When to use a Singleton in python?

Question

There are many questions related to the use of the Singleton pattern in python, and although this question might repeat many of the aspects already discussed, I have not found the answer to the following specific question.

Let's assume I have a class MyClass which I want to instantiate only exactly once. In python I can do this as follows in the code myclass.py:

class MyClass(object): 
    def foo(self):
        ....


instance = MyClass()

Then in any other program I can refer to the instance simply with

import myclass
myclass.instance.foo()

Under what circumstances is this approach enough? Under what circumstances is the use of a Singleton pattern useful/mandatory?

Answer 1

The singleton pattern is more often a matter of convenience than of requirement. Python is a little bit different than other languages in that it is fairly easy to mock out singletons in testing (just clobber the global variable!) by comparison to other languages, but it is neverthess a good idea to ask yourself when creating a singleton: am I doing this for the sake of convenience or because it is stricly necessary that there is only one instance? Is it possible that there may be more than one in the future?

If you create a class that really will be only constructed once, it may make more sense to make the state a part of the module, and to make its methods into module-level functions. If there is a possibility that the assumption of exactly one instance may change in the future, then it is often much better to pass the singleton instance around rather than referencing the singleton through a global name.

For example, you can just as easily implement a "singleton" this way:

if __name__ == '__main__':
   instance = MyClass()
   doSomethingWith(instance)

In the above, "instance" is singleton by virtue of the fact that it is constructed only once, but the code that handles it is provided the instance rather than referencing module.instance, which makes it easier to reuse pieces of the code if, in some future situation, you need more than one MyClass.