Since it’s impossible to guarantee that programs will never have bugs after being released, we need to test them before delivery to reduce potential errors. This article introduces basic assert statements and Python’s built-in unittest library.
Assert #
assert is a keyword in Python used for simple testing. To use it, place an expression that should evaluate to True after assert:
assert len("Hi") == 2
If the condition is True, nothing happens. If it’s False, the program immediately stops and throws an AssertionError. While this alerts us to a problem, it also means the program halts right away, so we can’t know if there are additional issues later in the code.
Unit Test #
To solve the issue above, we use a dedicated testing library that allows us to run multiple test cases and clearly see which ones pass or fail.
unittest is a commonly used unit testing library in Python. Unit testing means verifying the smallest testable parts of an application, such as checking if a function behaves as expected.
Example: Suppose this is a function in our program calculator.py:
def adder(x, y):
return x + y
Then the corresponding test code might be:
import unittest
from calculator import adder
class TestAdder(unittest.TestCase):
def test_positive_with_positive(self):
self.assertEqual(adder(5, 3), 8)
def test_neg_with_neg(self):
self.assertEqual(adder(-10, -2), -12)
Each test case is a method under a test class, and its name must start with test_. Only methods prefixed with test_ will be run as test cases.
Once written, just run the file and you’ll see the results. Each . means a test case passed; F means it failed.
unittest provides more assertions beyond assertEqual. Also, since each test runs independently, you often need to create objects repeatedly. To avoid redundant code, use the setUp method—it runs once before each test case.
Example without setUp:
import unittest
from sentence import Sentence
class TestSentence(unittest.TestCase):
def test_str_count(self):
sentence = Sentence("hello world!")
self.assertEqual(sentence.str_count(), 12)
def test_word_count(self):
sentence = Sentence("hello world!")
self.assertEqual(sentence.word_count(), 2)
def test_upper(self):
sentence = Sentence("hello world!")
self.assertEqual(sentence.upper(), "HELLO WORLD!")
Example using setUp:
import unittest
from sentence import Sentence
class TestSentence(unittest.TestCase):
def setUp(self):
self.sentence = Sentence("hello world!")
def test_str_count(self):
self.assertEqual(self.sentence.str_count(), 12)
def test_word_count(self):
self.assertEqual(self.sentence.word_count(), 2)
def test_upper(self):
self.assertEqual(self.sentence.upper(), "HELLO WORLD!")