"""
PROBLEM:

When trying to search the position of an element inside a list, we should use the `in` operator to first check if the element exists, and then use the `index` method to obtain the index.

`in` (__contains__) runs a linear search to return the boolean.
`index`        also runs a linear search to return the index.

This makes the code slower, because we need to search for the same item twice.


FEATURE PROPOSAL:

Similar to str.find(), list.find() should be implemented, where -1 is returned when the element isn't present
"""


# Since there's no list.find(), this is my workaround to achieve making only one linear search per query
def find(container: list, index: int) -> int:
    """ Str.find() behavior but for lists """
    try:
        return container.index(index)
    except ValueError:
        return -1

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

This is my first issue, is this the right place to discuss enhancements?

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.

Please take this to Python ideas.  My understanding is that index() is supposed to be the one-way-to-do-it going forward.  The find() API proved to be problematic because -1 is a valid index and people were hitting bugs by failing to check for the -1 and then doing an unintended s[-1].

Thanks for the replies!.

Here's the link for discussion in the Python Ideas:
https://discuss.python.org/t/adding-the-method-find-to-list/4113