Implementing Simple Test Driven Development

Question

I am trying to test the calculator program using the python unittest on the following test scenarios. But when I try to instantiate the "CalculatorClass" using the init it gives the following error message in the bottom. I want to use init for instantiating. I am using Visual Studio Code. Could anyone help me with this?

This is the code of "Calculator_test.py"

import unittest

import Calculator_steps as CalculatorClass

class TestCalculator(unittest.TestCase):
    
    def __init__(self):
        self.calculator = CalculatorClass.Calculator()  # instantiate the CalculatorClass
    
    def test_DecimalValues(self):
        with self.assertRaises(Exception):
            self.calculator.addNumbers('3.55,8.76')
            
    def test_NonNumeric(self):
        with self.assertRaises(Exception):
            self.calculator.addNumbers('7,8,e')    
            
    def test_MoreThanThreeIntegers(self):
        with self.assertRaises(Exception):
            self.calculator.addNumbers('5,7,2,8')   
            
    def test_MoreThan100(self):
        with self.assertRaises(Exception):
            self.calculator.addNumbers('109,76,90')

Following is the code of Calculator_steps.py

class Calculator:

    def addNumbers(self, numberlist):
        total = 0 
        if numberlist=="":
            numberlist ="0"
        numbers = numberlist.split(",")
        if len(numbers)>3:
            raise TypeError("Only three values can be entered")
        for i in range(len(numbers)):
            if int(numbers[i])>100:
                raise TypeError("Only values less than 100 is allowed")
            try:
                total = total +int(numbers[i])
            except ValueError:
                TypeError("Only integers can be entered")

        return total

Following is the error message

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PS C:\Users\mudit> py -m unittest Calculator_tests.py
Traceback (most recent call last):
  File "C:\Users\mudit\AppData\Local\Programs\Python\Python310\lib\runpy.py", line 196, in _run_module_as_main
    return _run_code(code, main_globals, None,
  File "C:\Users\mudit\AppData\Local\Programs\Python\Python310\lib\runpy.py", line 86, in _run_code
    exec(code, run_globals)
  File "C:\Users\mudit\AppData\Local\Programs\Python\Python310\lib\unittest\__main__.py", line 18, in <module>
    main(module=None)
  File "C:\Users\mudit\AppData\Local\Programs\Python\Python310\lib\unittest\main.py", line 100, in __init__   
    self.parseArgs(argv)
  File "C:\Users\mudit\AppData\Local\Programs\Python\Python310\lib\unittest\main.py", line 147, in parseArgs
    self.createTests()
  File "C:\Users\mudit\AppData\Local\Programs\Python\Python310\lib\unittest\main.py", line 158, in createTests
    self.test = self.testLoader.loadTestsFromNames(self.testNames,
  File "C:\Users\mudit\AppData\Local\Programs\Python\Python310\lib\unittest\loader.py", line 220, in loadTestsFromNames
    suites = [self.loadTestsFromName(name, module) for name in names]
  File "C:\Users\mudit\AppData\Local\Programs\Python\Python310\lib\unittest\loader.py", line 220, in <listcomp>
    suites = [self.loadTestsFromName(name, module) for name in names]
  File "C:\Users\mudit\AppData\Local\Programs\Python\Python310\lib\unittest\loader.py", line 191, in loadTestsFromName
    return self.loadTestsFromModule(obj)
  File "C:\Users\mudit\AppData\Local\Programs\Python\Python310\lib\unittest\loader.py", line 124, in loadTestsFromModule
    tests.append(self.loadTestsFromTestCase(obj))
  File "C:\Users\mudit\AppData\Local\Programs\Python\Python310\lib\unittest\loader.py", line 93, in loadTestsFromTestCase
    loaded_suite = self.suiteClass(map(testCaseClass, testCaseNames))
  File "C:\Users\mudit\AppData\Local\Programs\Python\Python310\lib\unittest\suite.py", line 24, in __init__
    self.addTests(tests)
  File "C:\Users\mudit\AppData\Local\Programs\Python\Python310\lib\unittest\suite.py", line 57, in addTests
    for test in tests:
TypeError: TestCalculator.__init__() takes 1 positional argument but 2 were given

Answer 1

When inheriting from unittest.TestCase, you do cannot overwrite the __init__ method. Instead, anything you want to do before each test runs can be put in the setUp method, like so:

import unittest

import Calculator_steps as CalculatorClass

class TestCalculator(unittest.TestCase):
    

    def setUp(self):
        self.calculator = CalculatorClass.Calculator()  # instantiate the CalculatorClass
    
    def test_DecimalValues(self):
        with self.assertRaises(Exception):
            self.calculator.addNumbers('3.55,8.76')
            
    def test_NonNumeric(self):
        with self.assertRaises(Exception):
            self.calculator.addNumbers('7,8,e')    
            
    def test_MoreThanThreeIntegers(self):
        with self.assertRaises(Exception):
            self.calculator.addNumbers('5,7,2,8')   
            
    def test_MoreThan100(self):
        with self.assertRaises(Exception):
            self.calculator.addNumbers('109,76,90')

To expand, the unitttest module is designed in such a way that special things happen in the __init__ method of unittest.TestCase, and the module does not support overwriting the __init__ method. In theory, you could overwrite the __init__ method if you call super().__init__ at some point during the overwritten method, but why would you want to? The unittest module is designed so that you should never need to overwrite the __init__ method. You can use the setUp and setUpClass methods to do any instantiating needed before running your tests