Python unittest subtest (original) (raw)

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:

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 != 10

======================================================================
FAIL: 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

FAILED (failures=2)`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 #

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