A simple implementation of Merge Ranges in Python, using a for loop

forloop.py

def free_times(calendar_list):
    output = []

    # This is easier with a list sorted by start time
    # sort() is mergesort, with O(n) = n log n
    # which is fast enough to not manually sort.
    calendar_list.sort(key = lambda event: event[0])

    for index in range(len(calendar_list)):
        if (index > 0):
            if (calendar_list[index][0] <= calendar_list[index - 1][1]):
                start = calendar_list[index - 1][0]

                if (calendar_list[index][1] > calendar_list[index - 1][1]):
                    end = calendar_list[index][1]
                else:
                    end = calendar_list[index - 1][1]

                if (len(output) > 0):
                    output[len(output) - 1] = [start, end]
                else:
                    output.append([start, end])

            else:
                output.append(calendar_list[index])

        else:
            output.append(calendar_list[index])

    return output


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

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