python创造一个新的元类，用于给现有的类增加相应的方法

from collections import OrderedDict
def pop_by_index(self,i):
    count=0
    for key in self.keys():
        if count==i:
            mykey=key
        count+=1
    self.pop(mykey,None)
class DelIndexDict(type):
    def __new__(cls,name,bases,attrs):
        attrs['pop_by_index'] = pop_by_index
        return type.__new__(cls,name,bases,attrs)
class DelDict(OrderedDict,metaclass=DelIndexDict):
    pass