Source code for toolz.sandbox.parallel

from toolz.itertoolz import partition_all
from toolz.compatibility import reduce, map
from toolz.utils import no_default


[docs]def fold(binop, seq, default=no_default, map=map, chunksize=128, combine=None): """ Reduce without guarantee of ordered reduction. inputs: ``binop`` - associative operator. The associative property allows us to leverage a parallel map to perform reductions in parallel. ``seq`` - a sequence to be aggregated ``default`` - an identity element like 0 for ``add`` or 1 for mul ``map`` - an implementation of ``map``. This may be parallel and determines how work is distributed. ``chunksize`` - Number of elements of ``seq`` that should be handled within a single function call ``combine`` - Binary operator to combine two intermediate results. If ``binop`` is of type (total, item) -> total then ``combine`` is of type (total, total) -> total Defaults to ``binop`` for common case of operators like add Fold chunks up the collection into blocks of size ``chunksize`` and then feeds each of these to calls to ``reduce``. This work is distributed with a call to ``map``, gathered back and then refolded to finish the computation. In this way ``fold`` specifies only how to chunk up data but leaves the distribution of this work to an externally provided ``map`` function. This function can be sequential or rely on multithreading, multiprocessing, or even distributed solutions. If ``map`` intends to serialize functions it should be prepared to accept and serialize lambdas. Note that the standard ``pickle`` module fails here. Example ------- >>> # Provide a parallel map to accomplish a parallel sum >>> from operator import add >>> fold(add, [1, 2, 3, 4], chunksize=2, map=map) 10 """ if combine is None: combine = binop chunks = partition_all(chunksize, seq) # Evaluate sequence in chunks via map if default == no_default: results = map(lambda chunk: reduce(binop, chunk), chunks) else: results = map(lambda chunk: reduce(binop, chunk, default), chunks) results = list(results) # TODO: Support complete laziness if len(results) == 1: # Return completed result return results[0] else: # Recurse to reaggregate intermediate results return fold(combine, results, map=map, chunksize=chunksize)