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

These are the tests I have by the end of the chapter

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 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 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 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 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 expectated calculation in test_division divides by 0 when random_second_number is 0 but the result 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, 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 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

Would you like to test TypeError?

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how to handle Exceptions (Errors) in programs

what causes TypeError?