How does Python manage a 'for' loop internally?

Question

I\'m trying to learn Python, and I started to play with some code:

a = [3,4,5,6,7]
for b in a:
    print(a)
    a.pop(0)
<         


        

Answer 1

kjaquier and Felix have talked about the iterator protocol, and we can see it in action in your case:

>>> L = [1, 2, 3]
>>> iterator = iter(L)
>>> iterator

>>> next(iterator)
1
>>> L.pop()
3
>>> L
[1, 2]
>>> next(iterator)
2
>>> next(iterator)
Traceback (most recent call last):
  File "", line 1, in 
StopIteration

From this we can infer that list_iterator.__next__ has code that behaves something like:

if self.i < len(self.list):
    return self.list[i]
raise StopIteration

It does not naively get the item. That would raise an IndexError which would bubble to the top:

class FakeList(object):
    def __iter__(self):
        return self

    def __next__(self):
        raise IndexError

for i in FakeList():  # Raises `IndexError` immediately with a traceback and all
    print(i)

Indeed, looking at listiter_next in the CPython source (thanks Brian Rodriguez):

if (it->it_index < PyList_GET_SIZE(seq)) {
    item = PyList_GET_ITEM(seq, it->it_index);
    ++it->it_index;
    Py_INCREF(item);
    return item;
}

Py_DECREF(seq);
it->it_seq = NULL;
return NULL;

Although I don't know how return NULL; eventually translates into a StopIteration.