Object oriented testing in Python

Prerequisite: Object-Oriented Testing

Automated Object-Oriented Testing can be performed in Python using Pytest testing tool. In this article, we perform object-oriented testing by executing test cases for classes. We write a program that has one parent class called Product and three child classes – Snack, Beverage, and Staples. We implement all the classes and save them in a file called product.py. The classes have the following functions: 

• Snack – printDetails() and getExpDate()
• Beverage – printDetails() and getExpDate()
• Staples – printDetails() and getExpDate()

It is important to note that getExpDate() is an overridden function in this case.

Code in product.py file: 

Python3

# importing the modules
from datetime import date
from dateutil.relativedelta import relativedelta

# base class


class Product:

    name = ""

    # printing the class in the constructor
    def __init__(self):
        print("super class Product")

# getExpDate() returns the expiry date of product
# since every product has different expiry date
# therefore this method is overridden by child classes
    def getExpDate():

        # gives exp date
        print("Expiry date")
        pass


# derived class 1
class Snack(Product):

    # months
    shelfLife = 6
    price = 0

    # constructor - initializing variables
    def __init__(self, name, price):
        self.name = name
        self.price = price

    # prints the Snack product details
    def printDetails(self):
        print("name : " + self.name)
        print("price : " + str(self.price))
        print("shelf life : " + str(self.shelfLife) + " months")

    # calculates the expiry date using relativedelta library and returns
    def getExpDate(self, pkdDate):
        expDate = pkdDate + relativedelta(months=self.shelfLife)
        return expDate

# derived class 2
class Beverage(Product):

    # 2 years
    shelfLife = 2
    price = 0

    # constructor - initializing variables
    def __init__(self, name, price):
        self.name = name
        self.price = price

    # prints the Beverage product details
    def printDetails(self):
        print("name : " + self.name)
        print("price : " + str(self.price))
        print("shelf life : " + str(self.shelfLife) + " years")

    # calculates the expiry date using relativedelta
    # library and returns
    def getExpDate(self, pkdDate):
        expDate = pkdDate + relativedelta(years=self.shelfLife)
        return expDate


# derived class 3
class Staples(Product):

    # 1 year
    shelfLife = 1
    price = 0

    # constructor - initializing variables
    def __init__(self, name, price):
        self.name = name
        self.price = price

    # prints the Staples product details
    def printDetails(self):
        print("name : " + self.name)
        print("price : " + str(self.price))
        print("shelf life : " + str(self.shelfLife) + " year")

    # calculates the expiry date using relativedelta
    # library and returns
    def getExpDate(self, pkdDate):
        expDate = pkdDate + relativedelta(years=self.shelfLife)
        return expDate


def main():
    s = Snack('cookies', 60)
    s.printDetails()
    print(s.name + " will expire on " +
          str(s.getExpDate(date(2019, 10, 3))) + "months")
    # yyyy-mm-dd

    p = Product()

    st = Staples('rice', 300)
    st.printDetails()
    print(st.name + " will expire on " + str(st.getExpDate(date(2020, 1, 23))))

    b = Beverage('coffee', 250)
    b.printDetails()
    print(b.name + " will expire on " + str(b.getExpDate(date(2018, 12, 17))))

    print("done till here")


if __name__ == '__main__':
    main()

Output:

name : cookies
price : 60
shelf life : 6 months
cookies will expire on 2020-04-03months
super class Product
name : rice
price : 300
shelf life : 1 year
rice will expire on 2021-01-23
name : coffee
price : 250
shelf life : 2 years
coffee will expire on 2020-12-17
done till here

To perform object-oriented testing, we write test cases for each of the classes. The following things should be kept in mind while writing these test cases: 

• Create a separate test class for testing the functions of each class, for e.g. TestSnack, TestStaple, TestBeverage
• Write a test case for each function of a class
• Use assert keyword to add assertions in the test cases. The assert statement will return True if the test case passes or will return False if the test case fails

We have written test cases for each function present in all classes of product.py file except the Product class (parent class). This is because the Product class has just one function and that too has been overridden by the child classes, so writing a test case would make no difference. Therefore, we write 6 test cases in total where two test cases are written for each of the child classes.

Code in test_product.py file: 

Python3

# importing the modules
import pytest
from product import Snack, Staples, Beverage
from datetime import date


class TestSnack:

      # test case for print details of Snack
    def test_details(self):
        s = Snack('chips' , 50)
        print("testing details : snack")
        assert ('chips' , 50, 6)  == (s.name, s.price, s.shelfLife)

    # test case for calculating exiry date of Snack
    def test_expDate(self):
        s = Snack('wafers', 40)
        print("testing expiry date : snack")
        expdate = s.getExpDate(date(2019, 10, 3))
        assert expdate == date(2020, 4, 3)


class TestStaple:

      # test case for print details of Staples
    def test_details(self):
        st = Staples('rice' , 300)
        print("testing details : staples")
        assert ('rice' , 300, 1)  == (st.name, st.price, st.shelfLife)

    # test case for calculating exiry date of Staples
    def test_expDate(self):
        st = Staples('wheat flour', 400)
        print("testing expiry date : staples")
        expdate = st.getExpDate(date(2020, 1, 23))
        assert expdate == date(2021, 1, 23)



class TestBeverage:

      # test case for print details of Beverage
    def test_details(self):
        b = Beverage('coffee' , 250)
        print("testing details : beverage")
        assert ('coffee' , 250, 2)  == (b.name, b.price, b.shelfLife)

    # test case for calculating exiry date of Beverage
    def test_expDate(self):
        b = Beverage('green tea', 400)
        print("testing expiry date : beverage")
        expdate = b.getExpDate(date(2018, 12, 17))
        assert expdate == date(2020, 12, 17)

Note: The function name and the test file name should always start with the word ‘test‘.

To execute the above test cases, create two separate files, product.py and test_product.py in a single folder. To execute write the following command:

pytest

OR

pytest -v

pytest -v  shows the verbose output.

The output is shown below: 

If we change the value in one of the assertion statements of a test case, it will lead the test case to fail. Refer to the output shown below: 

In the output shown above, test_expDate() test case in class TestBeverage failed as the assertion statement resulted in a False expression.