为Enum的子类重载\uUu init_Uu（）

Question

我试图重载枚举子类的__init__()方法。奇怪的是，普通类的模式不再适用于Enum。在

下面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，则相同的模式将中断：

^{pr2}$

Answer 1

有一个更好的答案，但我还是把它贴出来，因为它可能有助于理解这个问题。

文件给出了以下提示：

EnumMeta 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.

所以我们必须等待重新定义__new__，直到类被创建。通过一些难看的修补，这通过了测试：

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

Answer 2

必须重载_missing_钩子。WeekDay的所有实例都是在首次定义类时创建的；WeekDay(date(...))是索引操作，而不是创建操作，__new__最初查找绑定到整数0到6的预先存在的值。否则，它将调用_missing_，在这种情况下，您可以将date对象转换为这样一个整数。在

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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（注意：_missing_在python3.6之前不可用。）

---

在3.6之前，您似乎可以重写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__的推理总是让我头晕目眩。）

# 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