Python: use the same class instance in multiple modules

Question

I have a class called EventManager, and am calling this class in each module I have. I thought of creating a single instance of the EventManager in the main file, and use that instance in each module. My questions are:

• Is this the right way in my case? Note that I don't want to create multiple instances of the EventManager as it parses a file each time it is called, and that file can contain large amount of data.
• what is the best way to do it, given that I want less memory usage?

Answer 1

You could use a singleton. Here is a simple example:

def singleton(cls):
    return cls()

@singleton
class EventManager:
    def __init__(self):
        self.events = []
    def add_event(self, event):
        self.events.append(event)
    def print_events(self):
        print("Events : {}".format(self.events))

EventManager.add_event(0)
EventManager.add_event(1)
EventManager.add_event(2)
EventManager.print_events()

Output:

Events : [0, 1, 2]

Note that everything still works as expected when the singleton is imported and used in other modules.

---

EDIT

You asked :

The thing is that I initialize the class with a variable like EventManager(var), what shall I do in this case?

The class will be instantiated right after its definition, i.e. the first time the module containing EventManager is imported. Since EventManager is a singleton, I don't think there is a point initializing the singleton with a variable. Even though it is possible:

def singleton_with_args(*args, **kwargs):
    def wrapper(cls):
        return cls(*args, **kwargs)
    return wrapper

@singleton_with_args(0)
class EventManager:
    def __init__(self, first_event):
        self.events = [first_event]
    def add_event(self, event):
        self.events.append(event)
    def print_events(self):
        print("Events : {}".format(self.events))

EventManager.add_event(1)
EventManager.add_event(2)
EventManager.add_event(3)
EventManager.print_events()

Output:

Events : [0, 1, 2, 3]

It works but as I said, I don't think it's useful. You could as well use methods or module variables to initialize your singleton.