为什么即使我在循环中更改长度，Python 的“for”也会迭代列表的长度？

Question

I came across this question using python.

l = [1,2,3,4]

for i in l:
    x = l.pop(0)
    print(x)
    l.insert(0,x)

And the result gives me 1,1,1,1 . I was wondering why this could happen? Shouldn't I have infinite 1's?

Answer 1

There is a point worth to pay attention to. when your for loop start it navigate through your list, starting from the first item which is "1" in your list. it pop ( remove the item) and print the removed item, after this stage you have 2,3.4 in your list. and finally you add the item you have removed back to your list. when the loop execute again it does exactly what did before, in fact your "i" variable in the loop has finite values (1,2,3.4) and the code inside the loop repeat only 4 times!. I hope you get the point but if not let's trace your code as below

i=1   x=1  print(1) l = [1,2,3,4]
i=2   x=1  print(1) l = [1,2,3,4]
i=3   x=1  print(1) l = [1,2,3,4]
i=4   x=1  print(1) l = [1,2,3,4]

add print(l) at the end of your code to see what exactly happens to your list for each "i" in your for loop.