How does python iterator for list actually works?

Question

Let's say we have the following list and we are creating an iterator for it:

lst = [1,2,3]

itr = iter(lst)

Next lets say we are changing our list with completely different values:

lst = ['a', 'b', 'c']

And if I we run the following loop:

for x in itr:

   print x

We will get

'1,2,3'

. But why? As far as I understand, iterator doesn't copy all values from iterating object. At least iterator for list from three elements has the same size as a list of 100000 elements.

sys.getsizeof(i)

returns

64

. How can iterator be so small by size and keep 'old' values of list?

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Answer 1

The iterator itself contains a reference to the list. Since lst is rebound instead of mutated, this reference does not change.

>>> lst = [1, 2, 3]
>>> itr = iter(lst)
>>> lst[:] = ['a', 'b', 'c']
>>> for x in itr:
...   print x
... 
a
b
c

How does python iterator for list actually works?

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