[Python-Dev] PEP 318: Singleton decorator

[Ka-Ping Yee]

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