Exploring Singletons

Introduction

This won’t be an extensive exploration since it seems to be relatively straightforward. This is a test of creating a singleton in python. The motivation for it is that I want to allow plugins to add themselves to aggregating composites without having to worry if others have alread created one.

The Idea

To create the singleton you just need to rely on the fact that when you make multiple calls to the same module in python, the interpreter only creates one instance of it. So you can create a function in it that instantiates the object and even if multiple modules import it, they each get the same object.

The Function Test

I created a file called singleton_source.py that creates a function called get that creates and returns an instance of the SingletonTest class. I then created two files test_1.py and test_2.py that get the singletons from the singleton source.

test_1.py:

import singleton_source

t1 = singleton_source.get()

test_1 = singleton_source.test_instance

test_2.py:

import singleton_source

t2 = singleton_source.get()

test_2 = singleton_source.test_instance

from ape.documentation.developer.appendices.explore_singletons.test_1 import t1
from ape.documentation.developer.appendices.explore_singletons.test_2 import t2

assert t1 is t2

Testing Without the Function

I think the reason why the example I saw used the function call was so that you can refresh it, but it seems to me that if you aren’t going to, you should just be able to create an instance in the module without using a function. Using the same modules as above:

test_1.py:

import singleton_source

t1 = singleton_source.get()

test_1 = singleton_source.test_instance

test_2.py:

import singleton_source

t2 = singleton_source.get()

test_2 = singleton_source.test_instance

from ape.documentation.developer.appendices.explore_singletons.test_1 import test_1
from ape.documentation.developer.appendices.explore_singletons.test_2 import test_2

assert test_1 is test_2

I think I’ll use the second version for the plugins.