CODEWITHSUNDEEP
pythonpython-3.xsingleton
dom0
dom0

Reputation: 7486

Share a singleton across modules

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?

Upvotes: 13

Views: 7682

Answers (3)

dom0
dom0

Reputation: 7486

I found out that the problem is that main is loaded twice when it's executed from the command line, one time as __main__ and the second time when imported by x as main.

I found a very evil hack to circumvent the second load:

sys.modules["main"] = sys.modules["__main__"]

Separating the main module and the singleton class is not favorable in my case.

Upvotes: 1

joeln
joeln

Reputation: 3643

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.

Upvotes: 7

Eric
Eric

Reputation: 97591

I reckon the problem is that your Singleton class is somehow being reloaded, and thus loses its _instance field in the second module.

I think this will work:

singleton.py:

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

a.py:

from singleton import Singleton
uvw = Singleton()
print(id(uvw))
uvw.x += 10
print(uvw.x)

b.py:

from singleton import Singleton
uvw = Singleton()
print(id(uvw))
uvw.x += 10
print(uvw.x)

main.py

import a
import b

Upvotes: 8

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