A short introduction to python collections

Use the right data structure for the job with the containers from Python’s collections module.

A collection can be defined as a container data type. They have different characteristics primarily regarding their declaration and their usage.

Python has 4 collection data types: list, tuple, set and dict (dictionary). These types are general and have some limitations, that’s why other options are available in a built-in module named collections. Let’s go through some of these well known structures and some of the specialized data structures in detail.

Common Container Data Structures

These are built-in types that you can use without importing anything. These containers can solve most problems but sometimes they are not the best tool for the job.

List

• Declared in square brackets []
• Ordered
• Mutable
• Stores duplicated values
• Elements can be accessed with indexes

Tuple

• Declared in parenthesis ()
• Ordered
• Inmutable
• Stores duplicated values
• Elements can be accessed with indexes

Set

• Declared in braces{}. Empty declared with set()
• Unordered
• Mutable
• Doesn’t store duplicated values
• Not indexed

Dictionary

• Declared in braces with key and value separated by colon{key:value, ...}. Empty declared with {}
• Unordered
• Mutable
• Doesn’t store duplicated keys
• Not indexed

Specialized Collections Data Structures

The collections module implements many specialized data structures that with properties that differ from the built-in container data types. These are useful to solve more specific programming problems in a Pythonic and efficient manner.

Here are some of them.

namedtuple()

• A factory function to create subclasses of tuple with named fields
  • A name is assigned to every value
• Elements can be assigned with index or names
• namedtuple() docs

deque

• An optimized list that allows easy and efficient insertion and deletion
• Not only can it perform append and pop but also it can do appendleft and popleft
• deque docs

ChainMap

• It’s essentially a list of dictionaries
• Shows a single view of multiple mappings
• Indexed
• ChainMap docs

Counter

• A subclass of dictionary that counts hashable objects
• Has several methods to related to counting
• Counter docs

OrderedDict

• A subclass of dictionary that remembers the order in which keys were inserted
• Ordered
• OrderedDict docs

defaultdict

• A subclass of dictionary that has a factory function to provide values when a missing key is called
• If a value is provided for a missing key, the key-value is added to the dictionary
• defaultdict docs

Base Classes

On top of the specialized classes, collectionsalso has some base classes. These are the starting points if you want to subclass dictionary, list or string.

UserDict

• A wrapper class around dictionary that can be used as a base for inheritance.
• The dictionary is stored in an attribute that’s named data
• UserDict docs

UserList

• A wrapper class around list that can be used as a base for inheritance.
• The list is stored in an attribute that’s named data
• UserList docs

UserString

• A wrapper class around string that can be used as a base for inheritance.
• The string is stored in an attribute that’s named data
• UserString docs