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
本身。
注意
枚举成员案例
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
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
变量：
>>> 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()
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>
若想要访问枚举成员通过 name ，使用项访问：
>>> Color['RED'] <Color.RED: 1> >>> Color['GREEN'] <Color.GREEN: 2>
若有枚举成员且需要其
name
or
value
:
>>> member = Color.RED >>> member.name 'RED' >>> member.value 1
拥有 2 同名成员的枚举是无效的：
>>> 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>
注意
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.
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
若确切值不重要，可以使用
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']
注意
_generate_next_value_()
method must be defined before any members.
Iterating over the members of an enum does not provide the aliases:
>>> list(Shape) [<Shape.SQUARE: 2>, <Shape.DIAMOND: 1>, <Shape.CIRCLE: 3>]
The special attribute
__members__
is a read-only ordered mapping of names to members. It includes all names defined in the enumeration, including the aliases:
>>> for name, member in Shape.__members__.items(): ... name, member ... ('SQUARE', <Shape.SQUARE: 2>) ('DIAMOND', <Shape.DIAMOND: 1>) ('CIRCLE', <Shape.CIRCLE: 3>) ('ALIAS_FOR_SQUARE', <Shape.SQUARE: 2>)
__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']
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
Most of the examples above use integers for enumeration values. Using integers is short and handy (and provided by default by the 函数式 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
范例。
新的
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.)
Enumerations can be pickled and unpickled:
>>> from test.test_enum import Fruit >>> from pickle import dumps, loads >>> Fruit.TOMATO is loads(dumps(Fruit.TOMATO)) True
The usual restrictions for pickling apply: picklable enums must be defined in the top level of a module, since unpickling requires them to be importable from that module.
注意
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.
Enum
类可调用，提供以下 API 功能：
>>> 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
相当于：
>>> 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__)
警告
若
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, )
What the new enum class will record as its name.
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'
或名称迭代器：
['RED', 'GREEN', 'BLUE']
或 (name, value) 对迭代器：
[('CYAN', 4), ('MAGENTA', 5), ('YELLOW', 6)]
或映射：
{'CHARTREUSE': 7, 'SEA_GREEN': 11, 'ROSEMARY': 42}
name of module where new enum class can be found.
where in module new enum class can be found.
type to mix in to new enum class.
number to start counting at if only names are passed in.
3.5 版改变： start 参数被添加。
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
枚举：
>>> 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]
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.
3.11 版新增。
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
被使用。
注意
Any operation on an
IntFlag
member besides the bit-wise operations will lose the
IntFlag
成员资格。
Bit-wise operations that result in invalid
IntFlag
values will lose the
IntFlag
membership. See
FlagBoundary
了解细节。
3.6 版新增。
Changed in version 3.11.
样本
IntFlag
类：
>>> 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>
注意
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
因为
IntFlag
成员也是子类的
int
they can be combined with them (but may lose
IntFlag
membership:
>>> Perm.X | 4 <Perm.R|X: 5> >>> Perm.X | 8 9
注意
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>]
3.11 版新增。
最后变体是
Flag
。像
IntFlag
,
Flag
members can be combined using the bitwise operators (&, |, ^, ~). Unlike
IntFlag
, they cannot be combined with, nor compared against, any other
Flag
enumeration, nor
int
. While it is possible to specify the values directly it is recommended to use
auto
as the value and let
Flag
select an appropriate value.
3.6 版新增。
像
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: 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>]
3.11 版新增。
注意
对于大多数新代码，
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.
While
IntEnum
属于
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
而不是
int
.
一些规则：
当子类化
Enum
, mix-in types must appear before
Enum
itself in the sequence of bases, as in the
IntEnum
example above.
Mix-in types must be subclassable. For example,
bool
and
range
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
属性为
not the same
as the enum member itself, although it is equivalent and will compare equal.
%-style formatting:
%s
and
%r
call the
Enum
类的
__str__()
and
__repr__()
respectively; other codes (such as
%i
or
%h
for IntEnum) treat the enum member as its mixed-in type.
格式化字符串文字
,
str.format()
，和
format()
will use the enum’s
__str__()
方法。
注意
因为
IntEnum
,
IntFlag
，和
StrEnum
are designed to be drop-in replacements for existing constants, their
__str__()
method has been reset to their data types
__str__()
方法。
__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__()
，采用
__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
__dunder__
名称
¶
__members__
是只读有序映射的
member_name
:
member
项。只可用于 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.
_sunder_
名称
¶
_name_
– 成员名称
_value_
– 成员的值；设置/修改可以在
__new__
_missing_
– a lookup function used when a value is not found; may be overridden
_ignore_
– a list of names, either as a
list
或
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_
– 用于
函数式 API
和通过
auto
to get an appropriate value for an enum member; may be overridden
注意
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.
3.6 版新增：
_missing_
,
_order_
,
_generate_next_value_
3.7 版新增：
_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']
注意
在 Python 2 代码中
_order_
attribute is necessary as definition order is lost before it can be recorded.
Private names are not converted to enum members, but remain normal attributes.
Changed in version 3.11.
Enum
成员类型
¶
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'
3.5 版改变。
Changed in version 3.11.
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
Enum
类和成员
¶
Enum classes that are mixed with non-
Enum
类型 (譬如
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
类具有的方法
¶
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']
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
，和
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
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
(见
ssl.Options
for an example of when
KEEP
is needed).
Enums have a custom metaclass that affects many aspects of both derived
Enum
classes and their instances (members).
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__()
).
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
，和
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.
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:
使用实例化的
auto
对于值
使用实例化的
object
作为值
use a descriptive string as the value
use a tuple as the value and a custom
__new__()
to replace the tuple with an
int
值
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.
auto
¶
使用
auto
将看起来像：
>>> class Color(Enum): ... RED = auto() ... BLUE = auto() ... GREEN = auto() ... >>> Color.GREEN <Color.GREEN: 3>
object
¶
使用
object
将看起来像：
>>> 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>
使用字符串作为值将看起来像：
>>> class Color(Enum): ... RED = 'stop' ... GREEN = 'go' ... BLUE = 'too fast!' ... >>> Color.GREEN <Color.GREEN: 'go'>
__new__()
¶
使用自动编号
__new__()
将看起来像：
>>> 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
，添加
*args
到签名：
>>> 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'
注意
__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.
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
引发错误，若发现成员名称重复而不是创建别名：
>>> class DuplicateFreeEnum(Enum): ... def __init__(self, *args): ... cls = self.__class__ ... if any(self.value == e.value for e in cls): ... a = self.name ... e = cls(self.value).name ... raise ValueError( ... "aliases not allowed in DuplicateFreeEnum: %r --> %r" ... % (a, e)) ... >>> class Color(DuplicateFreeEnum): ... RED = 1 ... GREEN = 2 ... BLUE = 3 ... GRENE = 2 ... Traceback (most recent call last): ... ValueError: aliases not allowed in DuplicateFreeEnum: 'GRENE' --> 'GREEN'
注意
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.
若
__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
范例展示
_ignore_
属性的用法：
>>> 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)>]
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.