how to make a calculator part 2

test_division_handles_zero_division_error

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

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 activate the virtual environment
```
source .venv/bin/activate
```
Attention

on Windows without Windows Subsystem for Linux use .venv/bin/activate.ps1 NOT source .venv/bin/activate
```
.venv/scripts/activate.ps1
```
the terminal shows
```
(.venv) .../pumping_python/calculator
```

I use pytest-watch to run the tests

pytest-watch

the terminal shows

rootdir: .../pumping_python/calculator
collected 4 items

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

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

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

RED: make it fail

I change the last assertion 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),
            'undefined: I cannot divide by 0'
        )


# Exceptions seen

the terminal shows ZeroDivisionError

ZeroDivisionError: float division by zero

GREEN: make it pass

I add it 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 'undefined: I cannot divide by 0'

the test passes.

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 part of test_division will raise ZeroDivisionError

REFACTOR: make it better

I add a return statement to the a_random_number function in test_calculator.py to make it happen
```
6def a_random_number():
7    return 0
8    return random.triangular(-1000.0, 1000.0)
```
the terminal shows ZeroDivisionError
```
>           self.random_first_number/self.random_second_number
        )
E       ZeroDivisionError: division by zero
```
the expectation calculation in test_division divides by 0 when random_second_number is 0 but the result should be 'undefined: I cannot divide by 0'

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),
              'undefined: I cannot divide by 0'
          )

the test passes

I remove the return statement from a_random_number to go back to testing with a range of 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 and exit the tests with ctrl+c on the keyboard, the terminal shows
```
(.venv) ...\pumping_python\calculator
```
I deactivate the virtual environment
```
deactivate
```
the terminal goes back to the command line, (.venv) is no longer on the left side
```
...\pumping_python\calculator
```
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 the following
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

How many questions can you answer after going through this chapter?

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?