Mutation and Iteration

• avoid mutating a list as you are iterating over it

代码：

def remove_dups(L1,L2):
for e in L1:
if e in L2:
L1.remove(e)

L1=[1,2,4,5]
L2=[2,3,1,6]
remove_dups(L1,L2)

L1

》[2, 4, 5]

L2

》[2, 3, 1, 6]

L1 is [2,4,5], not [4,5] why?
Python uses an internal counter to keep  track of index it is in the loop
mutating changes the list length but Python doesn't update the counter
loop never sees element 2

所以上述代码是有问题的，可改为：
def remove_dups_new(L1,L2):
　　L1_copy = L1[:]
　　for e in L1_copy:
　　　　if e in L2:
　　　　　　L1.remove(e)