Implementing Custom Containers

Contributed by: David Beazley and Brian K. Jones

Difficulty level: Easy

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Problem

You want to implement a custom class that mimics the behavior of a common built-in container type, such as a list or dictionary. However, you’re not entirely sure what methods need to be implemented to do it.

Solution

The collections library defines a variety of abstract base classes that are extremely useful when implementing custom container classes. To illustrate: suppose you want your class to support iteration. To do that, simply start by having it inherit from collections.Iterable, as follows:

import collections

class A(collections.Iterable):
    pass

The special feature about inheriting from collections.Iterable is that it ensures you implement all of the required special methods. If you don’t, you’ll get an error upon instantiation:

>>> a = A()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: Can't instantiate abstract class A with abstract methods __iter__
>>>

To fix this error, simply give the class the required __iter__() method and implement it as desired (see Recipes 4.2 and 4.7 in Python Cookbook, 3rd ed.).

Other notable classes defined in collections include Sequence, MutableSequence, Mapping, MutableMapping, Set, and MutableSet. Many of these classes form hierarchies with increasing levels of functionality (e.g., ...