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General purpose tools for sorting with limited memory.

Project description

Experimental in-memory Pythonic MapReduce inspired by Spotify’s luigi framework.

The Word Count Example

Currently the only MapReduce implementation is in-memory and serial, but an implementation with parallelized map and reduce phases will be added.

from collections import Counter
import json
import re
import sys

from tinymr.memory import MemMapReduce

class WordCount(MemMapReduce):

    The go-to MapReduce example.

    Don't worry, a better example is on its way:

    # Counting word occurrence does not benefit from sorting post-map or
    # post-reduce and our `final_reducer()` doesn't care about key order
    # so we can disable sorting for a speed boost.
    sort_map = False
    sort_reduce = False
    sort_final_reduce = False

    def __init__(self):

        Stash a regex to strip off punctuation so we can grab it later.

        self.pattern = re.compile('[\W_]+')

    def mapper(self, line):

        Take a line of text from the input file and figure out how many
        times each word appears.

        An alternative, simpler implementation would be:

            def mapper(self, item):
                for word in item.split():
                    word = self.pattern.sub('', word)
                    if word:
                        yield word.lower(), 1

        This simpler example is more straightforward, but holds more data
        in-memory.  The implementation below does more work per line but
        potentially has a smaller memory footprint.  Like anything
        MapReduce the implementation benefits greatly from knowing a lot
        about the input data.

        # Normalize all words to lowercase
        line = line.lower().strip()

        # Strip off punctuation
        line = [self.pattern.sub('', word) for word in line]

        # Filter out empty strings
        line = filter(lambda x: x != '', line)

        # Return an iterable of `(word, count)` pairs
        return Counter(line).items()

    def reducer(self, key, values):

        Just sum the number of times each word appears in the entire

        At this phase `key` is a word and `values` is an iterable producing
        all of the values for that word from the map phase.  Something like:

            key = 'the'
            values = (1, 1, 2, 2, 1)

        The word `the` appeared once on 3 lines and twice on two lines for
        a total of `7` occurrences, so we yield:

            ('the', 7)

        yield key, sum(values)

    def output(self, pairs):

        Normally this phase is where the final dataset is written to disk,
        but since we're operating in-memory we just want to re-structure as
        a dictionary.

        `pairs` is an iterator producing `(key, iter(values))` tuples from
        the reduce phase, and since we know that we only produced one key
        from each reduce we want to extract it for easier access later.

        return {k: tuple(v)[0] for k, v in pairs}

wc = WordCount()
with open('LICENSE.txt') as f:
    out = wc(f)
    print(json.dumps(out, indent=4, sort_keys=True))

Truncated output:

    "a": 1,
    "above": 2,
    "advised": 1,
    "all": 1,
    "and": 8,
    "andor": 1

Word Count Workflow

Internally, the workflow looks like this:

Input data:

$ head -10 LICENSE.txt

New BSD License

Copyright (c) 2015, Kevin D. Wurster
All rights reserved.

Redistribution and use in source and binary forms, with or without
modification, are permitted provided that the following conditions are met:

* Redistributions of source code must retain the above copyright notice, this
  list of conditions and the following disclaimer.


Count occurrences of each word in every line.

# Input line
line = 'Copyright (c) 2015, Kevin D. Wurster'

# Sanitized words
words = ['Copyright', 'c', '2015', 'Kevin', 'D', 'Wurster']

# Return tuples with word as the first element and count as the second
pairs = [('Copyright', 1), ('c', 1), ('2015', 1), ('Kevin', 1), ('D', 1), ('Wurster', 1)]


Organize all of the (word, count) pairs by word. The word keys are kept at this point in case the data is sorted. Sorting grabs the second to last key, so the data could be partitioned on one key and sorted on another with (word, sort, count). The second to last key is used for sorting so the keys that appear below match the word only because a sort key was not given.

Words that appear in the input text on multiple lines have multiple (word, count) pairs. A count of 2 would indicate a word that appeared twice on a single line, but our input data does not have this condition. Truncated output below. The dictionary values are lists containing tuples to allow for a sort key, which is explained elsewhere.

    '2015': [(1,)]
    'above': [(1,)]
    'all': [(1,)]
    'and': [(1,), (1,), (1,)]
    'are': [(1,), (1,)]
    'binary': [(1,)]
    'bsd': [(1,)]
    'c': [(1,)]
    'code': [(1,)]


Sum count for each word.

# The ``reducer()`` receives a key and an iterator of values
key = 'the'
values = (1, 1, 1)
def reducer(key, values):
    yield key, sum(values)


The reducer does not _have_ to produces the same key it was given, so the data is partitioned by key again, which is superfluous for this wordcount example. Again the keys are kept in case the data is sorted and only match word because an optional sort key was not given. Truncated output below.

    '2015': [(1,)]
    'above': [(1,)]
    'all': [(1,)]
    'and': [(3,)]
    'are': [(2,)]
    'binary': [(1,)]
    'bsd': [(1,)]
    'c': [(1,)]
    'code': [(1,)]


The default implementation is to return (key, iter(values)) pairs from the final_reducer(), which would look something like:

values = [
    ('the', (3,)),
    ('in', (1,),

But a dictionary is much more useful, and we know that we only got a single value for each word in the reduce phase, so we can extract that value and produce a dictionary.

return {k: tuple(v)[0] for k, v in values}

The tuple() call is included because the data in the value key is _always_ an iterable object but _may_ be an iterator. Calling tuple() expands the iterable and lets us get the first element.


$ git clone
$ cd tinymr
$ pip install -e .\[dev\]
$ py.test tests --cov tinymr --cov-report term-missing





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