python使用技巧

s = 'zhangkaiguozhangxuanzhuangbaosui'

d={}
# for i ,c in enumerate(s):
#     d[c] = d.get(c,[])+[i]
#
# print(d)

#第二种方法 ，效果更好

for i,c in enumerate(s):
    d.setdefault(c,[]).append(i)
print(d)

创建一个向量类

import math

_EPSTION=0.0000001
class Vector:

    def __init__(self,lst:list):
        self._values = list(lst)

    @classmethod
    def zero(cls,dim):
        return cls([0]*dim)

    def norm(self):
        return math.sqrt(sum(e**2 for e in self))

    def normalize(self):
        # return Vector([e/self.norm() for e in self]) 这样self.norm()重复计算
        # return 1/self.norm()*Vector(self._values)
        if self.norm()<_EPSTION:
            raise ZeroDivisionError("除数不能为零")
        return Vector(self._values)/self.norm()
    
    def dot(self,another):
        '''向量点乘,返回结果为标量'''
        assert len(self) == len(another), \
            "Error in adding.length of vectors must be same"
        return sum(a*b for a,b in zip(self,another))
        
    
    def __repr__(self):
        return f"Vector:({self._values})"
    def __str__(self):
        return "({})".format(",".join(str(e) for e in self._values))
    def __getitem__(self, index):
        return self._values[index]

    def __add__(self, other):
        assert len(self)==len(other),\
        "Error in adding.length of vectors must be same"
        # return Vector([a+b for a,b in zip(self._values,other._values)])
        return Vector([a+b for a,b in zip(self,other)]) #返回一个新的向量类

    def __sub__(self, other):
        assert len(self) == len(other), \
            "Error in adding.length of vectors must be same"
        return Vector([a-b for a,b in zip(self,other)])

    def __pos__(self):
        return 1*self
    def __neg__(self):
        return -1*self
    def __mul__(self,k):
        return Vector([k*e for e in self])
    def __rmul__(self, k):
        return self*k
    def __truediv__(self, k):
        return (1/k)*self
    def __iter__(self):
        return self._values.__iter__()

    def __len__(self):
        '''返回向量长度'''
        return len(self._values)



if __name__ == '__main__':
    v=Vector([0,0])
    v2=Vector([3,4])
    print(v.norm())
    print(v2.norm())
    try:
        print(v.normalize().norm())
    except ZeroDivisionError as e:
        print(e)