using bisect on list of tuples but compare using first value only

Question

I read that question about how to use bisect on a list of tuples, and I used that information to answer that question. It works, but I\'d like a more generic so

Answer 1

This is a (quick'n'dirty) bisect_left implementation that allows an arbitrary key function:

def bisect(lst, value, key=None):
    if key is None:
        key = lambda x: x
    def bis(lo, hi=len(lst)):
        while lo < hi:
            mid = (lo + hi) // 2
            if key(lst[mid]) < value:
                lo = mid + 1
            else:
                hi = mid
        return lo
    return bis(0)

> from _operator import itemgetter
> test_array = [(1, 2), (3, 4), (4, 3), (5.2, 6), (5.2, 7000), (5.3, 8), (9, 10)]
> print(bisect(test_array, 5, key=itemgetter(0)))
3

This keeps the O(log_N) performance up since it does not assemble a new list of keys. The implementation of binary search is widely available, but this was taken straight from the bisect_left source. It should also be noted that the list needs to be sorted with regard to the same key function.