Python 从枚举返回None

我有一个简单的枚举：

from enum import Enum


class ColorEnum(Enum):
    RED = 1
    GREEN = 2
    BLUE = 3

    @classmethod
    def _missing_(cls, value):
        if value is None:
            return None
        super()._missing_(value)

正如您所看到的，如果将None传递给ColorEnum init，它将返回None。这适用于Python3.6。但当Python 3.7出现ValueError故障时，它不适用于Python 3.7：

ValueError:None不是有效的ColorEnum

对于Python 3.6，方法

\uuuuu new\uuuu

如下所示：

class Enum(metaclass=EnumMeta):
    """Generic enumeration.

    Derive from this class to define new enumerations.

    """
    def __new__(cls, value):
        # all enum instances are actually created during class construction
        # without calling this method; this method is called by the metaclass'
        # __call__ (i.e. Color(3) ), and by pickle
        if type(value) is cls:
            # For lookups like Color(Color.RED)
            return value
        # by-value search for a matching enum member
        # see if it's in the reverse mapping (for hashable values)
        try:
            if value in cls._value2member_map_:
                return cls._value2member_map_[value]
        except TypeError:
            # not there, now do long search -- O(n) behavior
            for member in cls._member_map_.values():
                if member._value_ == value:
                    return member
        # still not found -- try _missing_ hook
        return cls._missing_(value)

对于Python 3.7：

class Enum(metaclass=EnumMeta):
    """Generic enumeration.

    Derive from this class to define new enumerations.

    """
    def __new__(cls, value):
        # all enum instances are actually created during class construction
        # without calling this method; this method is called by the metaclass'
        # __call__ (i.e. Color(3) ), and by pickle
        if type(value) is cls:
            # For lookups like Color(Color.RED)
            return value
        # by-value search for a matching enum member
        # see if it's in the reverse mapping (for hashable values)
        try:
            return cls._value2member_map_[value]
        except KeyError:
            # Not found, no need to do long O(n) search
            pass
        except TypeError:
            # not there, now do long search -- O(n) behavior
            for member in cls._member_map_.values():
                if member._value_ == value:
                    return member
        # still not found -- try _missing_ hook
        try:
            exc = None
            result = cls._missing_(value)
        except Exception as e:
            exc = e
            result = None
        if isinstance(result, cls):
            return result
        else:
            ve_exc = ValueError("%r is not a valid %s" % (value, cls.__name__))
            if result is None and exc is None:
                raise ve_exc
            elif exc is None:
                exc = TypeError(
                        'error in %s._missing_: returned %r instead of None or a valid member'
                        % (cls.__name__, result)
                        )
            exc.__context__ = ve_exc
            raise exc

---

因此，如果没有传递给init，我希望没有。我如何存档它？

当您实例化一个

Enum

时，您会期望返回一个Enum实例。也就是说，

foo=ColorEnum（bar）

以

foo

作为

None

结尾将是非常令人惊讶的行为。您不应该期望

ColorEnum（…）

返回

None

，您应该直接将

None

分配给

foo

。例如：

foo=bar和ColorEnum（bar）

或

尝试：foo=ColorEnum（bar）除了ValueError:foo=None

。当您实例化

Enum

时，您会期望返回一个Enum实例。也就是说，

foo=ColorEnum（bar）

以

foo

作为

None

结尾将是非常令人惊讶的行为。您不应该期望

ColorEnum（…）

返回

None

，您应该直接将

None

分配给

foo

。例如：

foo=bar和ColorEnum（bar）

或

尝试：foo=ColorEnum（bar），除了value错误：foo=None

。