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#!/usr/bin/env python
from itertools import chain, ifilter, izip_longest
def spreader_generator(blockpool, spread):
"""Returns an iter over an iter of iters, where the inner elements are interleaved at spread intervals.
The tail has no such guarantees, as we're an eager bin packer.
>>> jobs = [["a1", "a2", "a3"], ["b1"], ["c1", "c2", "c3"]]
>>> list(spreader_generator(jobs, 1))
['a1', 'a2', 'a3', 'b1', 'c1', 'c2', 'c3']
>>> list(spreader_generator(jobs, 2))
['a1', 'b1', 'a2', 'c1', 'a3', 'c2', 'c3']
>>> list(spreader_generator(jobs, 3))
['a1', 'b1', 'c1', 'a2', 'c2', 'a3', 'c3']
>>> list(spreader_generator(jobs, 4))
['a1', 'b1', 'c1', 'a2', 'c2', 'a3', 'c3']
"""
# This sentinel object is unique to this function and can't equal anything
# a user can put into the lists, so it's a safe "nothing" value for filler.
sentinel = object()
# We need a real iterator for our feeders to share
blockpool_iter = iter(blockpool)
# The feeders pick off blocks from blockpool_iter lazily and return
# the elements of each group in turn
feeders = [chain.from_iterable(blockpool_iter) for _ in range(spread)]
# Now we'll zip our lazily-distributed spread-wide groups of jobs into stripes
stripes = izip_longest(*feeders, fillvalue=sentinel)
# and return all the values of the stripes in order
flattened_spread = chain.from_iterable(stripes)
# And we won't return the non-values from feeders that get the short straw
not_sentinel = lambda x: x is not sentinel
return ifilter(not_sentinel, flattened_spread)