Runs the reduce stage of MapReduce.
Merge-reads input files and runs reducer function on them.
Args:
job_name: mapreduce job name as string.
reader_spec: specification of reduce function.
output_writer_spec: specification of output write to use with reduce
function.
params: mapper parameters to use as dict.
filenames: list of filenames to reduce.
combiner_spec: Optional. Specification of a combine function. If not
supplied, no combine step will take place. The combine function takes a
key, list of values and list of previously combined results. It yields
combined values that might be processed by another combiner call, but will
eventually end up in reducer. The combiner output key is assumed to be the
same as the input key.
shards: Optional. Number of output shards. Defaults to the number of
input files.
Returns:
filenames from output writer.