Python unittest subtest
Summary: in this tutorial, you’ll learn how to define parameterized tests using unittest‘s subTest() context manager.
Introduction to the unittest subtest context manager
First, create a new module called pricing.py and define a calculate() function as follows:
def calculate(price, tax=0, discount=0):
return round((price - discount) * (1+tax), 2)Code language: Python (python)
The calculate() function calculates the net price from the price, tax, and discount.
Second, create the test_pricing.py test module to test the calculate() function:
import unittest
from pricing import calculate
class TestPricing(unittest.TestCase):
def test_calculate(self):
pass
Code language: Python (python)
To test the calculate() function, you need to come up with multiple test cases, for example:
- Has price with no tax and no discount
- Has price with tax but no discount
- Has price with no tax and discount
- Has price with tax and discount
To cover these cases, you need to have various test methods. Or you can define a single test method and supply test data from a list of cases. For example:
import unittest
from pricing import calculate
class TestPricing(unittest.TestCase):
def test_calculate(self):
items = (
{‘case’: ‘No tax, no discount’, ‘price’: 10, ‘tax’: 0, ‘discount’: 0, ‘net_price’: 10},
{‘case’: ‘Has tax, no discount’, ‘price’: 10, ‘tax’: 0.1, ‘discount’: 0, ‘net_price’: 10},
{‘case’: ‘No tax, has discount’, ‘price’: 10, ‘tax’: 0, ‘discount’: 1, ‘net_price’: 10},
{‘case’: ‘Has tax, has discount’, ‘price’: 10, ‘tax’: 0.1, ‘discount’: 1, ‘net_price’: 9.9},
)
for item in items:
with self.subTest(item[‘case’]):
net_price = calculate(
item[‘price’],
item[‘tax’],
item[‘discount’]
)
self.assertEqual(
net_price,
item[‘net_price’]
)
Code language: Python (python)
Run the test:
python -m unittest test_pricing -vCode language: Python (python)
Output:
test_calculate (test_pricing.TestPricing) ... FAIL
======================================================================
FAIL: test_calculate (test_pricing.TestPricing)
———————————————————————-
Traceback (most recent call last):
File “D:\python-unit-testing\test_pricing.py”, line 26, in test_calculate
self.assertEqual(
AssertionError: 11.0 != 10
———————————————————————-
Ran 1 test in 0.002s
FAILED (failures=1)
Code language: plaintext (plaintext)
The problem with this approach is that the test stops after the first failure. To resolve this, the unittest provides you with the subTest() context manager. For example:
import unittest
from pricing import calculate
class TestPricing(unittest.TestCase):
def test_calculate(self):
items = (
{‘case’: ‘No tax, no discount’, ‘price’: 10, ‘tax’: 0, ‘discount’: 0, ‘net_price’: 10},
{‘case’: ‘Has tax, no discount’, ‘price’: 10, ‘tax’: 0.1, ‘discount’: 0, ‘net_price’: 10},
{‘case’: ‘No tax, has discount’, ‘price’: 10, ‘tax’: 0, ‘discount’: 1, ‘net_price’: 10},
{‘case’: ‘Has tax, has discount’, ‘price’: 10, ‘tax’: 0.1, ‘discount’: 1, ‘net_price’: 9.9},
)
for item in items:
with self.subTest(item[‘case’]):
net_price = calculate(
item[‘price’],
item[‘tax’],
item[‘discount’]
)
self.assertEqual(
net_price,
item[‘net_price’]
)
Code language: Python (python)
Run the test:
python -m unittest test_pricing -vCode language: Python (python)
Output:
test_calculate (test_pricing.TestPricing) ...
======================================================================
FAIL: test_calculate (test_pricing.TestPricing) [Has tax, no discount]
----------------------------------------------------------------------
Traceback (most recent call last):
File "D:\python-unit-testing\test_pricing.py", line 26, in test_calculate
self.assertEqual(
AssertionError: 11.0 != 10FAIL: test_calculate (test_pricing.TestPricing) [No tax, has discount]
———————————————————————-
Traceback (most recent call last):
File “D:\python-unit-testing\test_pricing.py”, line 26, in test_calculate
self.assertEqual(
AssertionError: 9 != 10
Ran 1 test in 0.001s
Code language: plaintext (plaintext)
By using the subTest() context manager, the test didn’t stop after the first failure. Also, it shows a very detailed message after each failure so that you can examine the case.
The subTest() context manager syntax
The following shows the subTest() context manager syntax:
def subTest(self, msg=_subtest_msg_sentinel, **params):Code language: Python (python)
The subTest() returns a context manager. The optional message parameter identifies the closed block of the subtest returned by the context manager.
If a failure occurs, it’ll mark the test case as failed. However, it resumes execution at the end of the enclosed block, allowing further test code to be executed.
Summary
- Use the unittest
subTest()context manager to parameterize tests.