枚举类型可以看作是一种标签或是一系列常量的集合,通常用于表示某些特定的有限集合,例如星期、月份、状态等。Python 的原生类型(Built-in types)里并没有专门的枚举类型,但是我们可以通过很多方法来实现它,例如字典、类等:
WEEKDAY = { 'MON': 1, 'TUS': 2, 'WEN': 3, 'THU': 4, 'FRI': 5 } class Color: RED = 0 GREEN = 1 BLUE = 2 复制代码
上面两种方法可以看做是简单的枚举类型的实现,如果只在局部范围内用到这样的枚举变量是没有问题的,但问题在于它们都是可变的(mutable),也就是说可以在其它地方被修改,从而影响其正常使用:
WEEKDAY['MON'] = WEEKDAY['FRI'] print(WEEKDAY) # {'FRI': 5, 'TUS': 2, 'MON': 5, 'WEN': 3, 'THU': 4} 复制代码
通过类定义的枚举甚至可以实例化,变得不伦不类:
c = Color() print(c.RED) # 0 Color.RED = 2 print(c.RED) # 2 复制代码
当然也可以使用不可变类型(immutable),例如元组,但是这样就失去了枚举类型的本意,将标签退化为无意义的变量:
COLOR = ('R', 'G', 'B') print(COLOR[0], COLOR[1], COLOR[2]) # R G B 复制代码
为了提供更好的解决方案,Python 通过 PEP 435 在 3.4 版本中添加了 enum 标准库,3.4 之前的版本也可以通过 pip install enum
下载兼容支持的库。enum
提供了 Enum
/IntEnum
/unique
三个工具,用法也非常简单,可以通过继承 Enum
/IntEnum
定义枚举类型,其中 IntEnum
限定枚举成员必须为(或可以转化为)整数类型,而 unique
方法可以作为修饰器限定枚举成员的值不可重复。
创建枚举
通过子类化 enum 类来定义枚举,代码如下:
import enum class BugStatus(enum.Enum): new = 7 incomplete = 6 invalid = 5 wont_fix = 4 in_progress = 3 fix_committed = 2 fix_released = 1 print('\nMember name: {}'.format(BugStatus.wont_fix.name)) print('Member value: {}'.format(BugStatus.wont_fix.value)) # output # Member name: wont_fix # Member value: 4 复制代码
在解析 Enum 类时,会将每个成员转换成实例,每个实例都有 name 和 value 属性,分别对应成员的名称和值。
迭代枚举
直接看代码:
import enum class BugStatus(enum.Enum): new = 7 incomplete = 6 invalid = 5 wont_fix = 4 in_progress = 3 fix_committed = 2 fix_released = 1 for status in BugStatus: print('{:15} = {}'.format(status.name, status.value)) # output # new = 7 # incomplete = 6 # invalid = 5 # wont_fix = 4 # in_progress = 3 # fix_committed = 2 # fix_released = 1 复制代码
成员按照在类中的定义顺序生成。
比较枚举
由于枚举成员未被排序,因此它们仅支持通过 is
和 ==
进行比较。
import enum class BugStatus(enum.Enum): new = 7 incomplete = 6 invalid = 5 wont_fix = 4 in_progress = 3 fix_committed = 2 fix_released = 1 actual_state = BugStatus.wont_fix desired_state = BugStatus.fix_released print('Equality:', actual_state == desired_state, actual_state == BugStatus.wont_fix) print('Identity:', actual_state is desired_state, actual_state is BugStatus.wont_fix) print('Ordered by value:') try: print('\n'.join(' ' + s.name for s in sorted(BugStatus))) except TypeError as err: print(' Cannot sort: {}'.format(err)) # output # Equality: False True # Identity: False True # Ordered by value: # Cannot sort: '<' not supported between instances of 'BugStatus' and 'BugStatus' 复制代码
大小比较引发 TypeError
异常。
继承 IntEnum
类创建的枚举类,成员间支持大小比较,代码如下:
import enum class BugStatus(enum.IntEnum): new = 7 incomplete = 6 invalid = 5 wont_fix = 4 in_progress = 3 fix_committed = 2 fix_released = 1 print('Ordered by value:') print('\n'.join(' ' + s.name for s in sorted(BugStatus))) # output # Ordered by value: # fix_released # fix_committed # in_progress # wont_fix # invalid # incomplete # new 复制代码
唯一枚举值
具有相同值的枚举成员将作为对同一成员对象的别名引用,在迭代过程中,不会被打印出来。
import enum class BugStatus(enum.Enum): new = 7 incomplete = 6 invalid = 5 wont_fix = 4 in_progress = 3 fix_committed = 2 fix_released = 1 by_design = 4 closed = 1 for status in BugStatus: print('{:15} = {}'.format(status.name, status.value)) print('\nSame: by_design is wont_fix: ', BugStatus.by_design is BugStatus.wont_fix) print('Same: closed is fix_released: ', BugStatus.closed is BugStatus.fix_released) # output # new = 7 # incomplete = 6 # invalid = 5 # wont_fix = 4 # in_progress = 3 # fix_committed = 2 # fix_released = 1 # # Same: by_design is wont_fix: True # Same: closed is fix_released: True 复制代码
因为 by_design 和 closed 是其他成员的别名,所以没有被打印。在枚举中,第一个出现的值是有效的。
如果想让每一个成员都有唯一值,可以使用 @unique
装饰器。
import enum @enum.unique class BugStatus(enum.Enum): new = 7 incomplete = 6 invalid = 5 wont_fix = 4 in_progress = 3 fix_committed = 2 fix_released = 1 # This will trigger an error with unique applied. by_design = 4 closed = 1 # output # Traceback (most recent call last): # File "enum_unique_enforce.py", line 11, in <module> # class BugStatus(enum.Enum): # File ".../lib/python3.6/enum.py", line 834, in unique # (enumeration, alias_details)) # ValueError: duplicate values found in <enum 'BugStatus'>: # by_design -> wont_fix, closed -> fix_released 复制代码
如果成员中有重复值,会有 ValueError
的报错。
以编程方式创建枚举
在一些情况下,通过编程的方式创建枚举,比直接在类中硬编码更方便。如果采用这种方式,还可以传递成员的 name 和 value 到类的构造函数。
import enum BugStatus = enum.Enum( value='BugStatus', names=('fix_released fix_committed in_progress ' 'wont_fix invalid incomplete new'), ) print('Member: {}'.format(BugStatus.new)) print('\nAll members:') for status in BugStatus: print('{:15} = {}'.format(status.name, status.value)) # output # Member: BugStatus.new # # All members: # fix_released = 1 # fix_committed = 2 # in_progress = 3 # wont_fix = 4 # invalid = 5 # incomplete = 6 # new = 7 复制代码
参数 value
代表枚举的名称,names
表示成员。如果给 name 传递的参数是字符串,那么会对这个字符串从空格和逗号处进行拆分,将拆分后的单个字符串作为成员的名称,然后再对其赋值,从 1 开始,以此类推。
为了更好地控制与成员关联的值, names
可以使用元组或将名称映射到值的字典替换字符串。什么意思,看下面的代码:
import enum BugStatus = enum.Enum( value='BugStatus', names=[ ('new', 7), ('incomplete', 6), ('invalid', 5), ('wont_fix', 4), ('in_progress', 3), ('fix_committed', 2), ('fix_released', 1), ], ) print('All members:') for status in BugStatus: print('{:15} = {}'.format(status.name, status.value)) # output # All members: # new = 7 # incomplete = 6 # invalid = 5 # wont_fix = 4 # in_progress = 3 # fix_committed = 2 # fix_released = 1 复制代码
在这里例子中,names
是一个列表,列表中的元素是元组。
非整数成员值
枚举成员值不限于整数。实际上,任何类型的对象都可以作为枚举值。如果值是元组,则成员将作为单独的参数传递给__init__()
。
import enum class BugStatus(enum.Enum): new = (7, ['incomplete', 'invalid', 'wont_fix', 'in_progress']) incomplete = (6, ['new', 'wont_fix']) invalid = (5, ['new']) wont_fix = (4, ['new']) in_progress = (3, ['new', 'fix_committed']) fix_committed = (2, ['in_progress', 'fix_released']) fix_released = (1, ['new']) def __init__(self, num, transitions): self.num = num self.transitions = transitions def can_transition(self, new_state): return new_state.name in self.transitions print('Name:', BugStatus.in_progress) print('Value:', BugStatus.in_progress.value) print('Custom attribute:', BugStatus.in_progress.transitions) print('Using attribute:', BugStatus.in_progress.can_transition(BugStatus.new)) # output # Name: BugStatus.in_progress # Value: (3, ['new', 'fix_committed']) # Custom attribute: ['new', 'fix_committed'] # Using attribute: True 复制代码
在此示例中,每个成员值是一个元组,其中包含数字和列表。
对于更复杂的情况,元组可能就不那么方便了。由于成员值可以是任何类型的对象,因此如果有大量需要键值对数据结构的枚举值场景,字典就派上用场了。
import enum class BugStatus(enum.Enum): new = { 'num': 7, 'transitions': [ 'incomplete', 'invalid', 'wont_fix', 'in_progress', ], } incomplete = { 'num': 6, 'transitions': ['new', 'wont_fix'], } invalid = { 'num': 5, 'transitions': ['new'], } wont_fix = { 'num': 4, 'transitions': ['new'], } in_progress = { 'num': 3, 'transitions': ['new', 'fix_committed'], } fix_committed = { 'num': 2, 'transitions': ['in_progress', 'fix_released'], } fix_released = { 'num': 1, 'transitions': ['new'], } def __init__(self, vals): self.num = vals['num'] self.transitions = vals['transitions'] def can_transition(self, new_state): return new_state.name in self.transitions print('Name:', BugStatus.in_progress) print('Value:', BugStatus.in_progress.value) print('Custom attribute:', BugStatus.in_progress.transitions) print('Using attribute:', BugStatus.in_progress.can_transition(BugStatus.new)) # output # Name: BugStatus.in_progress # Value: (3, ['new', 'fix_committed']) # Custom attribute: ['new', 'fix_committed'] # Using attribute: True 复制代码
这个例子和上面用元组是等价的。