3. Data model

Types affect almost all aspects of object behavior. Even the importance ofobject identity is affected in some sense: for immutable types, operations thatcompute new values may actually return a reference to any existing object withthe same type and value, while for mutable objects this is not allowed. E.g.,after a = 1; b = 1, a and b may or may not refer to the same objectwith the value one, depending on the implementation, but after c = []; d =[], c and d are guaranteed to refer to two different, unique, newlycreated empty lists. (Note that c = d = [] assigns the same object to bothc and d.)