A First Example of Class Instances

A First Example of Class Instances#

Note

Source: Adapted from the C# edition (classes/a-first-class.rst). The Contact class is a direct Python translation. Python uses __init__ in place of a named constructor, and self in place of C#’s implicit this. There are no public/private keywords; the convention _name signals a non-public attribute.

We have used many built-in types and library classes. Now we define our own. As a first example, a contact has a name, phone number, and email address — three pieces of data that belong together as one entity.

Defining the Class#

class Contact:
    def __init__(self, name, phone, email):
        self.name = name
        self.phone = phone
        self.email = email

  • class Contact: declares a new type named Contact.

  • __init__ is the initialiser — Python calls it automatically when a new object is created.

  • self is the first parameter of every instance method. It refers to the specific object being created or operated on. You never pass it explicitly; Python fills it in.

  • self.name = name stores the argument in an instance variable — data that lives as long as the object exists.

Creating Instances#

c = Contact("Marie Ortiz", "773-508-7890", "mortiz2@luc.edu")

This creates a new Contact object and binds it to c. Each object has its own copy of the instance variables.

Accessing Instance Variables#

print(c.name)
print(c.phone)
print(c.email)

Output:

Marie Ortiz
773-508-7890
mortiz2@luc.edu

Adding a __str__ Method#

__str__ is called automatically whenever Python needs a string representation of the object (e.g., inside print()):

class Contact:
    def __init__(self, name, phone, email):
        self.name = name
        self.phone = phone
        self.email = email

    def __str__(self):
        return (f"Name:  {self.name}\n"
                f"Phone: {self.phone}\n"
                f"Email: {self.email}")

c = Contact("Marie Ortiz", "773-508-7890", "mortiz2@luc.edu")
print(c)

Output:

Name:  Marie Ortiz
Phone: 773-508-7890
Email: mortiz2@luc.edu

Adding Methods#

Instance methods work exactly like functions, but receive self as their first argument. We can add a method to update the email address:

def set_email(self, new_email):
    self.email = new_email

c.set_email("maria.ortiz@gmail.com")
print(c.email)

Output:

maria.ortiz@gmail.com

Privacy by Convention#

Python has no private keyword. The convention is to prefix an attribute with a single underscore (_name) to signal that it is intended for internal use and should not be accessed directly from outside the class. Double underscore (__name) invokes name mangling, which makes accidental access harder.

For the Contact class this is a matter of taste; we leave the attributes public since this is a simple data container.

Using a List of Contacts#

contacts = [
    Contact("Marie Ortiz", "773-508-7890", "mortiz2@luc.edu"),
    Contact("Otto Heinz",  "773-508-9999", "oheinz@luc.edu"),
]

for contact in contacts:
    print(contact)
    print()