 2-stream merge
 Read two sorted streams of items from external source (e.g. disk, or network), and write one stream of sorted items to external sink.
 Common algorithm: keep 1 buffered item from each source, select minimal of them, write it, fetch another item from that stream from which the written item was.
 N-stream merge
 The same as above, but reading from    N    sources.
 Common algorithm: same as above, but keep buffered items and their source descriptors in a heap.


Assume streams are very big. You must not suck them whole in the memory, but read them as streams.




import heapq
import sys
 
sources = sys.argv[1:]
for item in heapq.merge(open(source) for source in sources):
    print(item)