How to Implement a Mock MapReduce for Word Count in Python

from collections import defaultdict
import re

def mapper(text):
    """Split text into words and emit (word, 1) pairs."""
    words = re.findall(r'\b\w+\b', text.lower())
    return [(word, 1) for word in words]

def reducer(pairs):
    """Group word-count pairs and sum counts."""
    counts = defaultdict(int)
    for word, count in pairs:
        counts[word] += count
    return dict(counts)

def map_reduce_word_count(texts):
    """Run mock MapReduce word count over a list of text documents."""
    # MAP phase
    mapped = []
    for text in texts:
        mapped.extend(mapper(text))

    # SHUFFLE phase (group by key)
    grouped = defaultdict(list)
    for word, count in mapped:
        grouped[word].append(count)

    # REDUCE phase
    return reducer([(word, sum(counts)) for word, counts in grouped.items()])

if __name__ == "__main__":
    documents = [
        "the cat sat on the mat",
        "the dog sat on the log",
        "cat and dog"
    ]
    result = map_reduce_word_count(documents)
    print(sorted(result.items()))