Sometimes Python leans to the DWIM side: >>> 42 is not None True >>> 'oo' in 'foobar' True One could argue that both answers should be False, because:

Sometimes Python leans to the DWIM side: >>> 42 is not None True >>> 'oo' in 'foobar' True One could argue that both answers should be False, because: >>> 42 is (not None) False >>> 'oo' in iter('foobar') False