python-mastery/Exercises/ex5_6.md

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Exercise 5.6

Objectives:

• Learn how to use Python’s unittest module

Files Created: teststock.py

In this exercise, you will explore the basic mechanics of using Python’s unittest modules.

(a) Preliminaries

In previous exercises, you created a file stock.py that contained a Stock class. In a separate file, teststock.py, define the following testing code:

# teststock.py

import unittest
import stock

class TestStock(unittest.TestCase):
    def test_create(self):
        s = stock.Stock('GOOG', 100, 490.1)
        self.assertEqual(s.name, 'GOOG')
        self.assertEqual(s.shares, 100)
        self.assertEqual(s.price, 490.1)

if __name__ == '__main__':
    unittest.main()

Make sure you can run the file:

 bash % python3 teststock.py
 .
 ------------------------------------------------------------------```
 Ran 1 tests in 0.001s

 OK
 bash %

(b) Unit testing

Using the code in teststock.py as a guide, extend the TestStock class with tests for the following:

• Test that you can create a Stock using keyword arguments such as Stock(name='GOOG',shares=100,price=490.1).
• Test that the cost property returns a correct value
• Test that the sell() method correctly updates the shares.
• Test that the from_row() class method creates a new instance from good data.
• Test that the __repr__() method creates a proper representation string.
• Test the comparison operator method __eq__()

(c) Unit tests with expected errors

Suppose you wanted to write a unit test that checks for an exception. Here is how you can do it:

class TestStock(unittest.TestCase):
    ...
    def test_bad_shares(self):
        s = stock.Stock('GOOG', 100, 490.1)
        with self.assertRaises(TypeError):
             s.shares = '50'
    ...

Using this test as a guide, write unit tests for the following failure modes:

• Test that setting shares to a string raises a TypeError
• Test that setting shares to a negative number raises a ValueError
• Test that setting price to a string raises a TypeError
• Test that setting price to a negative number raises a ValueError
• Test that setting a non-existent attribute share raises an AttributeError

In total, you should have around a dozen unit tests when you’re done.

Important Note

For later use in the course, you will want to have a fully working stock.py and teststock.py file. Save your work in progress if you have to, but you are strongly encouraged to copy the code from Solutions/5_6 if things are still broken at this point.

We’re going to use the teststock.py file as a tool for improving the Stock code later. You’ll want it on hand to make sure that the new code behaves the same way as the old code.

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