How do I remove 'None' items from the end of a list in Python

Discarding from the end of a list is efficient.

while lst[-1] is None:
    del lst[-1]

Add a safeguard for IndexError: pop from empty list if necessary. It depends on your specific application whether proceeding with an empty list should be considered normal or an error condition.

while lst and lst[-1] is None:
    del lst[-1]

If you don't want to modify the list, you can just find the first index from the right that isn't None and slice to it:

def shrink(l):
    for i in range(len(l) - 1, -1, -1):
        if l[i] is not None:
            return l[:i + 1]
    return l[:0]

If you do want to modify the list in-place, you can just delete the slice:

def shrink(l):
    for i in range(len(l) - 1, -1, -1):
        if l[i] is not None:
            break
    else:
        i = -1
    del l[i + 1:]