From the PEP8 department...

Since a lot of people trip over this, let me repeat an important and useful passage from PEP8, the Python styleguide:

- Comparisons to singletons like None should always be done with
  'is' or 'is not', never the equality operators.

  Also, beware of writing "if x" when you really mean "if x is not None"
  -- e.g. when testing whether a variable or argument that defaults to
  None was set to some other value.  The other value might have a type
  (such as a container) that could be false in a boolean context!

In particular, you should avoid code like

  ob = retrieveObjectSomehow(..., default=None)
  if ob:
      # you'll never get here if ob is an IContainer and empty

If you know that None is the fallback, then check for it explicitly:

  ob = retrieveObjectSomehow(..., default=None)
  if ob is not None:

I don't know why people frequently trip over this, I blame tutorials that draw a misleading picture of Python's polymorphism.