Hello London!

Thank you for being here!

I'm a Computer Science Engineer, addicted to Python, clean code and books.

I'm also writing my own, it's going to be out in Jan. 2016.

About me

The plan:

What drew me to TDD?

Why do we need it?

A story about TDD

A little disclaimer

• This is my own view
• Simple examples
• No definitions

What drew me to TDD?

It all happened when I moved to London...

Mark Henwood

If someone is

achieving great results:

Observe and learn

def is_positive(n):
    # We assume n is integer.
    return n > 0

Example #1

How do we test this function?

def is_positive(n):
    # We assume n is integer.
    return n > 0


eq(False, is_positive(0))
eq(False, is_positive(-3))
eq(True, is_positive(3))

def is_positive(n):
    # We assume n is integer.
    return n > 1


eq(False, is_positive(0))   # still passing
eq(False, is_positive(-3))  # still passing
eq(True, is_positive(3))    # still passing

def is_positive(n):
    # We assume n is integer.
    return n > 1


eq(False, is_positive(0))   # still passing
eq(False, is_positive(-1))  # still passing
eq(True, is_positive(1))    # NOW FAILS!

Much better!

The boundary cannot jiggle any more!

def is_positive(n):
    # We assume n is integer.
    return n > 0


eq(False, is_positive(0))

for n in range(1, 10 ** 4):
    eq(False, is_positive(-n))
    eq(True, is_positive(n))

Even better!

But unit tests need to be FAST...

So, can we test everything?

def is_positive(n):
    # We assume n is integer.
    if n == 10 ** 16:
        return 'Hola!'
    return n > 0

def get_squares(v):
    # assumes v is a list of integers
    if not v:
        return []
    return [n ** 2 for n in v]

Example #2

How do we test this function?

def get_squares(v):
    # assumes v is a list of integers
    if not v:
        return []
    return [n ** 2 for n in v]


eq([1, 0, 4, 9], get_squares([-1, 0, 2, -3]))
eq([], get_squares([]))

def get_squares(v):
    # assumes v is a list of integers
    if not v:
        return []
    return [n ** 2 for n in v]


def get_squares2(v):
    # assumes v is a list of integers
    return [n ** 2 for n in v]

# this would cause us to write get_squares2
eq([1, 0, 4, 9], get_squares2([-1, 0, 2, -3]))

# this would automatically pass, thanks to
# the list comprehension
eq([], get_squares2([]))

Better than solving these problems, is to avoid introducing them in the first place.

And that's where TDD comes into the game

So let's hear a story about TDD

Once Upon a Time...

He was in love with a beautiful princess. One day, at the pond, the frog took courage, jumped out of the water and told her:

I am a prince under a spell, kiss me,

break the spell and marry me!

Prince? Intriguing!

But she was a Geek...

TDD?

Let me get back

to you on this...

And he ran off to learn TDD from the masters...

TDD

Test Driven Development

TDD is a Software Development Process based on the repetition of a very short development cycle.

Def:

Steps

• Short at first
• With experience: longer
• Trouble? Go back to short

Where does it start?

The frog thought the training was completed

What changes?

With TDD

What

How

TDD common aspects

• KISS
• YAGNI
• Architecture design during refactoring
• Three strikes and refactor
  (Test-Driven Development with Python - H. Percival)
• Triangulation

Triangulation

eq(4, square(-2))


def square(n):
    # we can cheat, it is
    # the only requirement
    return 4

First step

Main Benefits

• Refactor with Confidence
• Readability
• Loose Coupling
• Easier to test and maintain
• Test first => Better understanding of requirements
• Small units => easier debugging and tests as docs
• Higher speed:
  It takes less to write tests and code
  than to write code and debug

Main Shortcomings

• Whole company needs to believe
• Blind spots
• Badly written tests are hard to maintain

How do you test legacy code?

Better way...

Changing a horribly long view

def get(request, *args, **kwargs):    
    
    # imagine many lines of code here...    

    data = get_data(**search_params)

    # here data is prepared, worked on
    # put in the context dictionary
    # and the view finally renders
    # a template

    return render(template_name, context, extra_params)

We need to insert pagination, filtering, sorting

def get(request, *args, **kwargs):
    
    # same as before
    
    original_data = get_data(**search_params)

    filtered_data = filter_data(
        original_data, **filter_params)

    sorted_data = sort_data(
        filtered_data, **sort_params)

    data = paginate_data(
        sorted_data, **pagination_params)
    
    # same as before

    return render(template_name, context, extra_params)

We code pagination, filtering and sorting with TDD

Introducing a new functionality in existing code

We need to add a feature to a long piece of untested code.

def very_long_function(*args, **kwargs):
    
    # nasty piece of code that does
    # a lot of things.

    return result

We cannot test it (no time, badly written, etc.)

One possible solution:

def test_new_functionality():
    # preparation stage

    result = very_long_function(*args, **kwargs)

    eq(expected_result, result)


def very_long_function(*args, **kwargs):
    
    # same nasty piece of code
    # with NEW FUNCTIONALITY IN

    return result

After all these examples,

the frog was in ZEN-Mode

He went back to the princess and passed the exam.

Nothing changed!

He was just a talking frog after all!

What's the moral of the story?

And so should you