5.4 Handling Errors

Estimated time: 10 minutes

Overview

Questions

• FIXME

Objectives

• FIXME

How do we Handle Testing for Errors?

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But what do we do if our code is expected to throw an error? How would we test for that?

Let’s try our code with a negative number, which we’ve already identified as a good test case, from within the python interpreter:

PYTHON

>>> from mymath.factorial import factorial
>>> factorial(-1)

We can see that we get the result of 1, which is incorrect, since the factorial function is undefined for negative numbers.

Perhaps what we want in this case is to test for negative numbers as an invalid input, and display an exception if that is the case. How would we implement that, and how would we test for the presence of an exception?

In our implementation let’s add a check at the start of our function, which is known as a precondition. The precondition will check the validity of our input data before we do any processing on it, and this approach to checking function input data is considered good practice.

Edit the mymath/factorial.py file again, and add at the start, below the docstring:

PYTHON

if n < 0:
    raise ValueError('Only use non-negative integers.')

If we run it now, we should see our error:

PYTHON

>>> from mymath.factorial import factorial
>>> factorial(-1)

OUTPUT

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/steve/factorial-example/mymath/factorial.py", line 9, in factorial
    raise ValueError('Only use non-negative integers.')
ValueError: Only use non-negative integers.

Sure enough, we get our exception as desired. But how do we test for this in a unit test, since this is an exception, not a value? Fortunately, unit test frameworks have ways to check for this.

Let’s add a new test to tests/test_factorial.py:

PYTHON

    def test_negative(self):
        with self.assertRaises(ValueError):
          factorial(-1)

So here, we use unittest’s built-in assertRaises() (instead of assertEquals()) to test for a ValueError exception occurring when we run factorial(-1). We also use Python’s with here to test for this within the call to factorial(). So if we re-run our tests again, we should see them all succeed:

BASH

python -m unittest -v tests/test_factorial.py

You should see:

OUTPUT

test_3 (tests.test_factorial.TestFactorialFunctions) ... ok
test_5 (tests.test_factorial.TestFactorialFunctions) ... ok
test_negative (tests.test_factorial.TestFactorialFunctions) ... ok

----------------------------------------------------------------------
Ran 3 tests in 0.000s

OK

Brief Summary

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So we now have the beginnings of a test suite! And every time we change our code, we can rerun our tests. So the overall process of development becomes:

• Add new functionality (or modify new functionality) to our code
• Potentially add new tests to test any new functionality
• Re-run all our tests

Key Points

• FIXME