[Python-Dev] PEP 318: Singleton decorator (original) (raw)

Ka-Ping Yee python-dev at zesty.ca
Tue Mar 30 06:17:08 EST 2004

Hi folks.

Earlier messages suggested a nice singleton decorator, which is shown in the draft PEP:

def singleton(cls):
    return cls()

class MyClass [singleton]:
    ...

This has been mentioned as an argument against requiring or recommending that decorators accept and return callables.

But i don't think this is a good way to write a singleton, because then the user of the class has to get instances by saying "MyClass" instead of "MyClass()". Better would be:

def singleton(cls):
    instances = {}
    def getinstance():
        if cls not in instances:
            instances[cls] = cls()
        return instances[cls]
    return getinstance

Then, code that gets an instance makes more sense. MyClass() still instantiates MyClass -- it just always returns the same instance.

I respectfully suggest that the PEP use the latter implementation in its singleton example instead.

-- ?!ng

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