python - First common element from two lists

Question

x = [8,2,3,4,5]
y = [6,3,7,2,1]

How to find out the first common element in two lists (in this case, "2") in a concise and elegant way? Any list can be empty or there can be no common elements - in this case None is fine.

I need this to show python to someone who is new to it, so the simpler the better.

UPD: the order is not important for my purposes, but let's assume I'm looking for the first element in x that also occurs in y.

Answer 1

This should be straight forward and almost as effective as it gets (for more effective solution check Ashwini Chaudharys answer and for the most effective check jamylaks answer and comments):

result = None
# Go trough one array
for i in x:

    # The element repeats in the other list...
    if i in y:

        # Store the result and break the loop
        result = i
        break

Or event more elegant would be to encapsulate the same functionality to function_{using PEP 8 like coding style conventions}:

def get_first_common_element(x,y):
    ''' Fetches first element from x that is common for both lists
        or return None if no such an element is found.
    '''
    for i in x:
        if i in y:
            return i

    # In case no common element found, you could trigger Exception
    # Or if no common element is _valid_ and common state of your application
    # you could simply return None and test return value
    # raise Exception('No common element found')
    return None

And if you want all common elements you can do it simply like this:

>>> [i for i in x if i in y]
[1, 2, 3]