Using collections in Python
You read about built-in collections in Chapter 2, Python Structures. You saw list, dict, tuple, and set, but sometimes, those collections are not enough. The Python Standard Library comes with modules and collections that provide a number of advanced structures that can greatly simplify our code in common situations. Now, you will explore how you can use Counter, defaultdict, and ChainMap.
The counter class
Counter is a class that allows us to count hashable objects. It has keys and values as a dictionary (it actually inherits from dict) to store objects as keys and the number of occurrences in values. A Counter object can be created either with the list of objects that you want to count or with a dictionary that already contains the mapping of objects to their count. Once you have a counter instance created, you can get information about the count of objects, such as getting the most common ones or the count of a specific object.
