is checks that both operands are the same object. == calls __eq__() on the left operand, passing the right. Normally this method implements equality comparison, but it is possible to write a class that uses it for other purposes (but it never should).
Note that is and == will give the same results for certain objects (string literals, integers between -1 and 256 inclusive) on some implementations, but that does not mean that the operators should be considered substitutable in those situations.
