Agile and Test-Driven Development

TDD Worked Example

Robert Haines, University of Manchester, UK

Adapted from "Test-Driven Development By Example", Kent Beck

Introduction

Very simple example
Implement a function to return the nth number in the Fibonacci sequence
- 0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, 233, ...
- http://oeis.org/A000045

$ F_0 = 0, \\ F_1 = 1, \\ F_n = F_{n-1} + F_{n-2} $

Step 0a: Local python setup

You need to do this if you're using python on the command line.

Create two directories
- src
- test
Add the src directory to PYTHONPATH
```
$ export PYTHONPATH=`pwd`/src
```

Step 0b: IPython setup

You need to do this if you're using this IPython Notebook.

The run_tests() method, below, is called at the end of each step to run the tests.



In [ ]:

    
import unittest

def run_tests():
    suite = unittest.TestLoader().loadTestsFromTestCase(TestFibonacci)
    unittest.TextTestRunner().run(suite)

Step 1: Write a test (and run it)



In [ ]:

    
class TestFibonacci(unittest.TestCase):

    def test_fibonacci(self):
        self.assertEqual(0, fibonacci(0), "fibonacci(0) should equal 0")

run_tests()

Step 1: Implement and re-test



In [ ]:

    
def fibonacci(n):
    return 0

run_tests()

Step 2: Write a test (and run it)



In [ ]:

    
class TestFibonacci(unittest.TestCase):

    def test_fibonacci(self):
        self.assertEqual(0, fibonacci(0), "fibonacci(0) should equal 0")
        self.assertEqual(1, fibonacci(1), "fibonacci(1) should equal 1")

run_tests()

Step 2: Implement and re-test



In [ ]:

    
def fibonacci(n):
    if n == 0: return 0
    return 1

run_tests()

Step 3: Write a test (and run it)



In [ ]:

    
class TestFibonacci(unittest.TestCase):

    def test_fibonacci(self):
        self.assertEqual(0, fibonacci(0), "fibonacci(0) should equal 0")
        self.assertEqual(1, fibonacci(1), "fibonacci(1) should equal 1")
        self.assertEqual(1, fibonacci(2), "fibonacci(2) should equal 1")

run_tests()

Step 3: It works!

The current code outputs 1 whenever n is not 0. So this behaviour is correct.

Step 4: Write a test (and run it)



In [ ]:

    
class TestFibonacci(unittest.TestCase):

    def test_fibonacci(self):
        self.assertEqual(0, fibonacci(0), "fibonacci(0) should equal 0")
        self.assertEqual(1, fibonacci(1), "fibonacci(1) should equal 1")
        self.assertEqual(1, fibonacci(2), "fibonacci(2) should equal 1")
        self.assertEqual(2, fibonacci(3), "fibonacci(3) should equal 2")

run_tests()

Step 4: Implement and re-test



In [ ]:

    
def fibonacci(n):
    if n == 0: return 0
    if n <= 2: return 1
    return 2

run_tests()

Step 5: Write a test (and run it)



In [ ]:

    
class TestFibonacci(unittest.TestCase):

    def test_fibonacci(self):
        self.assertEqual(0, fibonacci(0), "fibonacci(0) should equal 0")
        self.assertEqual(1, fibonacci(1), "fibonacci(1) should equal 1")
        self.assertEqual(1, fibonacci(2), "fibonacci(2) should equal 1")
        self.assertEqual(2, fibonacci(3), "fibonacci(3) should equal 2")
        self.assertEqual(3, fibonacci(4), "fibonacci(4) should equal 3")

run_tests()

Step 5: Implement and re-test



In [ ]:

    
def fibonacci(n):
    if n == 0: return 0
    if n <= 2: return 1
    if n == 3: return 2
    return 3

run_tests()

Pause

How many tests are we going to write?
Just how big is the set of if statements going to get if we carry on like this?
Where do we stop?

Remember: