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Part 3: Automated Test Script 
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If you were to sit down and think about this problem, here's one
solution that you could come up with:

-  When I try squaring 3 I expect to get 9, but I got 27. When I try
   squaring 100 I expect to get 10,000, but I got a much larger number.
-  It takes me a minute or two to try these examples at the REPL, and
   getting into the REPL and trying these examples is kind of a hassle.
-  So I could write a Python script that tries using the Squarer class
   to square 3 and 100. Running the script will be easy and only take a
   few seconds, so when I make a change to Squarer it will only take me
   a few seconds to find out whether I've broken it or not.
-  I could make a habit of running this test script every time I make a
   change to Squarer. If I have a colleague who is working on the
   Squarer class, then they should run the script whenever *they* make
   a change to Squarer. In this way, we'll know right away if we've
   introduced an error into Squarer.

Here's a suitable test script:

.. raw:: html

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

::

    # test.py
    from squarer import Squarer

    class SquarerTest(object):

        @staticmethod
        def test_positive_numbers():

            squares = {
                1: 1,
                2: 4,
                3: 9,
                12: 144,
                100: 10000,
            }

            for num, square in squares.items():
                result = Squarer.calc(num)

                if result != square:
                    print("Squared {} and got {} but expected {}".format(num, result, square))
        @staticmethod
        def test_negative_numbers():

            squares = {
                -1: 1,
                -2: 4,
                -3: 9,
                -12: 144,
                -100: 10000,
            }

            for num, square in squares.items():
                result = Squarer.calc(num)

                if result != square:
                    print("Squared {} and got {} but expected {}".format(num, result, square))

    if __name__ == "__main__":
        SquarerTest.test_positive_numbers()
        SquarerTest.test_negative_numbers()

.. raw:: html

   </div>

The SquarerTest class has two static methods: test_positive_numbers
and test_negative_numbers. I'll explain the test_positive_numbers
method, and the test_negative_numbers method is nearly identical.

The goal of this script is to define the expected behavior of Squarer
and test Squarer against that expected behavior. So we begin by creating
a dictionary that defines that behavior:

.. raw:: html

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

::

        squares = {
            1: 1,
            2: 4,
            3: 9,
            12: 144,
            100: 10000,
        }

.. raw:: html

   </div>

Each *key* in this dictionary is a number. The corresponding *value*
is the value that we expect Squarer to produce when we square the key.
So one of the key/value pairs that we are testing is 12 and 144:
squaring 12 should produce 144.

Next we iterate through these number, square pairs:

.. raw:: html

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

::

        for num, square in squares.items():
            result = Squarer.calc(num)

            if result != square:
                print("Squared {} and got {} but expected {}\n".format(num, result, square))

.. raw:: html

   </div>

We use Squarer to square the number, and capture the result in a
variable named *result*. If *result* is not equal to the square that
we defined in *squares*, then we print a message describing the error.

Finally, we add a *__name__ == "__main__"* clause so that we
can run this script from the command line:

.. raw:: html

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

::

    if __name__ == "__main__":
        SquarerTest.test_positive_numbers()
        SquarerTest.test_negative_numbers()