Count the occurrences of each word

Count the occurrences of each word in a given sentence.
def word_count(S):
    counts = {}
    words = S.split()

    for word in words:
        if word in counts:
            counts[word] += 1
        else:
            counts[word] = 1

    return counts

# test
print(word_count('the quick brown fox jumps over the lazy dog.'))

Output:

{'the': 2, 'jumps': 1, 'brown': 1, 'lazy': 1, 'fox': 1, 'over': 1, 'quick': 1, 'dog.': 1}