Simple scheduling algorithm in python

schedule.py

import heapq

def schedule(tasks):
    heap = []
    def offer(begin, end):
        while heap and heap[0] <= begin:
            heapq.heappop(heap)
        heapq.heappush(heap, end)
        return len(heap)
    
    return max(offer(begin, end) for begin, end in sorted(tasks))

print schedule([(12,13), (13,15), (17,20), (13,14), (19, 21), (18, 20)])