Say you have the following code:
assert type(s) is str x = float(s) if x != x: print "Bad bad float!"
What value of "s" and then "x" can make the code complain? Do you see the really bad side of it?
The answer is in the next paragraph, so stop reading if you want to find out yourself.
The correct answer is that there is one float that is not equal to itself and that is float("nan"). Which I find terrible. It is so bad, it spoils set, dict, and everything there is. Any container that has it inside is no longer equal to itself.
Surprised? I was too! I only learned it while doing my Python compiler Nuitka and I made it a separate posting, because it really surprised me how this could possibly happen. A builtin type that breaks fundamental assumptions like "x == x".