Pytest testing expected exception

Pytest provides a tool that allows us to “expect and exception”. We’ll usually use it using a with context manager.

Here we can see two different ways to verify the exception.

import pytest
from fibonacci import fib

def test_fib():
    assert fib(10) == 55

def test_fib_negative():
    with pytest.raises(Exception) as err:
        fib(-1)
    assert err.type == ValueError
    assert str(err.value) == 'Invalid parameter -1'

def test_fib_negative_again():
    with pytest.raises(ValueError) as err:
        fib(-1)
    assert str(err.value) == 'Invalid parameter -1'

Output:

============================= test session starts ==============================
platform linux -- Python 3.8.6, pytest-6.1.2, py-1.9.0, pluggy-0.13.1
rootdir: /home/gabor/work/slides/python/examples/pytest/fib2
plugins: flake8-1.0.6, dash-1.17.0
collected 3 items

test_fibonacci.py ...                                                    [100%]

============================== 3 passed in 0.01s ===============================

Now that we are testing the exception let’s see how will this test protect us?

• Someone changing the text of the exception.
• Someone removing the exception.