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Performing Equivalence Class Testing using Pytest

  • Last Updated : 10 Sep, 2020

Prerequisite – Equivalence Class Testing

To perform automated Equivalence Class Testing, we can make use of Pytest or Unittest Python libraries. In this article, we will use the Pytest library to execute test cases for a simple program.

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Question :
Perform Equivalence Testing for a program that determines the type of triangle given the length of its three sides A, B, and C. The range of the length of the sides lies between 10 and 50(both inclusive).

Code in file :


# import math
# parent class for Error
class Error(BaseException):
# child class of Error named OutOfRangeError
class OutOfRangeError(Error):
    def __init__(self, message):
        self.message = message
# child class of Error named TriangleError        
class TriangleError(Error):
    def __init__(self, message):
        self.message = message
# checks if variables are in range
# if variables not in range then OutOfRangeError is raised
def checkRange(a, b, c):
    if a<10 or a>50:
        raise OutOfRangeError('A is out of the given range')
    if b<10  or b>50:
        raise OutOfRangeError('B is out of the given range'
    if c<10 or c>50:
        raise OutOfRangeError('C is out of the given range')
# checks if the given values of a, b, c can form a triangle
# if not, then Triangle error is raised
def checkTriangle(a, b, c):
    if a + b<= c or b + c<= a or c + a<= b:
        raise TriangleError('Triangle cannot be formed with these sides')
# determines the type of triangle        
def triangleType(a, b, c):
    checkRange(a, b, c)
    checkTriangle(a, b, c)
    # s = (a + b+c)/2
    # ar = math.sqrt(s*(s-a)*(s-b)*(s-c))
    # inradius = ar / s
    if(a == b and b == c):   
        return "Equilateral Triangle"
    elif(a == b or a == c or b == c):
        return "Isosceles Triangle"
        return "Scalene Triangle"
def main():
        print("Enter the sides of the triangle in range [10-50]")
        a = int(input('Enter Side A:'))
        b = int(input('Enter Side B:'))
        c = int(input('Enter Side C:'))
    except ValueError as v:
        print(v + " Raised :Input is not an integer.")
        checkRange(a, b, c)
    except OutOfRangeError as e:
        print("OutOfRangeError:" + e.message)
        checkTriangle(a, b, c)
    except TriangleError as e:
        print('TriangleError:' + e.message)
    typeOfTriangle = triangleType(a, b, c)
    print("The triangle is: " + typeOfTriangle)
if __name__ == "__main__":

Now, we need to write test cases for the above program using the Pytest library. Each test case is written in a separate function where we use pytest.raises function to check that the given input is valid or invalid. For the above program, we create 7 invalid classes and 1 valid class. The 7 invalid classes are :

  • a < 10
  • a > 50
  • b < 10
  • b > 50
  • c < 10
  • c > 50
  • Given values of a, b, c cannot form a triangle

The 1 valid class is :

  • 10 <= a, b, c <= 50

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

Code in file :


import pytest
# importing classes and function which we use in this file
from triangletype import OutOfRangeError
from triangletype import TriangleError
from triangletype import triangleType
# check if a < 10
def test_invalid_less_a():
    with pytest.raises(OutOfRangeError):
        triangleType(9, 20, 15)
# check if b < 10        
def test_invalid_less_b():
    with pytest.raises(OutOfRangeError):
        triangleType(20, 9, 15)
# check if c < 10        
def test_invalid_less_c():
    with pytest.raises(OutOfRangeError):
        triangleType(20, 15, 9)
# check if a > 50
def test_invalid_more_a():
    with pytest.raises(OutOfRangeError):
        triangleType(51, 30, 45)
# check if b > 50        
def test_invalid_more_b():
    with pytest.raises(OutOfRangeError):
        triangleType(30, 51, 45)
# check if c > 50        
def test_invalid_more_c():
    with pytest.raises(OutOfRangeError):
        triangleType(30, 45, 51)
# check if a, b, c can form a triangle or not        
def test_valid_invalidtriangle():
    with pytest.raises(TriangleError):
        triangleType(20, 15, 40)
# valid class - determines type of triangle        
def test_valid():
    assert triangleType(20, 15, 10) == "Scalene Triangle" 

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



pytest -v

pytest -v shows the verbose output.

The output is shown below :

As we can see in the above output, all the 8 test cases passed. However, if we edit a test case, for example, if we change the value of c variable to 90 in the test_valid() test case in file, then the test case fails :

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