Is there a simple, elegant way to define singletons? [duplicate]

Answer 1

There are also some interesting articles on the Google Testing blog, discussing why singleton are/may be bad and are an anti-pattern:

• Singletons are Pathological Liars
• Where Have All the Singletons Gone?
• Root Cause of Singletons

Answer 2

As the accepted answer says, the most idiomatic way is to just use a module.

With that in mind, here's a proof of concept:

def singleton(cls):
    obj = cls()
    # Always return the same object
    cls.__new__ = staticmethod(lambda cls: obj)
    # Disable __init__
    try:
        del cls.__init__
    except AttributeError:
        pass
    return cls

See the Python data model for more details on __new__.

Example:

@singleton
class Duck(object):
    pass

if Duck() is Duck():
    print "It works!"
else:
    print "It doesn't work!"

Notes:

1. You have to use new-style classes (derive from object) for this.

2. The singleton is initialized when it is defined, rather than the first time it's used.

3. This is just a toy example. I've never actually used this in production code, and don't plan to.

Answer 3

class Singeltone(type):
    instances = dict()

    def __call__(cls, *args, **kwargs):
        if cls.__name__ not in Singeltone.instances:            
            Singeltone.instances[cls.__name__] = type.__call__(cls, *args, **kwargs)
        return Singeltone.instances[cls.__name__]


class Test(object):
    __metaclass__ = Singeltone


inst0 = Test()
inst1 = Test()
print(id(inst1) == id(inst0))