Python 枚举类源码解析

1. enummeta

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