Does for loop call iter?

Question

Look at below sample:

a = [1, 2, 3, 4]
 
for i in a:
    print(a)

a is the list (iterable) not the iterator.

I'm not asking to know that __iter__ or iter() convert list to iterator!

I'm asking to know if for loop itself convert list implicitly then call __iter__ for iteration keeping list without removing like iterator?

Since stackoverflow identified my question as possible duplicate: The unique part is that I'm not asking about for loop as concept nor __iter__, I'm asking about the core mechanism of for loop and relationship with iter.

I'm not sure I understand "I'm asking to know if for loop itself convert list implicitly then call iter for iteration keeping list without removing like iterator?" What do you mean? — MSeifert, Apr 20 '17 at 12:21
I dont follow your question, you want to know how the for loop iterate through list? — danglingpointer, Apr 20 '17 at 12:22
@LethalProgrammer Yes. And if there is any relation with __iter__ implicitly. — anati, Apr 20 '17 at 12:23
Possible duplicate of [What exactly is Python's iterator protocol?](http://stackoverflow.com/questions/16301253/what-exactly-is-pythons-iterator-protocol) — Mad Physicist, Apr 20 '17 at 12:23
or read this: http://www.bogotobogo.com/python/python_iterators.php — hiro protagonist, Apr 20 '17 at 12:26
@anati. I am not sure what you found inappropriate in my comment. The only thing I can imagine is the word "dupe", which is a popular colloquialism for "duplicate" on this site. — Mad Physicist, Apr 20 '17 at 12:31

timgeb · Accepted Answer · 2017-04-20T12:31:55.120

I'm asking to know if for loop itself convert list implicitly then call iter for iteration keeping list without removing like iterator?

The for loop does not convert the list implicitly in the sense that it mutates the list, but it implicitly creates an iterator from the list. The list itself will not change state during iteration, but the created iterator will.

a = [1, 2, 3]
for x in a:
    print(x)

is equivalent to

a = [1, 2, 3]
it = iter(a) # calls a.__iter__
while True:
    try:
        x = next(it)
    except StopIteration:
        break
    print(x)

Here's proof that __iter__ actually gets called:

import random

class DemoIterable(object):
    def __iter__(self):
        print('__iter__ called')
        return DemoIterator()

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

    def __next__(self):
        print('__next__ called')
        r = random.randint(1, 10)
        if r == 5:
            print('raising StopIteration')
            raise StopIteration
        return r

Iteration over a DemoIterable:

>>> di = DemoIterable()
>>> for x in di:
...     print(x)
...
__iter__ called
__next__ called
9
__next__ called
8
__next__ called
10
__next__ called
3
__next__ called
10
__next__ called
raising StopIteration

Does for loop call __iter__?

1 Answers1

Does for loop call iter?