When writing Python functions that may be used by others, it is important to demonstrate that your functions work as intended. In my last post I discussed how a proper function specification establishes a sort of contract between developers and users of functions that delineates errors in implementation (user error) from bugs in the code (developer error). While this contract is provides an important chain of accountability, it does not actually contain any proof that the function works as advertised. This is where unit testing comes in. A unit test simply runs a function over a suite of test cases (set of inputs that produce known results) to verify performance. As its name implies, a single unit test is meant to test one basic component of a code. Large or complex codes will have many sets of unit tests, each testing different elements (usually individual functions). Unit testing provides users with proof that your code works as intended, and serves as a powerful tool for finding and removing any errors in your code.

In this post I’ll provide guidance on the development of unit tests and demonstrate an example for a simple python function. Material in this post represents my interpretation of content taught by Professor Walker White at Cornell, in a Python Fundamentals course that I proctored in the summer of 2020.

## An example script

To illustrate unit testing I’ll examine the a function called “check_satisficing.py” which tests whether a set of performance objectives meets a set of satisficing criteria (for background on satisficing, see this post). The function returns a boolean and has three parameters:

- objs: a numpy array containing the two objective values
- criteria: a numpy array two criteria,
- directions: a list of strings that specify whether the criteria is meant to be a lower or upper bound.

Note that the actual code for this function only takes eight lines, the rest is the function specification and precondition checks.

```
def check_satisficing(objs, criteria, directions):
"""
Returns whether a set of objectives meets a set of satisficing criteria
Value return has type bool
Parameters:
objs: objective values from a given solution
criteria: satisficing criteria for each objective
directions: a list of strings containing "ge" or "le" for each
criteria, where ge indicates satisfication for values greater than
or equal to the criteria and le indicates satisfaction for values
less than or equal to the criteria
Examples:
objs = [.5, .5], criteria = [0.4, 0.4], directions = ['ge', 'ge']
returns True
objs = [.5, .5], criteria = [0.4, 0.4], directions = ['le', 'le']
returns False
objs = [.4, .4], criteria = [0.5, 0.5], directions = ['le', 'le']
returns True
objs = [.5, .5], criteria = [0.4, 0.4], directions = ['ge', 'le']
returns False
objs = [.5, .5], criteria = [0.4, 0.4], directions = ['le', 'ge']
returns False
objs = [.5, .5], criteria = [0.5, 0.5], directions = ['ge', 'ge']
returns True
objs = [.5, .5], criteria = [0.5, 0.5], directions = ['le', 'le']
returns True
Preconditions:
objs is a numpy array of floats with length 2
criteria is a numpy array of floats with length 2
directions is a list of strings with length 2 containing either
"ge" or "le"
"""
# check preconditions
assert len(objs) == 2, 'objs has length ' + repr(len(objs)) + ', should be 2'
assert len(criteria) == 2, 'criteria has length ' + repr(len(criteria)) + \
', should be 2'
assert len(directions) == 2, 'directions has length ' + \
repr(len(directions)) + ', should be 2'
# check to make sure
for i in range(2):
assert type(objs[i])== np.float64, 'objs element ' + str(i) + ': ' + \
repr(objs[i]) + ', is not a numpy array of floats'
assert type(criteria[i])== np.float64, 'criteria element ' + str(i) + \
': ' + repr(criteria[i]) + ', is not a numpy array of floats'
assert type(directions[i])== str, 'directions element ' + str(i) + \
': ' + repr(directions[i]) + ', is not a string'
assert directions[i] == 'ge' or directions[i] == 'le', 'directions ' + \
str(i) + ' is ' + repr(directions[i]) + ', should be either "ge" or "le"'
# loop through objectives and check if each meets the criteria
meets_criteria = True
for i in range(2):
if directions[i] == 'ge':
meets_criteria = meets_criteria and (objs[i] >= criteria[i])
else:
meets_criteria = meets_criteria and (objs[i] <= criteria[i])
return meets_criteria
```

## Developing test cases

If you read my last post, you may notice that this specification includes an extra section called “Examples”. These examples show the user how the function is supposed to perform. They also represent the suite of test cases used to validate the function. Creating test cases is more of an art than a science, and test cases will be unique to each function you write. However, there is a set of basic rule you can follow to guide your implementation of unit testing which I’ll outline below.

- Pick the simplest cases first. In this example, the simplest cases are when both objectives are both above or below the criteria
- Move to more complex cases. In this example, a more complex case could be when one objective is above and the other below, or vice versa
- Think about “weird” possibilities. One “weird” case for this code could be when one or both objectives are equal to the criteria
- Never test a precondition violation. Precondition violations are outside the scope of the function and should not be included in unit tests

Test cases should be exhaustive and even simple codes may have many test cases. In my example above I provide seven, can you think of any more that are applicable to this code?

## Implementing a testing strategy

After you’ve developed your test cases, it’s time to implement your unit test. For demonstration purposes I’ve written my own unit test code which can be used to test the cases developed above. This function simply utilizes assert statements to check if each test input generates the correct output. A danger of writing your own testing function is that the test function itself may have errors. In practice, it’s easiest to use an established tool such as PyTest to perform unit testing (for in-depth coverage of PyTest see Bernardo’s post from 2019).

```
def test_check_satisficing():
"""
Unit test for the function check_satisficing
"""
import numpy as np
from check_satisficing import check_satisficing
print("Testing check_satisficing")
# test case 1:
objs1 = np.array([0.5, 0.5])
criteria1 = np.array([0.4, 0.4])
directions1 = ['ge','ge']
result1 = True
assert (check_satisficing(objs1, criteria1, directions1)) == result1, \
'Test 1 failed ' + repr(objs1) + ', ' + repr(criteria1) + ', ' + \
repr(directions1) + ' returned False, should be True'
# test case 2:
objs2 = np.array([0.5, 0.5])
criteria2 = np.array([0.4, 0.4])
directions2 = ['ge','le']
result2 = False
assert (check_satisficing(objs2, criteria2, directions2)) == result2, \
'Test 2 failed ' + repr(objs2) + ', ' + repr(criteria2) + ', ' + \
repr(directions2) + ' returned True, should be False'
# test case 3:
objs3 = np.array([0.4, 0.4])
criteria3 = np.array([0.5, 0.5])
directions3 = ['le','le']
result3= True
assert (check_satisficing(objs3, criteria3, directions3)) == result3, \
'Test 3 failed ' + repr(objs3) + ', ' + repr(criteria3) + ', ' + \
repr(directions3) + ' returned False, should be True'
# test case 4:
objs4 = np.array([0.5, 0.5])
criteria4 = np.array([0.4, 0.4])
directions4 = ['ge','le']
result4 = False
assert (check_satisficing(objs4, criteria4, directions4)) == result4, \
'Test 4 failed ' + repr(objs4) + ', ' + repr(criteria4) + ', ' + \
repr(directions4) + ' returned True, should be False'
# test case 5
objs5 = np.array([0.5, 0.5])
criteria5 = np.array([0.4, 0.4])
directions5 = ['le','ge']
result5 = False
assert (check_satisficing(objs5, criteria5, directions5)) == result5, \
'Test 5 failed ' + repr(objs5) + ', ' + repr(criteria5) + ', ' + \
repr(directions5) + ' returned True, should be False'
# test case 6:
objs6 = np.array([0.5, 0.5])
criteria6 = np.array([0.5, 0.5])
directions6 = ['ge','ge']
result6 = True
assert (check_satisficing(objs6, criteria6, directions6)) == result6, \
'Test 6 failed ' + repr(objs6) + ', ' + repr(criteria6) + ', ' + \
repr(directions6) + ' returned False, should be True'
# test case 7:
objs7 = np.array([0.5, 0.5])
criteria7 = np.array([0.5, 0.5])
directions7 = ['le','le']
result7 = True
assert (check_satisficing(objs7, criteria7, directions7)) == result7, \
'Test 7 failed ' + repr(objs7) + ', ' + repr(criteria7) + ', ' + \
repr(directions7) + ' returned False, should be True'
print("check_satisficing has passed all tests!")
return
```

## Concluding thoughts

When developing a new Python function, it’s good practice to code the test cases while you’re writing the function and test each component during the development cycle. Coding in this matter will allow you to identify and fix errors early an can save a lot of time and headache. Creating test cases during code development may also improve the quality of your code by helping you conceptualize and avoid potential bugs.