check-code: catch Python 'is' comparing number or string literals The Python 'is' operator compares object identity, so it should definitely not be applied to string or number literals, which Python implementations are free to represent with a temporary object. This should catch the following kinds of bogus expressions (examples): x is 'foo' x is not 'foo' x is "bar" x is not "bar" x is 42 x is not 42 x is -36 x is not -36 As originally proposed by Martin Geisler, amended with catching negative numbers.