Hi João, ideas like this can also be proposed first on the python-ideas mailing list but as you said in your post there is already a method to do this and it raises ValueError when it is not found which is a common idiom in Python. Other objects don't often have two versions of the same method, one that raises an exception on error and one that returns a sentinel, most only have the one that raises.


Notice that your example is not simpler with your proposal:

# Example driver code:
index = find(list, possible_element)
if index_of_element == -1:
    pass # Not found
else:
    pass # Found


is a hard to read than and actually longer:

try:
    index = list.index(possible_element)
except ValueError:
    index = -1

if you really care about the number of lines you could use, while I don't recommend it:

try: index = list.index(possible_element)
except ValueErrort: index = -1

Also adding a new method to list would mean adding it to all sequence objects and that is asking for a lot.