I want to use enums in python like in code below [java]. I am a greenhorn in Python. I have the following code in Java and want to replicate the functionality in Python:
class Direction {
public enum Direction {LEFT, RIGHT, UP, DOWN}
public void navigate[Direction direction]
switch[direction]{
case Direction.LEFT:
System.out.print["left"];
break;
case Direction.RIGHT:
System.out.print["right"];
break;
case Direction.UP:
System.out.print["up"];
break;
case Direction.DOWN:
System.out.print["down"];
break;
}
}
How can I enforce users to only provide an enum to a Python method?
asked Feb 15, 2016 at 10:54
3
Python is dynamic and duck typed - variables can change type and you can't force types on methods.
You can, however, check for types in the body of a method using isinstance[]
.
isinstance[]
will allow users to subclass your enum
for future extensibility. - See comments
E.g.
# Python 2.x: pip install enum34
from enum import Enum
class Direction[Enum]:
LEFT = "left"
RIGHT = "right"
UP = "up"
DOWN = "down"
def move[direction]:
# Type checking
if not isinstance[direction, Direction]:
raise TypeError['direction must be an instance of Direction Enum']
print direction.value
>>> move[Direction.LEFT]
left
>>> move["right"]
TypeError: direction must be an instance of Direction Enum
answered Feb 15, 2016 at 12:34
2
The "pythonic" thing to do is to follow the principle of duck-typing: Try to accept the value you are passed without making too big of a fuss. In this case, instead of enforcing the type I would simply check for equality with each enum value, and raise an error for anything that cannot be handled:
def navigate[direction]:
"""Turn toward `direction` [an enum of type `Direction`]"""
if direction == Direction.left:
print["Left"]
elif direction == Direction.right:
[etc., etc.]
else:
# Hmm, `direction` does not compare equal to any enum value:
raise ValueError["Invalid direction "+ str[direction]]
answered Feb 15, 2016 at 13:00
alexisalexis
46.9k15 gold badges98 silver badges155 bronze badges
1
Not the answer you're looking for? Browse other questions tagged python python-2.7 enums or ask your own question.
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]
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] >>> 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[]]
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
But
Flag
also allows us to combine several members into a single variable:
>>> weekend = Weekday.SATURDAY | Weekday.SUNDAY >>> weekend
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[]
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[3]
If you want to access enum members by name, use item access:
>>> Color['RED'] >>> 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.ALIAS_FOR_SQUARE >>> Shape[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 : 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] [, , ]
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', ] ['DIAMOND', ] ['CIRCLE', ] ['ALIAS_FOR_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 == 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.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: cannot extend
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 >>> Animal.ANT >>> list[Animal] [, , , ]
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=, start=1, ]value
What the new enum class will record as its name.
namesThe 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:
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.
qualnamewhere in module new enum class can be found.
typetype to mix in to new enum class.
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 + 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[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 >>> 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.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] [, ]
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 >>> 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
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 >>> bool[Color.BLACK] False
Flag
members can also be iterated over:
>>> purple = Color.RED | Color.BLUE >>> list[purple] [, ]
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.
Note
Because IntEnum
,
IntFlag
, and StrEnum
are designed to be drop-in replacements for existing constants, their __str__[]
method has been reset to their data types __str__[]
method.
When to use __new__[]
vs.
__init__[]
¶
__new__[]
must be used whenever you want to customize the actual value of the Enum
member. Any other modifications may go in either __new__[]
or __init__[]
, with __init__[]
being preferred.
For example, if you want to pass several items to the constructor, but only want one of them to be the value:
>>> class Coordinate[bytes, Enum]: ... """ ... Coordinate with binary codes that can be indexed by the int code. ... """ ... def __new__[cls, value, label, unit]: ... obj = bytes.__new__[cls, [value]] ... obj._value_ = value ... obj.label = label ... obj.unit = unit ... return obj ... PX = [0, 'P.X', 'km'] ... PY = [1, 'P.Y', 'km'] ... VX = [2, 'V.X', 'km/s'] ... VY = [3, 'V.Y', 'km/s'] ... >>> print[Coordinate['PY']] Coordinate.PY >>> print[Coordinate[3]] Coordinate.VY
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: 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]
Plain Enum
classes always evaluate as True
.
Enum
classes with methods¶
If you give your enum subclass extra methods, like the Planet class below, those methods will show up in a dir[]
of the member, but not of the class:
>>> dir[Planet] ['EARTH', 'JUPITER', 'MARS', 'MERCURY', 'NEPTUNE', 'SATURN', 'URANUS', 'VENUS', '__class__', '__doc__', '__members__', '__module__'] >>> dir[Planet.EARTH] ['__class__', '__doc__', '__module__', 'mass', 'name', 'radius', 'surface_gravity', 'value']
Combining members of Flag
¶
Iterating over a combination of Flag
members will only return the members that
are comprised of a single bit:
>>> class Color[Flag]: ... RED = auto[] ... GREEN = auto[] ... BLUE = auto[] ... MAGENTA = RED | BLUE ... YELLOW = RED | GREEN ... CYAN = GREEN | BLUE ... >>> Color[3] # named combination >>> Color[7] # not named combination
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] [, , ]
negating a flag or flag set returns a new flag/flag set with the corresponding positive integer value:
>>> Color.BLUE >>> ~Color.BLUE
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[7] # or Color[-1] >>> Color[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 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 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
Using
object
¶
Using object
would look like:
>>> class Color[Enum]: ... RED = object[] ... GREEN = object[] ... BLUE = object[] ... >>> Color.GREEN
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 "" % [self.__class__.__name__, self._name_] ... >>> 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
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
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.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 > class Grade[OrderedEnum]: ... A = 5 ... B = 4 ... C = 3 ... D = 2 ... F = 1 ... >>> Grade.C >> 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] [, ] >>> list[Period][-2:] [, ]
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.