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Introduction to Python Class

Creating Classes

Look the code below. It defines a class. it also declares methods which are functions that are defined inside of a class. One of the methods, __init__, is automatically called when a new object is created by the class. One of the methods, __str__, is automatically called when you print an object of the class. These methods start and end with two underscores.

Run the following code

class Book: """ Represents a book object """ # initializes the values in a new object called self def __init__(self, title, author): self.title = title # set title in self to the passed title self.author = author # set author in self to the passsed author # returns a string with information about the object self def __str__(self): return "title: " + self.title + " author: " + self.author def main(): # calls the __init__ method b2 = Book("A Wrinkle in Time", "M. L'Engle") # calls the __str__ method print(b2) # calls the __init__ method b1 = Book("Goodnight Moon", "Margaret Wise Brown") # calls the __str__ method print(b1) main()

Creating More Objects

Once you have defined a class you can use it to create many objects.

Change the following main function to add a new person object.

class Person: """ Represents a person object """ # initializes the values in a new object called self def __init__(self, first, last): self.first = first # set first in self to the passed first self.last = last # set last in self to the passed last # returns a string with information about the object self def __str__(self): return self.first + " " + self.last def main(): # calls the __init__ method p1 = Person("Barbara", "Ericson") # calls the __str__ method print(p1) # create an object for another person (calls the __init__ method) # print the new object (calls the __str__ method) main()

Add a Method to a Class

You can add a new method to a class by adding a new function inside the class. For example, you can add the initials method to the Person class. The name of the function doesn’t need to have any underscores in it. It only needs to start and end with double underscores if it is a special method like __init__ or __str__. It does need to take the current object which is by convention referred to as self.

The following Person class has an initials method that returns a string with the first letter in the first name and the first letter in the last name in lowercase.

class Person: """ Represents a person object """ # initializes the values in a new object called self def __init__(self, first, last): self.first = first # set first in self to the passed first self.last = last # set last in self to the passed last # returns a string with information about the object self def __str__(self): return self.first + " " + self.last # returns the first characters of the first and last name in lowercase def initials(self): return self.first[0].lower() + self.last[0].lower() def main(): # calls the __init__ method p1 = Person("Barbara", "Ericson") # calls the __str__ method print(p1) # calls the initials method print(p1.initials()) main() ==== from unittest.gui import TestCaseGui class myTests(TestCaseGui): def testOne(self): p1 = Person("Barbara", "Ericson") self.assertEqual(p1.initials(),'be',"testing initials for Barbara Ericson") p2 = Person("Enoch", "Obe") self.assertEqual(p2.initials(),"eo", "testing initials for Enoch Obe") myTests().main()

Glossary

class

A template that can be used to construct an object. Defines the attributes and methods that will make up the object.

attribute

A variable that is part of a class.

constructor

An optional specially named method ( __init__ ) that is called at the moment when a class is being used to construct an object. Usually this is used to set up initial values for the object.

inheritance

When we create a new class (child) by extending an existing class (parent). The child class has all the attributes and methods of the parent class plus additional attributes and methods defined by the child class.

method

A function that is contained within a class and the objects that are constructed from the class. Some object-oriented patterns use ‘message’ instead of ‘method’ to describe this concept.

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