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
