Given two lists with date ranges, return merged list with ranges from higher priority list preferred over ranges from lower priority list in case of intersection. Partial intervals are allowed.

merge_interval

def merge_interval(low_priority_lst,high_priority_lst):
        '''
	Given two lists with date ranges, return merged list with high_priority_lst ranges preferred over
	preferred over low_priority_lst ranges in case of intersection.  Partial intervals are allowed.
	'''
	if low_priority_lst == [] or low_priority_lst == None: return high_priority_lst
	if high_priority_lst == [] or high_priority_lst == None: return low_priority_lst
	
	# k way merge interval, also may need to mark which array it belongs to
	# case :               |-------|
	#        |-------|            
	if low_priority_lst[0][0] > high_priority_lst[0][1]:
	 return [high_priority_lst[0]] + 
		merge_interval(low_priority_lst,high_priority_lst[1:])
	# case :   |-------|
	#                     |-------|  	 
	elif low_priority_lst[0][1] < high_priority_lst[0][0]:
		return [low_priority_lst[0]] + 
			merge_interval(low_priority_lst[1:],high_priority_lst)
	# case :|-------|
	#            |-------|  
	elif low_priority_lst[0][0] < high_priority_lst[0][0]:
		return [[low_priority_lst[0][0],high_priority_lst[0][0]]] + 
			merge_interval([[high_priority_lst[0][0],low_priority_lst[0][1]]] + low_priority_lst[1:],
					high_priority_lst)
	# case :      |-------|
	#        |-------|  
	elif low_priority_lst[0][1] > high_priority_lst[0][1]:
		return [high_priority_lst[0]] + 
			merge_interval([[high_priority_lst[0][1],low_priority_lst[0][1]]] + low_priority_lst[1:] ,
					high_priority_lst[1:])
	# case :  |-------| |---| |----|
	#        |-----------------| 
	else:
		return merge_interval(low_priority_lst[1:],high_priority_lst)