Hashable objects for maps in Python

Published: Oct 8, 2019 by marc

Maps in Python are called dict. To create one, you have to choose an hashable object as key and a value.
What is an hashable object?
An hashable object is an object with a hash which never changes during its lifetime. Such objects are called immutable.
Python offers the following hashable built-in objects.

Hashable built-in objects

• bool
• int
• float
• tuple
• str
• frozenset
• bytes
• complex

Non-Hashable built-in objects

• set
• dict
• list
• byte array

Example with an hashable object

Create a simple dict with a int key and a string value. Then print the content of the key 1.

>>> mydict = {}
>>> mydict[1] = "Alice"
>>> print(mydict[1])
Alice

Example with non-hashable object

In some cases we need a dict that as a list as key. For example:

>>> mylist = ["Alice","Bob"]
>>> mydict = {}
>>> mydict[mylist] = 1
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unhashable type: 'list'

In order to make a list (or another non-hashable object) hashable, you can use tuple.

>>> mylist = ["Alice","Bob"]
>>> mydict = {}
>>> mydict[tuple(mylist)] = 1
>>> print(mydict)
{('Alice', 'Bob'): 1}