为 Enum 的子类重载 __init__()

Question

我正在尝试重载__init__()枚举子类的方法。奇怪的是，适用于普通类的模式不再适用于 Enum。

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 Enum 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

你必须使_missing_钩子过载。的所有实例WeekDay都是在第一次定义类时创建的；WeekDay(date(...))是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。如果失败，它会调用_missing_，您可以在其中将date对象转换为这样的整数。

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)

几个例子：

>>> 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，您似乎可以覆盖EnumMeta.__call__以进行相同的检查，但我不确定这是否会产生意想不到的副作用。（推理__call__总是让我头晕。）

# 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