python +=

In [25]: a=[]


In [26]: b=[]


In [27]: id(a)
Out[27]: 51344568

In [29]: a+=b

In [31]: id(a)                         ## id 没变， a +=不会创建新list
Out[31]: 51344568


In [32]: a=a+b                  ## id 改变，    创建了新list


In [33]: id(a)
Out[33]: 51345968