checking member existence python

i frequently wish check an vigilant member not. an instance origination essay function. purpose, hasattr this:

class foo(object):
    @classmethod
    def singleton(self):
        hasattr(self, 'instance'):
            self.instance = foo()
        relapse self.instance

but also this:

class foo(object):
    @classmethod
    def singleton(self):
        try:
            relapse self.instance
        solely attributeerror:
            self.instance = foo()
            relapse self.instance

is slight improved other?

edit: total @classmethod ... nonetheless note doubt not essay nonetheless check coming member an object.

edit: example, customary use be:

s = foo.singleton()

then s an vigilant form foo, same any time. and, typically, slight called times.