Modify a list while iterating

I know you should not add/remove items while iterating over a list. But can I modify an item in a list I'm iterating over if I do not change the list length?

You're not modifying the list in any way at all. What you are modifying is the elements in the list; That is perfectly fine. As long as you don't directly change the actual list, you're fine.

There's no need to iterate over the indices. In fact, that's unidiomatic. Unless you are actually trying to change the list itself, simply iterate over the list by value.

If the answer is yes, will the following snippet be valid?

lovely_numbers = [[41, 32, 17], [26, 55]]
for numbers_pair in lovely_numbers:
    numbers_pair.pop()
print(lovely_numbers)  # [[41, 32], [26]]

Absolutely. For the exact same reasons as I said above. Your not modifying lovely_numbers itself. Rather, you're only modifying the elements in lovely_numbers.

You are not modifying the list, so to speak. You are simply modifying the elements in the list. I don't believe this is a problem.

To answer your second question, both ways are indeed allowed (as you know, since you ran the code), but it would depend on the situation. Are the contents mutable or immutable?

For example, if you want to add one to every element in a list of integers, this would not work:

>>> x = [1, 2, 3, 4, 5]
>>> for i in x:
...     i += 1
... 
>>> x
[1, 2, 3, 4, 5] 

Indeed, ints are immutable objects. Instead, you'd need to iterate over the indices and change the element at each index, like this:

>>> for i in range(len(x)):
...     x[i] += 1
...
>>> x
[2, 3, 4, 5, 6]

If your items are mutable, then the first method (of directly iterating over the elements rather than the indices) is more efficient without a doubt, because the extra step of indexing is an overhead that can be avoided since those elements are mutable.