A classic error of python programming reference list

 Consider the following codes

class User:
    def __init__(self, name, hobby=[]):
        self.name = name
        self.hobby = hobby

    def add_hobby(self, hobby):
        self.hobby.append(hobby)

    def remove_hobby(self, hobby):
        self.hobby.remove(hobby)

    def __str__(self):
        return self.name + ',' + '-'.join(self.hobby)


if __name__ == '__main__':
    u1 = User(' Bob ' ) 
    u1.add_hobby ( ' football ' )
     Print (U1)   # Xiaoming, soccer 
    Print (ID (u1.hobby))   # 2074461954696 

    U2 = the User ( ' small strong ' ) 
    u2.add_hobby ( ' beauty ' )
     Print (U2 )   # Xiaoqiang, football - beauty 
    Print (U1)   # Xiao Ming, football - beauty 
    Print (the above mentioned id (u2.hobby))   # 2074461954696

 

 

analysis:

　　We created two User objects are u1, u2, but my hobby when modifying the properties of u2, u1 has caused a change in the properties of the object hobby

　　The fundamental reason is because the creation u1, u2 when the object does not pass this hobby list object, the default on the air, and this air is globally unique, point to the same memory address, so modifications can also cause changes in u2 u1, and

　　Therefore, when we pass the reference list, should special attention. This is a pit.