Assertions

Overview

Teaching: 10 min
Exercises: 0 min

Questions

• How can we compare observed and expected values?

Objectives

• Assertions are one line tests embedded in code.

• Assertions can halt execution if something unexpected happens.

• Assertions are the building blocks of tests.

Assertions are the simplest type of test. They are used as a tool for bounding acceptable behavior during runtime. The assert keyword in python has the following behavior:

>>> assert True == False

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  AssertionError

>>> assert True == True

That is, assertions raise an AssertionError if the comparison is false. It does nothing at all if the comparison is true. Assertions are therefore a simple way of writing tests.

>>> assert mean([1,2,3]) == 2

Testing Near Equality

Assertions are also helpful for catching abnormal behaviors, such as those that arise with floating point arithmetic. Using the assert keyword, how could you test whether some value is almost the same as another value?

• My package, mynum, provides the number a.
• Use the assert keyword to check whether the number a is greater than 2.
• Use the assert keyword to check that a is equal to 2 within an error of 0.003.

from mynum import a
# greater than 2 assertion here
# 0.003 assertion here

Solution

The simplest solution is

from mynum import a
assert a > 2
assert (2 - 0.003) < a < (2 + 0.003)

However what if we make a typo in the second assertion? We could be testing for the wrong thing! A better solution would be to use the function assert_allclose from NumPy:

from numpy.testing import assert_allclose
from mynum import a
assert a > 2
assert_allclose(a, 2, atol=0.003, rtol=0)

Tools to use: numpy.testing

The NumPy numerical computing library has a module which provides functions for testing floating point numbers, numpy.testing, which you should use whenever you are dealing with floating point numbers.

Key Points

• Assertions are one line tests embedded in code.

• The assert keyword is used to set an assertion.

• Assertions halt execution if the argument is false.

• Assertions do nothing if the argument is true.

• The numpy.testing module provides tools numeric testing.

• Assertions are the building blocks of tests.