[ python-Feature Requests-1185121 ] itertools.imerge: merge sequences

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Feature Requests item #1185121, was opened at 2005-04-18 07:11
Message generated for change (Settings changed) made by rhettinger
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>Category: Python Library
>Group: None
Status: Open
Resolution: None
Priority: 5
Submitted By: Jurjen N.E. Bos (jneb)
>Assigned to: Raymond Hettinger (rhettinger)
Summary: itertools.imerge: merge sequences

Initial Comment:
(For the itertools library, so Python 2.2 and up)
This is a suggested addition to itertools, proposed name imerge.
usage: imerge(seq0, seq1, ..., [key=<key function>])
result: imerge assumes the sequences are all in sorted order, and 
produces a iterator that returns pairs of the form (value, index),
where value is a value of one of the sequences, and index is the 
index number of the given sequence.
The output the imerge is in sorted order (taking into account the 
key function), so that identical values in the sequences will be 
produced from left to right.
The code is surprisingly short, making use of the builtin heap 
module.
(You may disagree with my style of argument handling; feel free to 
optimize it.)
def imerge(*iterlist, **key):
	"""Merge a sequence of sorted iterables.
	
	Returns pairs [value, index] where each value comes from 
iterlist[index], and the pairs are sorted
	if each of the iterators is sorted.
	Hint use groupby(imerge(...), operator.itemgetter(0)) to get 
the items one by one.
	"""
	if key.keys() not in ([], ["key"]): raise TypeError, "Excess 
keyword arguments for imerge"
	key = key.get("key", lambda x:x)
	from heapq import heapreplace, heappop
	#initialize the heap containing (inited, value, index, 
currentItem, iterator)
	#this automatically makes sure all iterators are initialized, 
then run, and finally emptied
	heap = [(False, None, index, None, iter(iterator)) for index, 
iterator in enumerate(iterlist)]
	while heap:
		inited, item, index, value, iterator = heap[0]
		if inited: yield value, index
		try: item = iterator.next()
		except StopIteration: heappop(heap)
		else: heapreplace(heap, (True, key(item), index, item, 
iterator))

If you find this little routine worth its size, please put it into 
itertools.

- Jurjen

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