What happens when you try to delete a list element while iterating over it

Question

I'm iterating a list as follows:

some_list = [1, 2, 3, 4]
another_list = [1, 2, 3, 4]

for idx, item in enumerate(some_list):
    del some_list[idx]

for item in another_list:
    another_list.remove(item)

When I print out the contents of the lists

>>> some_list
[2, 4]
>>> another_list
[2, 4]

I'm aware that Python doesn't support modifying a list while iterating over it and the right way is to iterate over copy of list instead. But I want to know what exactly happens behind the scenes i.e. Why is the output of the above snippet [2, 4]?

Answer 1

You can use a self-made iterator that shows (in this case prints) the state of the iterator:

class CustomIterator(object):
    def __init__(self, seq):
        self.seq = seq
        self.idx = 0

    def __iter__(self):
        return self

    def __next__(self):
        print('give next element:', self.idx)
        for idx, item in enumerate(self.seq):
            if idx == self.idx:
                print(idx, '--->', item)
            else:
                print(idx, '    ', item)
        try:
            nxtitem = self.seq[self.idx]
        except IndexError:
            raise StopIteration
        self.idx += 1
        return nxtitem

    next = __next__  # py2 compat

Then use it around the list you want to check:

some_list = [1, 2, 3, 4]

for idx, item in enumerate(CustomIterator(some_list)):
    del some_list[idx]

This should illustrate what happens in that case:

give next element: 0
0 ---> 1
1      2
2      3
3      4
give next element: 1
0      2
1 ---> 3
2      4
give next element: 2
0      2
1      4

It only works for sequences though. It's more complicated for mappings or sets.