TDD joyful coding

Why don't you just write unit tests and stop yelling?

Because you find it boring to test your code since you think it is already working fine. Also, you are most likely to write tests that will make your code pass, because your code wasn't designed to be testable.

Isn't it stupid to write tests for code that doesn't exist?

I thought so, but since I watched a demo about how it works, I started to understand how it can help you have more fun while coding

TDD steps

Write test to make it fail
write code to make it pass
Refactor the code
Repeat the steps 1,2,3 until you can't make it fail

This article will be a demo for Queue implementation:

First iteration

Step 1

#tests.py
class TestQueue(unittest.TestCase):
    def test_queue(self):
        q = MyQueue()

Step 2

#queue.py
class MyQueue:
    pass

Step 3

#tests.py
class TestQueue(unittest.TestCase):
    def test_queue(self):
        q = Queue()

#queue.py
class Queue:
    pass

Second iteration

Step 1

#tests.py
class TestQueue(unittest.TestCase):
    def test_new_queue_is_empty(self):
        q = Queue()
        self.assertTrue(q.is_empty())

Step 2

#queue.py
class Queue:
    def is_empty(self) -> bool:
        return True

Third iteration

Step 1

#tests.py
class TestQueue(unittest.TestCase):
    ...
    def test_queue_after_one_enqueue_not_empty(self):
        q = Queue()
        q.enqueue(3)
        self.assertFalse(q.is_empty())

Step 2

#queue.py
class Queue:
    __empty = True

    def is_empty(self) -> bool:
        return self.__empty

    def enqueue(self, element: int):
        self.__empty = False

Step 3

#tests.py
class TestQueue(unittest.TestCase):
    def setUp(self) -> None:
        self.q = Queue()

    def test_new_queue_is_empty(self):
        self.assertTrue(self.q.is_empty())

    def test_queue_after_one_enqueue_not_empty(self):
        self.q.enqueue(3)
        self.assertFalse(self.q.is_empty())

Fourth iteration

Step 1

#tests.py
class TestQueue(unittest.TestCase):
    ...
    def test_dequeue_from_empty_queue(self):
        self.assertRaises(UnderFlowException, self.q.dequeue)

Step 2

#queue.py
class UnderFlowException(Exception):
    def __str__(self):
        return "Can't dequeue from empty queue"

class Queue:
    ...
    def dequeue(self):
        if self.is_empty():
            raise UnderFlowException()
        return -1

Fifth iteration

Step 1

#tests.py
class TestQueue(unittest.TestCase):
    ...
    def test_is_empty_after_one_enqueue_one_dequeue(self):
        self.q.enqueue(10)
        self.q.dequeue()
        self.assertTrue(self.q.is_empty())

Step 2

#queue.py
    ...
    def dequeue(self):
        if self.is_empty():
            raise UnderFlowException()
        self.__empty = True
        return -1

Sixth iteration

Step 1

#tests.py
class TestQueue(unittest.TestCase):
    ...
    def test_is_not_empty_two_enqueue_one_dequeue(self):
        self.q.enqueue(10)
        self.q.enqueue(20)
        self.q.dequeue()
        self.assertFalse(self.q.is_empty())

Step 2

#queue.py
class Queue:
    __size = 0

    def is_empty(self) -> bool:
        return self.__size == 0

    def enqueue(self, element: int):
        self.__size += 1

    def dequeue(self):
        if self.is_empty():
            raise UnderFlowException()
        self.__size -= 1
        return -1

Seventh iteration

Step 1

#tests.py
class TestQueue(unittest.TestCase):
    ...
    def test_dequeue_value(self):
        self.q.enqueue(10)
        self.assertEqual(self.q.dequeue(), 10)
        self.q.enqueue(20)
        self.assertEqual(self.q.dequeue(), 20)

Step 2

#queue.py
class Queue:
    __size = 0
    __element = -1

    def is_empty(self) -> bool:
        return self.__size == 0

    def enqueue(self, element: int):
        self.__size += 1
        self.__element = element

    def dequeue(self) -> int:
        if self.is_empty():
            raise UnderFlowException()
        self.__size -= 1
        return self.__element

Eighth iteration

Step 1

#tests.py
class TestQueue(unittest.TestCase):
    ...
    def test_dequeue_value_after_two_enqueues(self):
        self.q.enqueue(10)
        self.q.enqueue(20)
        self.assertEqual(self.q.dequeue(), 10)

Step 2

#queue.py
class Queue:
    __size = 0
    __elements = []

    def is_empty(self) -> bool:
        return self.__size == 0

    def enqueue(self, element: int):
        self.__size += 1
        self.__elements.append(element)

    def dequeue(self) -> int:
        if self.is_empty():
            raise UnderFlowException()
        self.__size -= 1
        return self.__elements.pop(0)

Step 3

#queue.py
class Queue:
    __elements = []

    def is_empty(self) -> bool:
        return len(self.__elements) == 0

    def enqueue(self, element: int):
        self.__elements.append(element)

    def dequeue(self) -> int:
        if self.is_empty():
            raise UnderFlowException()
        return self.__elements.pop(0)