#################
Part 2: Why test?
#################

It's likely you already have some familiarity with software testing in
general, and writing tests in Python specifically. In this section,
we're going to revisit the basic question: why write software tests? Our
discussion here will form the foundation of the rest of the lesson.

It's highly recommended for you to type along with these examples, creating the
squarer.py file and the corresponding test files!

A Simple Example
----------------

Suppose that you have written a simple class for squaring numbers:

.. raw:: html

   <div
   style#"background: #ffffff; overflow: auto; width: auto; border: solid gray; border-width: .1em .1em .1em .8em; padding: .2em .6em;">

::

    # squarer.py
    class Squarer(object):

        @staticmethod
        def calc(operand):
            return operand*operand

.. raw:: html

   </div>

Squarer is a simple class, with a single static method: calc. The calc
method calculates the square of its argument in the usual way, and
returns it to the caller.

Let's use the Squarer class from the REPL (the Python shell): 

::

    $ python -i
    >>> from squarer import Squarer
    >>> Squarer.calc(3)
        9
    >>> Squarer.calc(100)
        10000

Squarer works well: squaring 3 yields 9. Squaring 100 yields 10,000. The
Squarer class is working perfectly.

Now imagine that another engineer working on this project
has a "bright idea" about how to improve the Squarer class: squaring
involves powers, therefore the Squarer class should be using the power
operator. They make the following modification to your code:

.. raw:: html

   <div
   style#"background: #ffffff; overflow: auto; width: auto; border: solid gray; border-width: .1em .1em .1em .8em; padding: .2em .6em;">

::

    # squarer.py
    class Squarer(object):

        @staticmethod
        def calc(operand):
            # return operand*operand  # OLD
            return operand**operand

.. raw:: html

   </div>

Then at some point later you're using the squarer class to square
numbers:

::

    $ python -i
    >>> from squarer import Squarer
    >>> Squarer.calc(3)
        27
    >>> Squarer.calc(100)
        100000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000

The square of 3 is definitely not 27. And squaring 100 should give
10,000, not
100000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000!
Your code is definitely broken: an error has been introduced into the
Squarer class.