Checking for member existence in Python

Question

I regularly want to check if an object has a member or not. An example is the creation of a singleton in a function. For that purpose, you can use hasattr like

Answer 1

I just tried to measure times:

class Foo(object):
    @classmethod
    def singleton(self):
        if not hasattr(self, 'instance'):
            self.instance = Foo()
        return self.instance



class Bar(object):
    @classmethod
    def singleton(self):
        try:
            return self.instance
        except AttributeError:
            self.instance = Bar()
            return self.instance



from time import time

n = 1000000
foo = [Foo() for i in xrange(0,n)]
bar = [Bar() for i in xrange(0,n)]

print "Objs created."
print


for times in xrange(1,4):
    t = time()
    for d in foo: d.singleton()
    print "#%d Foo pass in %f" % (times, time()-t)

    t = time()
    for d in bar: d.singleton()
    print "#%d Bar pass in %f" % (times, time()-t)

    print

On my machine:

Objs created.

#1 Foo pass in 1.719000
#1 Bar pass in 1.140000

#2 Foo pass in 1.750000
#2 Bar pass in 1.187000

#3 Foo pass in 1.797000
#3 Bar pass in 1.203000

It seems that try/except is faster. It seems also more readable to me, anyway depends on the case, this test was very simple maybe you'd need a more complex one.