A simple implementation of Merge Ranges in Python, using reduce

reduce.py

from functools import reduce

def merge_arrays(accumulator, new):
    # Since Python doesn't have multiline lambdas
    # we define our reducer function here

    last = len(accumulator) - 1

    if (new[0] <= accumulator[last][1]):
        start = accumulator[last][0]

        end = new[1] if (new[1] > accumulator[last][1]) else accumulator[last][1]

        accumulator[last] = [start, end]

    else:
        accumulator.append(new)

    return accumulator

def free_times(calendar_list):
    # This is easier with a list sorted by start time
    # sorted() is mergesort, with O(n) = n log n
    # which is fast enough to not manually sort.

    return reduce(
                merge_arrays,
                sorted(calendar_list, key = lambda event: event[0]),
                [calendar_list[0]]
                )


cal = [[3, 4], [9, 12], [1, 3], [6, 8], [9, 10]];

print(free_times(cal)) # [[1, 4], [6, 8], [9, 12]]