python programming if class had no accessor methods we could put values in the object but we could never retrieve them some classes have mutator methods and some don' for instancethe list class has mutator methodsincluding the reverse method there are some classes that don' have any mutator methods for instancethe str class does not have any mutator methods when class does not contain any mutator methodswe say that the class is immutable we can form new values from the data in an immutable classbut once an immutable object is createdit cannot be changed other immutable classes include int and float implementing class programming in an object-oriented language usually means implementing classes that describe objects which hold information that is needed by the program you are writing objects contain data and methods operate on that data class is the definition of the data and methods for specific type of object every class contains one special method called constructor the constructor' job is to create an instance of an object by placing references to data within the object itself for exampleconsider class called dog dog has namea birthdayand sound it makes when it barks when we create dog objectwe write code like that appearing in sect creating objects and calling methods boydog dog("mesa" "wooof"girldog dog("sequoia" "barkbark"print(boydog speak()print(girldog speak()print(boydog birthdate()print(girldog birthdate()boydog changebark("woofywoofy"print(boydog speak()once created in the memory of the computerdog objects looks like those appearing in fig each object is referenced by the variable reference assigned to iteither girldog or boydog in this case the objects themselves are collection of references that point to the information that is stored in the object each object has namemonthdayyearand speaktext references that point to the associated data that make up dog object to be able to create dog objects like these two objects we need dog class to define these objects in additionwe'll need to define speakbirthdateand changebark methods we can do this by writing class as shown in sect comments about each part of the class appear in the code the special variable self always points at the current object and must be the first parameter to each method in the class this copy belongs to 'acha