Question

我正在尝试重载枚举的子类的__init__()方法。奇怪的是，与普通类一起使用的模式在Enum中不再起作用。

以下显示了使用普通类的所需模式：

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，则相同的模式会中断：

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

Answer 1

您必须重载_missing_钩子。 WeekDay的所有实例都是在首次定义该类时创建的； WeekDay(date(...))是索引操作，而不是创建操作，__new__最初正在寻找绑定到整数0到6的预先存在的值。否则，它将调用_missing_，其中您可以将date对象转换成这样的整数。

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)

一些例子：

>>> 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

（注意：_missing_在Python 3.6之前不可用。）

在3.6之前，似乎您可以覆盖EnumMeta.__call__进行相同的检查，但是我不确定这是否会产生意外的副作用。（对__call__的回想总是使我的头有些旋转。）

# 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

Answer 2

有一个更好的答案，但是无论如何我都会发布此问题，因为这可能有助于理解该问题。

文档给出了以下提示：

EnumMeta会在创建Enum类本身时全部创建它们，然后放置自定义 new （）以确保没有新的只能通过返回现有成员实例来实例化。

因此，我们必须等待重新定义__new__，直到创建该类。经过一些丑陋的修补，这可以通过测试：

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 

wnew = WeekDay.__new__

def _new(cls, value):
    if isinstance(value, date):
        return wnew(cls, value.weekday()) # not date.weekday()
    else:
        return wnew(cls, value)

WeekDay.__new__ = _new

assert WeekDay(0) == WeekDay.MONDAY
assert WeekDay(date(2019, 3, 4)) == WeekDay.MONDAY # not 2019,4,3

重载init （）以获取Enum的子类

2 个答案:

重载__init __（）以获取Enum的子类

2 个答案:

重载init （）以获取Enum的子类