I have the following code:
s = [1,2,3]
t = reversed(s)
for i in t:
print(i)
# output: 3,2,1
If I pop one element from s (original), then the t (reversed) is emptied:
s = [1,2,3]
t = reversed(s)
s.pop()
for i in t:
print(i)
# expected output: 2, 1
# actual output (nothing):
Why does this happen?
Taking a look at the cpython code on GitHub, we can get some intuition as to why it no longer works.
The iterator that is returned essentially requires knowing the position of the last index and the length of the array. If the size of the array is changed, the iterator will no longer work.
This will not produce the correct results either, but the iterator does run:
s = [1,2,3]
t = reversed(s)
s.append(4)
for i in t:
print(i)
# output: [3, 2, 1]
s = [1,2,3]
t = reversed(s)
s.pop()
s.append(4)
for i in t:
print(i)
# output: [4, 2, 1]
It still works!
So there's an internal check to see whether or not the last index is still valid, and if it is, it's a simple for loop down to index 0.
If it doesn't work, the iterator returns empty.
line 3339: if (index>=0 && index < PyList_GET_SIZE(seq)) where seq is a pointer to the original list object. If the PyList_GET_SIZE check fails the reference is removed and NULL is returned.
calling reversed return a iterator over that list, which a special object that allow you iterate in reverse order over the original list, is not a new list and is a one time use only
>>> s= [1,2,3]
>>> t = reversed(s)
>>> t
<list_reverseiterator object at 0x00000261BE8F0C40>
>>> list(t)
[3, 2, 1]
>>> list(t)
[]
>>>
and because this iterator reference the original list, any change on it is reflected when you iterate over the iterator later.
Update
In particular and as MZ explain, if that change is such that the state of the list is different from when the iterator was created you get nothing if the size decreases or an incomplete version of the list if increased
>>> s= [1,2,3]
>>> t = reversed(s)
>>> s.insert(0,23)
>>> s
[23, 1, 2, 3]
>>> list(t)
[2, 1, 23]
>>> t = reversed(s)
>>> s.append(32)
>>> list(t)
[3, 2, 1, 23]
>>> s
[23, 1, 2, 3, 32]
>>> t = reversed(s)
>>> s.pop()
32
>>> list(t)
[]
>>>
reversed(s) is basically just a wrapper around s, so it's fast to initialize and doesn't require much memory. But...
reversed(s)is an iterator, not a reversed list.next(t)throws aStopIterationexception, which explains why nothing is printed. But I don't know whylist(t)is[]. I think this is a very good question.tisn't a reversed list, it's basically just a wrapped iterator arounds. Don't modify an iterator before you've consumed it, otherwise you get "surprising" results.