Overload __init__() for a subclass of Enum

Question

I'm trying to overload the __init__() method of a subclass of an enum. Strangely, the pattern that work with a normal class doesn't work anymore with Enum.

The following show the desired pattern working with a normal class:

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

The same pattern then breaks if used with an Enum:

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

You have to overload the _missing_ hook. All instances of WeekDay are created when the class is first defined; WeekDay(date(...)) is an indexing operation rather than a creation operation, and __new__ is initially looking for pre-existing values bound to the integers 0 to 6. Failing that, it calls _missing_, in which you can convert the date object into such an integer.

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)

A few examples:

>>> 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

(Note: _missing_ is not available prior to Python 3.6.)

---

Prior to 3.6, it seems you can override EnumMeta.__call__ to make the same check, but I'm not sure if this will have unintended side effects. (Reasoning about __call__ always makes my head spin a little.)

# 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

There is a much better answer, but I post this anyway as it might be helpful for understanding the issue.

The docs gives this hint:

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.

So we have to wait with redefining __new__ until the class is created. With some ugly patching this passes the test:

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