Python Morsels: Making a singleton in Python

Table of contents

1. What is a singleton?
2. Making a singletons in Python with a custom constructor
3. We could use a global object instead
4. Do we even need a class?
5. Python's modules act like singletons
6. Modules are the most typical Python singleton

What is a singleton?

A singleton class is a class that only allows creating one instance of itself.

None is an example of a singleton object in Python. The class of None is NoneType:

>>> type(None)<class 'NoneType'>

If we try to call the NoneType class, we'll get back an instance of that class.

>>> new_none=type(None)()

But every instance of the NoneType class is identical to every other instance of the NoneType class:

>>> new_noneisNoneTrue

So there's only one instance of NoneType in Python, and it's None.

Making a singletons in Python with a custom constructor

Here we have a class …