a,b = c,f(a)

当写形如a,b = c,f(a)时,注意,b=f(a)用的a依然是a=c之前的那个a,类似C语言的a++,而不是++a
举例:

>>> a = 3
>>> a,b = 100,a+1
>>> b
4
>>> a
100
>>> a = 3
>>> a = 100
>>> b = a+1
>>> a
100
>>> b
101