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

We can easily see the sequence of events by using a little helper function foo:

def foo():
    for i in l:
        l.pop()

and dis.dis(foo) to see the Python byte-code generated. Snipping away the not-so-relevant opcodes, your loop does the following:

          2 LOAD_GLOBAL              0 (l)
          4 GET_ITER
    >>    6 FOR_ITER                12 (to 20)
          8 STORE_FAST               0 (i)

         10 LOAD_GLOBAL              0 (l)
         12 LOAD_ATTR                1 (pop)
         14 CALL_FUNCTION            0
         16 POP_TOP
         18 JUMP_ABSOLUTE            6

That is, it get's the iter for the given object (iter(l) a specialized iterator object for lists) and loops until FOR_ITER signals that it's time to stop. Adding the juicy parts, here's what FOR_ITER does:

PyObject *next = (*iter->ob_type->tp_iternext)(iter);

which essentially is:

list_iterator.__next__()

this (finally^*) goes through to listiter_next which performs the index check as @Alex using the original sequence l during the check.

if (it->it_index < PyList_GET_SIZE(seq))

when this fails, NULL is returned which signals that the iteration has finished. In the meantime a StopIteration exception is set which is silently suppressed in the FOR_ITER op-code code:

if (!PyErr_ExceptionMatches(PyExc_StopIteration))
    goto error;
else if (tstate->c_tracefunc != NULL)
    call_exc_trace(tstate->c_tracefunc, tstate->c_traceobj, tstate, f);
PyErr_Clear();  /* My comment: Suppress it! */

so whether you change the list or not, the check in listiter_next will ultimately fail and do the same thing.

^{*For anyone wondering, listiter_next is a descriptor so there's a little function wrapping it. In this specific case, that function is wrap_next which makes sure to set PyExc_StopIteration as an exception when listiter_next returns NULL.}