Python 3 - Second Edition

7.3    Mutable Versus Immutable Data Types

Perhaps you’ve already come across the following seeming contradiction while trying out what we have just described:

>>> a = 1>>> b = 1>>> id(a) 9656320>>> id(b) 9656320>>> a is bTrue  

Why do a and b reference the same integer instance, as shown by the identity comparison, even though we explicitly created two instances with the value 1 in the first two lines?

To answer this question, you need to know that Python fundamentally distinguishes between two types of data types—namely, between mutable and immutable data types. As the names suggest, the difference between the two types is whether the value of an instance can change after its creation—that is, whether it’s mutable. Instances of a mutable type ...