Python: comparing to None

Posted on 01 August 2011 in Articles • 1 min read

Consider a class:

class Queue(object):
    def __init__(self):
        self._len = 0

    def __len__(self):
        return self._len

Next, the usage of the class is:

q = Queue()

# at this point, you want to check if q is None
if not q:
   doSomething()

The confusing thing is that doSomething() is actually called! And that is because len(q) == 0!

Instead, use the is None comparison:

if q is None:
    doSomething()