how to make a calculator 3

I want the divide function in the calculator project to return a message when 0 is given as the second number, so that the program can continue to work after ZeroDivisionError is raised.

preview

I have these tests by the end of the chapter

import random
import src.calculator
import unittest


def a_random_number():
    return random.triangular(-1000.0, 1000.0)


class TestCalculator(unittest.TestCase):

    random_first_number = a_random_number()
    random_second_number = a_random_number()

    def test_addition(self):
        self.assertEqual(
            src.calculator.add(
                self.random_first_number,
                self.random_second_number
            ),
            self.random_first_number+self.random_second_number
        )

    def test_subtraction(self):
        self.assertEqual(
            src.calculator.subtract(
                self.random_first_number,
                self.random_second_number
            ),
            self.random_first_number-self.random_second_number
        )

    def test_multiplication(self):
        self.assertEqual(
            src.calculator.multiply(
                self.random_first_number,
                self.random_second_number
            ),
            self.random_first_number*self.random_second_number
        )

    def test_division(self):
        try:
            self.assertEqual(
                src.calculator.divide(
                    self.random_first_number,
                    self.random_second_number
                ),
                self.random_first_number/self.random_second_number
            )
        except ZeroDivisionError:
            self.assertEqual(
                src.calculator.divide(self.random_first_number, 0),
                'brmph?! I cannot divide by 0. Try again...'
            )


# Exceptions seen
# AssertionError
# NameError
# AttributeError
# TypeError

open the project

I change directory to the calculator folder
```
cd calculator
```
the terminal shows I am in the calculator folder
```
.../pumping_python/calculator
```

I use pytest-watcher to run the tests

uv run pytest-watcher . --now

the terminal is my friend, and shows

rootdir: .../pumping_python/calculator
configfile: pyproject.toml
collected 4 items

tests/test_calculator.py ....                                 [100%]

======================== 4 passed in X.YZs =========================

I hold ctrl on the keyboard, then click on tests/test_calculator.py to open it in the editor

test_division_handles_zero_division_error

RED: make it fail

I change the assertRaises to assertEqual in test_division

      def test_division(self):
          self.assertEqual(
              src.calculator.divide(
                  self.random_first_number,
                  self.random_second_number
              ),
              self.random_first_number/self.random_second_number
          )
          self.assertEqual(
              src.calculator.divide(self.random_first_number, 0),
              'brmph?! I cannot divide by 0. Try again...'
          )


# Exceptions seen

the terminal is my friend, and shows ZeroDivisionError

ZeroDivisionError: float division by zero

GREEN: make it pass

I add ZeroDivisionError to the list of Exceptions seen in test_calculator.py

# Exceptions seen
# AssertionError
# NameError
# AttributeError
# TypeError
# ZeroDivisionError

I use the Explorer to open calculator.py from the src folder in the editor

I add an exception handler to the divide function

def divide(first_input, second_input):
    try:
        return first_input / second_input
    except ZeroDivisionError:
        return 'brmph?! I cannot divide by 0. Try again...'

the test passes.

REFACTOR: make it better

There is a problem, the test uses random numbers, which means at some point random_second_number will have a value of 0 and the first assertion of test_division will raise ZeroDivisionError

I add a return statement to the a_random_number function in test_calculator.py to make it happen

def a_random_number():
    return 0
    return random.triangular(-1000.0, 1000.0)

the terminal is my friend, and shows ZeroDivisionError

    def test_division(self):
        self.assertEqual(
            src.calculator.divide(
                self.random_first_number,
                self.random_second_number
            ),
>           self.random_first_number/self.random_second_number
        )
E       ZeroDivisionError: division by zero

the expectation in test_division when random_second_number is 0 should be 'brmph?! I cannot divide by 0. Try again...'

I add an exception handler to the test

  def test_division(self):
      try:
          self.assertEqual(
              src.calculator.divide(
                  self.random_first_number,
                  self.random_second_number
              ),
              self.random_first_number/self.random_second_number
          )
      except ZeroDivisionError:
          self.assertEqual(
              src.calculator.divide(self.random_first_number, 0),
              'brmph?! I cannot divide by 0. Try again...'
          )

the test passes.

I remove the return statement from a_random_number to go back to testing with a range of random numbers

import random
import src.calculator
import unittest


def a_random_number():
    return random.triangular(-1000.0, 1000.0)


class TestCalculator(unittest.TestCase):

the test is still green.

close the project

I close test_calculator.py and calculator.py in the editor
I click in the terminal where the tests are running, then use q on the keyboard to leave the tests. The terminal goes back to the command line.
I change directory to the parent of calculator
```
cd ..
```
the terminal is my friend, and shows
```
...\pumping_python
```
I am back in the pumping_python directory

review

I ran tests to show that

I can cause any Exception I want with the raise keyword
I can use the assertRaises method to catch Exceptions in tests and tested these
I can use assertRaisesRegex to catch Exceptions with messages
I can use try..except…else to make programs that can choose what to do when Exceptions are raised

code from the chapter

Do you want to see all the CODE I typed in this chapter?

what is next?

you know

how to make a Python test driven development environment manually
how to raise AssertionError
how to make functions
what causes AttributeError
how to pass values from tests to functions
what is None and NOT None
what is True and False in Python
how to write programs that make decisions
how to make a calculator
how to make a Python Test Driven Development environment automatically
how to test that an Exception is raised with assertRaises
how to handle Exceptions in programs with try…except…else

Would you like to test TypeError?

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