16.16. Group Work - Classes, constructors, attributes, and methods

It is best to use a POGIL approach with the following. In POGIL students work in groups on activities and each member has an assigned role. For more information see https://cspogil.org/Home.

Note

If you work in a group, have only one member of the group fill in the answers on this page. You will be able to share your answers with the group at the bottom of the page.

Learning Objectives

Students will know and be able to do the following.

Content Objectives:

• Identify the parts of a class

• Explain what a constructor (initializer) is and what it does

• Explain what the __str__ method does

• Explain the difference(s) between a function and method.

• Create new objects and name them

• Create new methods

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 is automatically called when a new object is created by the class. One of the methods is automatically called when you print an object of the class.

16.16.1. A Book Class

Run the following code

class Book: """ Represents a book object """ def __init__(self, title, author): self.title = title self.author = author def __str__(self): return "title: " + self.title + " author: " + self.author def main(): b2 = Book("A Wrinkle in Time", "M. L'Engle") print(b2) b1 = Book("Goodnight Moon", "Margaret Wise Brown") print(b1) main()

Look at the class definition. It starts with the keyword class. Then answer the following questions.

Q-2: What is the name of this class?

Q-3: What is the name of the method in the example above that is always called when an object is created?

Q-4: Describe in your own words what the __init__ method does.

Q-5: What is the name of the method that is called when the object is printed?

Q-6: Describe in your own words what the __str__ method does.

Q-7: How many attributes does an object of the Book class have?

Q-8: Click on all of the method names in the code below.Method names are the names of functions that are defined in a class.

class Book:
    """ Represents a book object """

    def __init__(self, title, author):
        self.title = title
        self.author = author

    def __str__(self):
        return "title: " + self.title + " author: " + self.author

    def get_author(self):
        return self.author

def main():
     b2 = Book("A Wrinkle in Time", "M. L'Engle")
     print(b2)
     b1 = Book("Goodnight Moon", "Margaret Wise Brown")
     print(b1)

main()

Q-9: Describe in your own words what is/are the difference(s) between a function and a method.

Q-10: What is the name of the first parameter in all of the methods?

Put the code blocks in order below to create a class Person with a constructor (__init__) method that takes a first and last name and a __str__ method to return a string with the first and last name separated with a space.

        class Person:
---
Class Person: #paired
---
    def __init__(self, first, last):
---
    def __init__(first, last): #paired
---
        self.first = first
        self.last = last
---
    def __str__(self):
---
    def __str__(): #paired
---
        return self.first + " " + self.last
---
        return first + " " + last #paired
        

Q-12: Describe in your own words what self means.

16.16.2. Create More Book Objects

Change the following main function to create a third book object called b3 with a title of “1984” and author “George Orwell”. Print out the values using the print function in the main.

class Book: """ Represents a book object """ def __init__(self, title, author): self.title = title self.author = author def __str__(self): return "title: " + self.title + " author: " + self.author def main(): b2 = Book("A Wrinkle in Time", "M. L'Engle") print(b2) b1 = Book("Goodnight Moon", "Margaret Wise Brown") print(b1) main()

Change the following main function to add a person object with your first and last name.

class Person: """ Represents a person object """ def __init__(self, first, last): self.first = first self.last = last def __str__(self): return self.first + " " + self.last def main(): p1 = Person("Barbara", "Ericson") print(p1) main()

16.16.3. Add a Method to a Class

Change the following Person class to add 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 """ def __init__(self, first, last): self.first = first self.last = last def __str__(self): return self.first + " " + self.last def main(): p1 = Person("Barbara", "Ericson") print(p1) print(p1.initials()) main()

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