Python for-in-loop stops iteration on list object created from for-in-loop

Question

[First-ever question from first-ever Stack-o-flow Python beginner user]

I have been trying to create a function that appends numbers 1 to 10 and then erase from 10 to 1 in python list object. I succeeded however, I bumped into a weird behavior of for-in loops.

Below is what I did & succeeded:

def a():
    li = []

    for i in range(1,11):
        li.append(i)
        print(li)

    for n in range(1,11):
        li.remove(li[-1])
        print(li)

a()

would print:

[1]
[1, 2]
[1, 2, 3]
[1, 2, 3, 4]
[1, 2, 3, 4, 5]
[1, 2, 3, 4, 5, 6]
[1, 2, 3, 4, 5, 6, 7]
[1, 2, 3, 4, 5, 6, 7, 8]
[1, 2, 3, 4, 5, 6, 7, 8, 9]
[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
[1, 2, 3, 4, 5, 6, 7, 8, 9]
[1, 2, 3, 4, 5, 6, 7, 8]
[1, 2, 3, 4, 5, 6, 7]
[1, 2, 3, 4, 5, 6]
[1, 2, 3, 4, 5]
[1, 2, 3, 4]
[1, 2, 3]
[1, 2]
[1]
[]

However, if I change the second part of its for-loop range(1,11) to li, the iteration stops as shown below.

def a():
    li = []

    for i in range(1,11):
        li.append(i)
        print(li)

    for n in li:         <-------------- THIS PART
        li.remove(li[-1])
        print(li)

a()

Would print:

[1]
[1, 2]
[1, 2, 3]
[1, 2, 3, 4]
[1, 2, 3, 4, 5]
[1, 2, 3, 4, 5, 6]
[1, 2, 3, 4, 5, 6, 7]
[1, 2, 3, 4, 5, 6, 7, 8]
[1, 2, 3, 4, 5, 6, 7, 8, 9]
[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
[1, 2, 3, 4, 5, 6, 7, 8, 9]
[1, 2, 3, 4, 5, 6, 7, 8]
[1, 2, 3, 4, 5, 6, 7]
[1, 2, 3, 4, 5, 6]
[1, 2, 3, 4, 5]

So, I checked its length of the object li.

def a():
    li = []
    for i in range(1,11):
        li.append(i)
        print(li)

    print("Length before passed: ", len(li))

    for n in li: <------------ THIS
        print("Length BEFORE REMOVE: ", len(li))

        li.remove(li[-1])

        print("Length AFTER REMOVE: ", len(li))
        print(li)
a()

Would print:

[1]
[1, 2]
[1, 2, 3]
[1, 2, 3, 4]
[1, 2, 3, 4, 5]
[1, 2, 3, 4, 5, 6]
[1, 2, 3, 4, 5, 6, 7]
[1, 2, 3, 4, 5, 6, 7, 8]
[1, 2, 3, 4, 5, 6, 7, 8, 9]
[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
Length before passed:  10
Length BEFORE REMOVE:  10
Length AFTER REMOVE:  9
[1, 2, 3, 4, 5, 6, 7, 8, 9]
Length BEFORE REMOVE:  9
Length AFTER REMOVE:  8
[1, 2, 3, 4, 5, 6, 7, 8]
Length BEFORE REMOVE:  8
Length AFTER REMOVE:  7
[1, 2, 3, 4, 5, 6, 7]
Length BEFORE REMOVE:  7
Length AFTER REMOVE:  6
[1, 2, 3, 4, 5, 6]
Length BEFORE REMOVE:  6
Length AFTER REMOVE:  5
[1, 2, 3, 4, 5] <---- And it stops here.

What is going on here? Why would python for-loop stop before its full cycle?

Thank you very much.

Answer 1

In your case, the iteration stops because you're iterating from the start, but you're removing elements from the end at the same time:

• The 1st iteration ends with 9 elements, so the loop proceeds to the 2nd element.
• The 2nd iteration ends with 8 elements, so the loop proceeds to the 3rd element
• ...
• The 5th iteration ends with 5 elements, and there's no 6th element to proceed to and the loop ends.

This is why mutating a list while iterating it is generally inadvisable.