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.