@Jiaaro is right. Using is with immutable data types is dangerous because it is not predictable because of Pythons interpreter optimization.
See this example:
10 * "a" is 10 * "a" # True
100 * "a" is 100 * "a" # False
In the second line it is faster to create a new object with a new id for the interpreter. So use the is operator only with mutable types.
