Identity, State, and Behavior

An object has an identity, a current state and behavior. Two books may store the same title and still be different objects.

As in the earlier object diagram, underlined labels identify objects; the compartment below each label shows a stored value. No line connects them because equal titles alone do not create a relationship.

Identity

is compares identity. == uses the type’s equality rule; our Book has not defined a custom rule.

class Book:
    def __init__(self, title):
        self.title = title

book1 = Book("Hamlet")
book2 = Book("Hamlet")

print(book1 is book2)
print(book1 == book2)
print(book1.title == book2.title)

Output

False
False
True

The books are separate objects, but their title strings are equal. id() also returns an identity number; treating it as a memory address is implementation-specific.

State

State is the data held by an object at a particular time. Here, None means no courses list was supplied. The conditional expression chooses a new empty list in that case; otherwise list(courses) copies the supplied list.

class Student:
    def __init__(self, name, courses=None):
        self.name = name
        self.courses = [] if courses is None else list(courses)

ada = Student("Ada")
grace = Student("Grace", ["CS101"])

print(ada.name, ada.courses)
print(grace.name, grace.courses)

Output

Ada []
Grace ['CS101']

Ada has an empty courses list; Grace has CS101. The initializer creates or copies a list, so their collections are independent.

Continue with the students above:

ada.courses.append("Math101")
print(ada.courses)

Output

['Math101']

Appending changes Ada’s state. Grace’s courses are unchanged.

Behavior

A method can update state or return information from it.

class Student:
    def __init__(self, name):
        self.name = name
        self.courses = []

    def enroll(self, course):
        self.courses.append(course)

    def full_name(self):
        return self.name

ada = Student("Ada Lovelace")
ada.enroll("CS101")

print(ada.full_name())
print(ada.courses)

Output

Ada Lovelace
['CS101']

enroll changes the courses list. full_name returns the stored name.

Try it: create another Ada object. Compare the objects with is, then compare their names with ==.

We can now separate values stored on individual objects from defaults supplied by their class.