The Collections Module

Python has a very complete set of built in standard types that support most programming tasks. These include strings and numbers, and also types that can be used to hold other objects – or “collection” types.

  • tuples
  • lists
  • dictionaries

You can get pretty darn far with just these basic types – but some problems require (or could be helped by) more complex collection types.

This was recognised by the Python development team, so a number of genreally useful collection types are provided in the collections module.

The collections module

The first step is to see what’s there by looking at the documentation:

https://docs.python.org/3/library/collections.html

You can see a pretty nice list of options (kind of in order of usefulness)

  • namedtuple(): factory function for creating tuple subclasses with named fields
  • deque: list-like container with fast appends and pops on either end
  • Counter: dict subclass for counting hashable objects
  • OrderedDict: dict subclass that remembers the order entries were added
  • defaultdict: dict subclass that calls a factory function to supply missing values
  • ChainMap: dict-like class for creating a single view of multiple mappings

These are just the regular builtin types, but in a form that they can be subclassed – to make your own custom version.

  • UserDict: wrapper around dictionary objects for easier dict subclassing
  • UserList: wrapper around list objects for easier list subclassing
  • UserString: wrapper around string objects for easier string subclassing

To get an idea what these all are, read the docs, or a nice overview Python Module of the Week:

https://pymotw.com/3/collections/

Using the collection types

To use these special types, they must be imported:

In [4]: from collections import defaultdict

Then you can use it – creating a defaultdict with a empty list as a default:

In [8]: dd = defaultdict(list)

In [9]: dd['this'].append(23)

In [10]: dd
Out[10]: defaultdict(list, {'this': [23]})

In [11]: dd['this'].append(4)

In [12]: dd['this'].append(4)

In [13]: dd
Out[13]: defaultdict(list, {'this': [23, 4, 4]})

In [14]: dd['that'].append(4)

And you’ll get a dict that will automatically put an empty list in when the key isn’t there yet. Kind of a handy replacement from having to call dict.setdefault each time.

Similarly for the others.

Take a bit of time to try them out – you may find them really useful.