Python 枚举类源码解析

元类编程，生成类的类，可以动态生成类。
用法： type(name, bases, dict)
name -> 类名： str
bases -> 基类： tuple
dict -> 属性： dict
EnumMeta元类是用于生成Enum类，后续类都继承Enum类。

    class EnumMeta(type):
        def __new__(metacls, cls, bases, classdict):
            # member_type    枚举成员的类型
            # first_enum        第一个继承的类型枚举类
            member_type, first_enum = metacls._get_mixins_(bases)    
            .......

_get_mixins_方法

    @staticmethod
    def _get_mixins_(bases):
        """
        返回用于创建枚举成员的类型，以及第一个继承的类型枚举类。
        """
        # 这里是Enum(metaclass=Enum)时用到的，bases=()
        if not bases:
            return object, Enum

        # 继承自Enum类的子类，由以下方法判断
        member_type = first_enum = None
        for base in bases:
            if  (base is not Enum and
                    issubclass(base, Enum) and
                    base._member_names_):
                raise TypeError("Cannot extend enumerations")
        # base is now the last base in bases
        if not issubclass(base, Enum):
            raise TypeError("new enumerations must be created as "
                    "`ClassName([mixin_type,] enum_type)`")

        # get correct mix-in type (either mix-in type of Enum subclass, or
        # first base if last base is Enum)
        if not issubclass(bases[0], Enum):
            member_type = bases[0]     # first data type
            first_enum = bases[-1]  # enum type
        else:
            for base in bases[0].__mro__:
                # most common: (IntEnum, int, Enum, object)
                # possible:    (<Enum 'AutoIntEnum'>, <Enum 'IntEnum'>,
                #               <class 'int'>, <Enum 'Enum'>,
                #               <class 'object'>)
                if issubclass(base, Enum):
                    if first_enum is None:
                        first_enum = base
                else:
                    if member_type is None:
                        member_type = base

        return member_type, first_enum