Introduction to Classes
Introduction to Classes
Fundamentals of Classes
Introduction
An object is something, anything, we see in our daily life. Practically anything you see around you is an object. A class is one or a group of characteristics that describes an object. A class is like a story that describes an object, but instead of using many words as in a novel or a dissertation, you make some type of list of words that serve as starting points to describe such an object.
Practical Learning: Introducing Classes
print("==================================================")
print("Loan Applicant: " + " " + " " + " ")
print('--------------------------------------------------')
print("Loan Values")
print("Principal:", 0)
print("Interest Rate: " + "0.00" + "%")
print("Periods:", 6, "Months")
print("==================================================")================================================== Loan Applicant: -------------------------------------------------- Loan Values Principal: 0 Interest Rate: 0.00% Periods: 6 Months ================================================== Press any key to continue . . .
Creating a Class
The primary formula to create a class is:
class class-name:
. . .
Start with the class keyword.
The Name of a Class
The class keyword is followed by a name. Normally, the name of a class primarily follows the same rules we reviewed for names of variables and functions. Other than that, you can add your own policy. In our lessons, the name of a class will start in uppercase. Examples are Vehicle, House, or Road. If a name is made of more than one word, each word will start in uppercase and the words will be combined (remember that there must not be a space (or a special character) in a name). Examples are CollegeMajor, NationalCensus, CommonUnderstanding. After the name of the class, type a colon.
Practical Learning: Creating a Class
class LoanContract:
The Body of a Class
After the colon that follows the name of a class, on the next line, create the content of the class. That section is the body of the class. That body of the class must be indented. In the body of the class, you can type almost any of the types of code we have written so far. For example, you can write one or more lines that call the print() function. Here is an example:
class Square:
print("A square is a geometric figure with four equal sides and four right angles.")
sqr = Square()
This would produce:
A square is a geometric figure with four equal sides and four right angles. Press any key to continue . . .
In the same way, you can write other such statements in the body of the class.
An Instance of a Class
Remember that the purpose of a class is to describe an object. To get an object, you must declare a variable of the class. That variable is also called an object. It is also called an instance of the class, or just an instance.
To create an object or an instance of a class, type a name for a variable. If the class was created in the same document where you want to use it, assign the name of the class to the variable. The name of the class must be followed by parentheses (you can end with a semi-colon (as it is done in many other languages) but this is not required). Here is an example:
class Square:
sqr = Square()
If you had created the class in a separate module or the class was supplied to you somehow, you must import it. If you import only the module, to access the class, type the name of the module, a period, and the name of the class. Here is an example:
import geometry sqr = geometry.Square()
As an alternative, you can indicate that, from the module, you want to import the class. This time, you can directly assign the name of the class to a variable. Here is an example:
from geometry import Square
sqr = Square()
Practical Learning: Creating an Object
class LoanContract:
lc = LoanContract()Introduction to the Attributes of a Class
Introduction to Member Variables
In Python (unlike many other languages, like C#, Visual Basic, Java, Pascal, etc), a member of a class is called an attribute. The primary type of attribute of a class is created exactly like a variable. This means that, to have a simple member in a class, you can simply declare a variable for it. Because Python is an inferred language, you can/should also initialize the variable. Here is an example:
class Square:
side = 228.93
Practical Learning: Adding a Member Variable to a Class
class LoanContract:
loanAmount = 6500
lc = LoanContract()Such a variable can be called a member variable.
Accessing a Member of an Object
After creating an object of a class, to access a member of the class, where necessary in a document, type the name of the object, a period, and the desired member. Here is an example:
class Square:
side = 228.93
print("A square is geometric figure with four equal sides and four right angles.")
sqr = Square()
print("Square Characteristics")
print("----------------------")
print("Side: ", sqr.side)
print("=================================")
This would produce:
A square is geometric figure with four equal sides and four right angles. Square Characteristics ---------------------- Side: 228.93 ================================= Press any key to continue . . .
In the same way, you can declare other variables in a class and access them outside the class. As you can initialize a member variable with a constant value, you can initialize a member variable with an expression. Here is an example:
class Square:
side = 228.93
perimeter = side * 4
print("A square is geometric figure with four equal sides and four right angles.")
sqr = Square()
print("Square Characteristics")
print("----------------------")
print("Side: ", sqr.side)
print("Perimeter: ", sqr.perimeter)
print("=================================")
This would produce:
A square is geometric figure with four equal sides and four right angles. Square Characteristics ---------------------- Side: 228.93 Perimeter: 915.72 ================================= Press any key to continue . . .
Practical Learning: Accessing a Member of an Object
class LoanContract:
loanAmount = 6500
lc = LoanContract()
print("Principal:", lc.loanAmount)Principal: 6500 ================================================== Press any key to continue . . .
class LoanContract:
firstName = "Alexandra"
lastName = "Colson"
loanAmount = 3685
interestRate = 9.95
periods = 38
lc = LoanContract()
print("==================================================")
print("Loan Applicant: " + lc.firstName + " " + lc.lastName)
print('--------------------------------------------------')
print("Loan Values")
print("Principal:", lc.loanAmount)
print("Interest Rate: " + str(lc.interestRate) + "%")
print("Periods:", lc.periods, "Months")
print("==================================================")================================================== Loan Applicant: Alexandra Colson -------------------------------------------------- Loan Values Principal: 3685 Interest Rate: 9.95% Periods: 38 Months ================================================== Press any key to continue . . .
class LoanContract:
firstName = ""
lastName = ""
loanAmount = 0.00
interestRate = 0.00
periods = 0
lc = LoanContract()
print("Watts' A Loan")
print("==================================================")
print('Enter the information about the loan applicant')
lc.firstName = input("First Name: ")
lc.lastName = input("Last Name: ")
print('--------------------------------------------------')
print("Enter the values of the loan")
lc.loanAmount = float(input("Current Value: "))
lc.interestRate = float(input("Interest Rate: "))
lc.periods = int(input("Number of Months: "))
print("==================================================")
print("Loan Applicant: " + lc.firstName + " " + lc.lastName)
print('--------------------------------------------------')
print("Loan Values")
print("Principal: ", lc.loanAmount)
print("Interest Rate: " + str(lc.interestRate) + "%")
print("Periods: ", lc.periods, "Months")
print("==================================================")Watts' A Loan ================================================== Enter the information about the loan applicant First Name: Stephen Last Name: Ignatavicius -------------------------------------------------- Enter the values of the loan Current Value: 28865 Interest Rate: 8.75 Number of Months: 60 ================================================== Loan Applicant: Stephen Ignatavicius -------------------------------------------------- Loan Values Principal: 28865.0 Interest Rate: 8.75% Periods: 60 Months ================================================== Press any key to continue . . .
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