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

Answer 1

Just for fun (probably not efficient), another version using itertools:

from itertools import dropwhile, product
from operator import __ne__

def accept_pair(f):
    "Make a version of f that takes a pair instead of 2 arguments."
    def accepting_pair(pair):
        return f(*pair)
    return accepting_pair

def get_first_common(x, y):
    try:
        # I think this *_ unpacking syntax works only in Python 3
        ((first_common, _), *_) = dropwhile(
            accept_pair(__ne__),
            product(x, y))
    except ValueError:
        return None
    return first_common

x = [8, 2, 3, 4, 5]
y = [6, 3, 7, 2, 1]
print(get_first_common(x, y))  # 2
y = [6, 7, 1]
print(get_first_common(x, y))  # None

It is simpler, but not as fun, to use lambda pair: pair[0] != pair[1] instead of accept_pair(__ne__).