Question

Given two modules, main and x with the following contents:

main:

class Singleton(object):
    _instance = None
    def __new__(cls, *args, **kwargs):
        if not cls._instance:
            cls._instance = super(Singleton, cls).__new__(cls, *args, **kwargs)
        cls._instance.x = 10
        return cls._instance
uvw = Singleton()

if __name__ == "__main__":
    print(id(uvw))
    uvw.x += 10
    print(uvw.x)
    import x

and x, respectively:

import main

print(id(main.uvw))
print(main.uvw.x)

I would now expect that executing main would yield the same IDs and the value twenty in both instances, but what I get is this:

$ python main.py
140592861777168
20
140592861207504
10

Is there any way I can ensure that uvw is the same object at both places?

Answer 1

Python loads each module by name once (unless reload(module) is called). If you run main.py, the module is __main__ (try printing uvw.__class__.__module__). When x imports main, the module called main is being loaded for the first time.

If you defined uvw in a third module, or in x — as long as it is imported in the same way into __main__ and x — it would be the same object.