关于python：为Enum的子类重载__init __（）

Overload init() for a subclass of Enum

我正试图重载枚举子类的__init__()方法。奇怪的是，使用普通类的模式不再适用于枚举。

下面显示了使用普通类的所需模式：

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class Integer:
def __init__(self, a):
"""Accepts only int"""
assert isinstance(a, int)
self.a = a

def __repr__(self):
return str(self.a)

class RobustInteger(Integer):
def __init__(self, a):
"""Accepts int or str"""
if isinstance(a, str):
super().__init__(int(a))
else:
super().__init__(a)

print(Integer(1))
# 1
print(RobustInteger(1))
# 1
print(RobustInteger('1'))
# 1

如果与枚举一起使用，则相同的模式将中断：

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from enum import Enum
from datetime import date

class WeekDay(Enum):
MONDAY = 0
TUESDAY = 1
WEDNESDAY = 2
THURSDAY = 3
FRIDAY = 4
SATURDAY = 5
SUNDAY = 6

def __init__(self, value):
"""Accepts int or date"""
if isinstance(value, date):
super().__init__(date.weekday())
else:
super().__init__(value)

assert WeekDay(0) == WeekDay.MONDAY
assert WeekDay(date(2019, 4, 3)) == WeekDay.MONDAY
# ---------------------------------------------------------------------------
# TypeError Traceback (most recent call last)
# /path/to/my/test/file.py in <module>()
# 27
# 28
# ---> 29 class WeekDay(Enum):
# 30 MONDAY = 0
# 31 TUESDAY = 1

# /path/to/my/virtualenv/lib/python3.6/enum.py in __new__(metacls, cls, bases, classdict)
# 208 enum_member._name_ = member_name
# 209 enum_member.__objclass__ = enum_class
# --> 210 enum_member.__init__(*args)
# 211 # If another member with the same value was already defined, the
# 212 # new member becomes an alias to the existing one.

# /path/to/my/test/file.py in __init__(self, value)
# 40 super().__init__(date.weekday())
# 41 else:
# ---> 42 super().__init__(value)
# 43
# 44

# TypeError: object.__init__() takes no parameters

号

你必须让_missing_钩子过载。WeekDay的所有实例都是在第一次定义类时创建的；WeekDay(date(...))是一个索引操作而不是创建操作，并且__new__最初查找绑定到整数0到6的预先存在的值。如果不成功，它将调用_missing_，您可以在其中将date对象转换为这样的整数。

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class WeekDay(Enum):
MONDAY = 0
TUESDAY = 1
WEDNESDAY = 2
THURSDAY = 3
FRIDAY = 4
SATURDAY = 5
SUNDAY = 6

@classmethod
def _missing_(cls, value):
if isinstance(value, date):
return cls(value.weekday())
return super()._missing_(value)

几个例子：

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>>> WeekDay(date(2019,3,7))
<WeekDay.THURSDAY: 3>
>>> assert WeekDay(date(2019, 4, 1)) == WeekDay.MONDAY
>>> assert WeekDay(date(2019, 4, 3)) == WeekDay.MONDAY
Traceback (most recent call last):
File"<stdin>", line 1, in <module>
AssertionError

号

(注意：在python 3.6之前，_missing_不可用。)

在3.6之前，您似乎可以覆盖EnumMeta.__call__进行相同的检查，但我不确定这是否会产生意外的副作用。(关于__call__的推理总是让我的头有点转。)

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# Silently convert an instance of datatime.date to a day-of-week
# integer for lookup.
class WeekDayMeta(EnumMeta):
def __call__(cls, value, *args, **kwargs):
if isinstance(value, date):
value = value.weekday())
return super().__call__(value, *args, **kwargs)

class WeekDay(Enum, metaclass=WeekDayMeta):
MONDAY = 0
TUESDAY = 1
WEDNESDAY = 2
THURSDAY = 3
FRIDAY = 4
SATURDAY = 5
SUNDAY = 6

码农家园

关于python：为Enum的子类重载init （）

Overload init() for a subclass of Enum

Overload __init__() for a subclass of Enum

Overload init() for a subclass of Enum