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Python native implementation of the Spark RDD interface.

Project description

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pysparkling

A native Python implementation of Spark’s RDD interface. The primary objective is not to have RDDs that are resilient and distributed, but to remove the dependency on the JVM and Hadoop. The focus is on having a lightweight and fast implementation for small datasets. It is a drop-in replacement for PySpark’s SparkContext and RDD.

Use case: you have a pipeline that processes 100k input documents and converts them to normalized features. They are used to train a local scikit-learn classifier. The preprocessing is perfect for a full Spark task. Now, you want to use this trained classifier in an API endpoint. You need the same pre-processing pipeline for a single document per API call. This does not have to be done in parallel, but there should be only a small overhead in initialization and preferably no dependency on the JVM. This is what pysparkling is for.

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Install

pip install pysparkling

Features

  • Supports multiple URI scheme: s3://, http:// and file://. Specify multiple files separated by comma. Resolves * and ? wildcards.
  • Handles .gz and .bz2 compressed files.
  • Parallelization via multiprocessing.Pool, concurrent.futures.ThreadPoolExecutor or any other Pool-like objects that have a map(func, iterable) method.
  • only dependencies: boto for AWS S3 and requests for http

The change log is in HISTORY.rst.

Examples

Word Count

from pysparkling import Context

counts = Context().textFile(
    'README.rst'
).map(
    lambda line: ''.join(ch if ch.isalnum() else ' ' for ch in line)
).flatMap(
    lambda line: line.split(' ')
).map(
    lambda word: (word, 1)
).reduceByKey(
    lambda a, b: a + b
)
print(counts.collect())

which prints a long list of pairs of words and their counts. This and a few more advanced examples are demoed in docs/demo.ipynb.

API

A usual pysparkling session starts with either parallelizing a list or by reading data from a file using the methods Context.parallelize(my_list) or Context.textFile("path/to/textfile.txt"). These two methods return an RDD which can then be processed with the methods below.

RDD

  • aggregate(zeroValue, seqOp, combOp): aggregate value in partition with seqOp and combine with combOp
  • aggregateByKey(zeroValue, seqFunc, combFunc): aggregate by key
  • cache(): synonym for persist()
  • cartesian(other): cartesian product
  • coalesce(): do nothing
  • collect(): return the underlying list
  • count(): get length of internal list
  • countApprox(): same as count()
  • countByKey: input is list of pairs, returns a dictionary
  • countByValue: input is a list, returns a dictionary
  • context(): return the context
  • distinct(): returns a new RDD containing the distinct elements
  • filter(func): return new RDD filtered with func
  • first(): return first element
  • flatMap(func): return a new RDD of a flattened map
  • flatMapValues(func): return new RDD
  • fold(zeroValue, op): aggregate elements
  • foldByKey(zeroValue, op): aggregate elements by key
  • foreach(func): apply func to every element
  • foreachPartition(func): apply func to every partition
  • getNumPartitions(): number of partitions
  • getPartitions(): returns an iterator over the partitions
  • groupBy(func): group by the output of func
  • groupByKey(): group by key where the RDD is of type [(key, value), …]
  • histogram(buckets): buckets can be a list or an int
  • id(): currently just returns None
  • intersection(other): return a new RDD with the intersection
  • isCheckpointed(): returns False
  • join(other): join
  • keyBy(func): creates tuple in new RDD
  • keys(): returns the keys of tuples in new RDD
  • leftOuterJoin(other): left outer join
  • lookup(key): return list of values for this key
  • map(func): apply func to every element and return a new RDD
  • mapPartitions(func): apply f to entire partitions
  • mapValues(func): apply func to value in (key, value) pairs and return a new RDD
  • max(): get the maximum element
  • mean(): mean
  • min(): get the minimum element
  • name(): RDD’s name
  • persist(): caches outputs of previous operations (previous steps are still executed lazily)
  • pipe(command): pipe the elements through an external command line tool
  • reduce(): reduce
  • reduceByKey(): reduce by key and return the new RDD
  • repartition(numPartitions): repartition
  • rightOuterJoin(other): right outer join
  • sample(withReplacement, fraction, seed=None): sample from the RDD
  • sampleStdev(): sample standard deviation
  • sampleVariance(): sample variance
  • saveAsTextFile(path): save RDD as text file
  • stats(): return a StatCounter
  • stdev(): standard deviation
  • subtract(other): return a new RDD without the elements in other
  • sum(): sum
  • take(n): get the first n elements
  • takeSample(n): get n random samples
  • toLocalIterator(): get a local iterator
  • union(other): form union
  • variance(): variance
  • zip(other): other has to have the same length
  • zipWithUniqueId(): pairs each element with a unique index

Context

A Context describes the setup. Instantiating a Context with the default arguments using Context() is the most lightweight setup. All data is just in the local thread and is never serialized or deserialized.

If you want to process the data in parallel, you can use the multiprocessing module. Given the limitations of the default pickle serializer, you can specify to serialize all methods with dill instead. For example, a common instantiation with multiprocessing looks like this:

c = Context(
    multiprocessing.Pool(4),
    serializer=dill.dumps,
    deserializer=dill.loads,
)

This assumes that your data is serializable with pickle which is generally faster than dill. You can also specify a custom serializer/deserializer for data.

  • __init__(pool=None, serializer=None, deserializer=None, data_serializer=None, data_deserializer=None): pool is any instance with a map(func, iterator) method
  • broadcast(var): returns an instance of Broadcast(). Access its value with value.
  • newRddId(): incrementing number [internal use]
  • parallelize(list_or_iterator, numPartitions): returns a new RDD
  • textFile(filename): load every line of a text file into an RDD filename can contain a comma separated list of many files, ? and * wildcards, file paths on S3 (s3://bucket_name/filename.txt) and local file paths (relative/path/my_text.txt, /absolut/path/my_text.txt or file:///absolute/file/path.txt). If the filename points to a folder containing part* files, those are resolved.
  • version: the version of pysparkling

fileio

The functionality provided by this module is used in Context.textFile() for reading and in RDD.saveAsTextFile() for writing. You can use this submodule for writing files directly with File(filename).dump(some_data), File(filename).load() and File.exists(path) to read, write and check for existance of a file. All methods transparently handle http://, s3:// and file:// locations and compression/decompression of .gz and .bz2 files.

Use environment variables AWS_SECRET_ACCESS_KEY and AWS_ACCESS_KEY_ID for auth and use file paths of the form s3://bucket_name/filename.txt.

  • File:
    • __init__(filename): filename is a URI of a file (can include http://, s3:// and file:// schemes)
    • dump(stream): write the stream to the file
    • [static] exists(path): check for existance of path
    • load(): return the contents as BytesIO
    • make_public(recursive=False): only for files on S3
    • [static] resolve_filenames(expr): given an expression with * and ? wildcard characters, get a list of all matching filenames. Multiple expressions separated by , can also be specified. Spark-style partitioned datasets (folders containing part-* files) are resolved as well to a list of the individual files.

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