What's wrong with `is not None`?

[Frans Englich]

That's what PyChecker tells me, at least. 

A line of:

if testReport is not None:

results in:

runner.py:587: Using is not None, may not always work

In what circumstances can `is not None` fail? How and why does it fail?

What is the equivalent expression which is more secure; `!= None`?


Cheers,

		Frans