLUE = 4
... PURPLE = RED | BLUE
... WHITE = RED | GREEN | BLUE
...
the following are true:
single-bit flags are canonical
multi-bit and zero-bit flags are aliases
only canonical flags are returned during iteration:
>>> list(Color.WHITE) [<Color.RED: 1>, <Color.GREEN: 2>, <Color.BLUE: 4>]
negating a flag or flag set returns a new flag/flag set with the corresponding positive integer value:
>>> Color.BLUE <Color.BLUE: 4> >>> ~Color.BLUE <Color.RED|GREEN: 3>
names of pseudo-flags are constructed from their members’ names:
>>> (Color.RED | Color.GREEN).name 'RED|GREEN'
multi-bit flags, aka aliases, can be returned from operations:
>>> Color.RED | Color.BLUE <Color.PURPLE: 5> >>> Color(7) # or Color(-1) <Color.WHITE: 7> >>> Color(0) <Color.BLACK: 0>
membership / containment checking: zero-valued flags are always considered to be contained:
>>> Color.BLACK in Color.WHITE True
otherwise, only if all bits of one flag are in the other flag will True be returned:
>>> Color.PURPLE in Color.WHITE True >>> Color.GREEN in Color.PURPLE False
There is a new boundary mechanism that controls how out-of-range / invalid
bits are handled: STRICT
, CONFORM
, EJECT
, and KEEP
:
STRICT –> raises an exception when presented with invalid values
CONFORM –> discards any invalid bits
EJECT –> lose Flag status and become a normal int with the given value
- KEEP –> keep the extra bits
keeps Flag status and extra bits
extra bits do not show up in iteration
extra bits do show up in repr() and str()
The default for Flag is STRICT
, the default for IntFlag
is EJECT
,
and the default for _convert_
is KEEP
(see ssl.Options
for an
example of when KEEP
is needed).
How are Enums and Flags different?¶
Enums have a custom metaclass that affects many aspects of both derived Enum
classes and their instances (members).
Enum Classes¶
The EnumType
metaclass is responsible for providing the
__contains__()
, __dir__()
, __iter__()
and other methods that
allow one to do things with an Enum
class that fail on a typical
class, such as list(Color)
or some_enum_var in Color
. EnumType
is
responsible for ensuring that various other methods on the final Enum
class are correct (such as __new__()
, __getnewargs__()
,
__str__()
and __repr__()
).
Flag Classes¶
Flags have an expanded view of aliasing: to be canonical, the value of a flag
needs to be a power-of-two value, and not a duplicate name. So, in addition to the
Enum
definition of alias, a flag with no value (a.k.a. 0
) or with more than one
power-of-two value (e.g. 3
) is considered an alias.
Enum Members (aka instances)¶
The most interesting thing about enum members is that they are singletons.
EnumType
creates them all while it is creating the enum class itself,
and then puts a custom __new__()
in place to ensure that no new ones are
ever instantiated by returning only the existing member instances.
Flag Members¶
Flag members can be iterated over just like the Flag
class, and only the
canonical members will be returned. For example:
>>> list(Color)
[<Color.RED: 1>, <Color.GREEN: 2>, <Color.BLUE: 4>]
(Note that BLACK
, PURPLE
, and WHITE
do not show up.)
Inverting a flag member returns the corresponding positive value, rather than a negative value — for example:
>>> ~Color.RED
<Color.GREEN|BLUE: 6>
Flag members have a length corresponding to the number of power-of-two values they contain. For example:
>>> len(Color.PURPLE)
2
Enum Cookbook¶
While Enum
, IntEnum
, StrEnum
, Flag
, and
IntFlag
are expected to cover the majority of use-cases, they cannot
cover them all. Here are recipes for some different types of enumerations
that can be used directly, or as examples for creating one’s own.
Omitting values¶
In many use-cases, one doesn’t care what the actual value of an enumeration is. There are several ways to define this type of simple enumeration:
use instances of
auto
for the valueuse instances of
object
as the valueuse a descriptive string as the value
use a tuple as the value and a custom
__new__()
to replace the tuple with anint
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: 3>
Using object
¶
Using object
would look like:
>>> class Color(Enum):
... RED = object()
... GREEN = object()
... BLUE = object()
...
>>> Color.GREEN
<Color.GREEN: <object object at 0x...>>
This is also a good example of why you might want to write your own
__repr__()
:
>>> class Color(Enum):
... RED = object()
... GREEN = object()
... BLUE = object()
... def __repr__(self):
... return "<%s.%s>" % (self.__class__.__name__, self._name_)
...
>>> Color.GREEN
<Color.GREEN>
Using a descriptive string¶
Using a string as the value would look like:
>>> class Color(Enum):
... RED = 'stop'
... GREEN = 'go'
... BLUE = 'too fast!'
...
>>> Color.GREEN
<Color.GREEN: 'go'>
Using a custom __new__()
¶
Using an auto-numbering __new__()
would look like:
>>> class AutoNumber(Enum):
... def __new__(cls):
... value = len(cls.__members__) + 1
... obj = object.__new__(cls)
... obj._value_ = value
... return obj
...
>>> class Color(AutoNumber):
... RED = ()
... GREEN = ()
... BLUE = ()
...
>>> Color.GREEN
<Color.GREEN: 2>
To make a more general purpose AutoNumber
, add *args
to the signature:
>>> class AutoNumber(Enum):
... def __new__(cls, *args): # this is the only change from above
... value = len(cls.__members__) + 1
... obj = object.__new__(cls)
... obj._value_ = value
... return obj
...
Then when you inherit from AutoNumber
you can write your own __init__
to handle any extra arguments:
>>> class Swatch(AutoNumber):
... def __init__(self, pantone='unknown'):
... self.pantone = pantone
... AUBURN = '3497'
... SEA_GREEN = '1246'
... BLEACHED_CORAL = () # New color, no Pantone code yet!
...
>>> Swatch.SEA_GREEN
<Swatch.SEA_GREEN: 2>
>>> Swatch.SEA_GREEN.pantone
'1246'
>>> Swatch.BLEACHED_CORAL.pantone
'unknown'
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 value is found instead of creating an alias:
>>> class DuplicateFreeEnum(Enum):
... def __init__(self, *args):
... cls = self.__class__
... if any(self.value == e.value for e in cls):
... a = self.name
... e = cls(self.value).name
... raise ValueError(
... "aliases not allowed in DuplicateFreeEnum: %r --> %r"
... % (a, e))
...
>>> class Color(DuplicateFreeEnum):
... RED = 1
... GREEN = 2
... BLUE = 3
... GRENE = 2
...
Traceback (most recent call last):
...
ValueError: aliases not allowed in DuplicateFreeEnum: 'GRENE' --> 'GREEN'
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: datetime.timedelta(0)>, <Period.day_1: datetime.timedelta(days=1)>]
>>> list(Period)[-2:]
[<Period.day_365: datetime.timedelta(days=365)>, <Period.day_366: datetime.timedelta(days=366)>]
Subclassing EnumType¶
While most enum needs can be met by customizing Enum
subclasses,
either with class decorators or custom functions, EnumType
can be
subclassed to provide a different Enum experience.