Counter

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Counter#

collections.Counter is a multiset / frequency map. A dict subclass where missing keys read as 0 and the type provides most_common, multiset arithmetic, and bulk update.

from collections import Counter

c = Counter("operator")
c.most_common(3)              # [('o', 2), ('p', 1), ('e', 1)]
c["o"]                        # 2
c["z"]                        # 0 (no KeyError)
c.update("ranger")            # add another iterable
c - Counter("rare")           # multiset subtraction

Counter supports +, -, & (min), | (max) over two counters. Negative counts are allowed during arithmetic but +counter drops them; -counter drops the positives.

For the top-N use case, prefer c.most_common(N) over sorted(c.items(), key=lambda kv: -kv[1])[:N]; the former uses heapq internally and is O(M + N log M) for M unique elements.

References#

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