Python collections.Counter

A Counter is a data type in the collections module. It is a dict subclass that stores counts for hashable objects. For each key-value pair, the key is the item being counted, and the value is the count of that object. A Counter‘s items are accessed just as in a dictionary, except accessing a missing key will return a 0 value rather than an error.

Note: A Counter will allow negative counts.

Syntax

myCollection = collections.Counter()

A Counter can also be initialized from an iterable or initialized from another mapping.

Additional Methods

A Counter supplies the following additional methods beyond those for a standard dictionary:

• .elements(): Returns an iterator for the keys of the Counter repeating each element as many times as its count. It will ignore items with a count of less than one.
• .most_common(n): Returns the n most common key-count pairs, in descending order of their counts. If n is omitted or None, the entire contents of the Collection are returned.
• .subtract(values): Where values is an iterable or mapping. Matching keys have their values subtracted from the elements in the Counter. Inputs and outputs can be negative or zero.
• .total(): Returns the sum of all the counts.

Other differences from a standard dictionary:

• A Counter does not implement the .fromkeys() method.
• The .update() method adds counter values rather than replacing them, making it the opposite of .subtract() above.

Codebyte Example

The following example creates a Counter from an iterable (a string) and outputs various characteristics of the Counter:

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