Asserting Exceptions with Pytest

January 2019 ยท 3 minute read

First time I had someone review my pull requests, she was pretty strict on tests. I couldn’t merge if the tests were failing, of course. But I also couldn’t merge if coverage had decreased by even 1%. TDD was still new to me so maintaining coverage was a challenge since I was only testing the bare minimum I could. I had to find out how to make my tests more robust and ensure as much of my code was tested as possible. One area that I wasn’t really sure how to test was the custom exceptions I had written. Here’s an example:

# login.py

def check_email_format(email):
    """check that the entered email format is correct"""
    pass

def test_email_exception():
    """test that exception is raised for invalid emails"""
    pass

This is probably something you want to do if you’re implementing a system with email authentication. The example is oversimplified, but it serves the purpose of this post well.

To test for raised exceptions, pytest offers a handy method: pytest.raises. Let’s see how to use it in our example:

import re
import pytest

def check_email_format(email):
    """check that the entered email format is correct"""
    if not re.match(r"(^[a-zA-Z0-9_.+-]+@[a-zA-Z0-9-]+\.[a-zA-Z0-9-.]+$)", email):
        raise Exception("Invalid email format")
    else:
        return "Email format is ok"

def test_email_exception():
    """test that exception is raised for invalid emails"""
    with pytest.raises(Exception):
        assert check_email_format("good@email.com")

The check_email_format method takes in an email and checks that it matches the regex pattern given. If it does, it returns "Email format is ok", otherwise, an exception is raised.

Using pytest.raises in a with block as a context manager, we can check that an exception is actually raised if an invalid email is given. Running the tests on the code as it is above should fail:

collected 1 item
login.py F                [100%]

==================== FAILURES ========================

    def test_email_exception():
        """test that exception is raised for invalid emails"""
        with pytest.raises(Exception):
>           assert check_email_format("good@email.com")
E           Failed: DID NOT RAISE <class 'Exception'>

login.py:16: Failed

Notice it says Failed: DID NOT RAISE <class 'Exception'>. If an exception is not raised, the test fails. I found this to be pretty awesome. We passed in a valid email format (according to our standards here) so the test works as expected. Now we can make it pass.

import re
import pytest

def check_email_format(email):
    """check that the entered email format is correct"""
    if not re.match(r"(^[a-zA-Z0-9_.+-]+@[a-zA-Z0-9-]+\.[a-zA-Z0-9-.]+$)", email):
        raise Exception("Invalid email format")
    else:
        return "Email format is ok"

def test_email_exception():
    """test that exception is raised for invalid emails"""
    with pytest.raises(Exception):
        assert check_email_format("bademail.com") # invalid email format to raise exception

Run your test: pytest login.py:

collected 1 item

login.py .              [100%]

====================== 1 passed in 0.05 seconds ======================

You can also add an extra check for the exception message:

import re
import pytest

def check_email_format(email):
    """check that the entered email format is correct"""
    if not re.match(r"(^[a-zA-Z0-9_.+-]+@[a-zA-Z0-9-]+\.[a-zA-Z0-9-.]+$)", email):
        raise Exception("Invalid email format")
    else:
        return "Email format is ok"

def test_email_exception():
    """test that exception is raised for invalid emails"""
    with pytest.raises(Exception) as e:
        assert check_email_format("bademail.com")
    assert str(e.value) == "Invalid email format"

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