heapq.merge()

heapq.merge(*iterables, key=None, reverse=False)

Merge multiple sorted inputs into a single sorted output (for example, merge timestamped entries from multiple log files). Returns an iterator over the sorted values.

Similar to sorted(itertools.chain(*iterables)) but returns an iterable, does not pull the data into memory all at once, and assumes that each of the input streams is already sorted (smallest to largest).

Has two optional arguments which must be specified as keyword arguments.

key specifies a key function of one argument that is used to extract a comparison key from each input element. The default value is None (compare the elements directly).

reverse is a boolean value. If set to True, then the input elements are merged as if each comparison were reversed.