Testing exceptions

The unittest support for asserting that exceptions are raised when expected is fairly weak. Like many other Python testing libraries, TestFixtures has tools to help with this.

The ShouldRaise context manager

If you are using a version of Python where the with statement can be used, it’s recommended that you use the ShouldRaise context manager.

Suppose we wanted to test the following function to make sure that the right exception was raised:

def the_thrower(throw=True):
    if throw:
        raise ValueError('Not good!')

The following example shows how to test that the correct exception is raised:

>>> from testfixtures import ShouldRaise
>>> with ShouldRaise(ValueError('Not good!')):
...     the_thrower()

If the exception raised doesn’t match the one expected, ShouldRaise will raise an AssertionError causing the tests in which it occurs to fail:

>>> with ShouldRaise(ValueError('Is good!')):
...     the_thrower()
Traceback (most recent call last):
AssertionError: ValueError('Not good!',) raised, ValueError('Is good!',) expected

If you’re not concerned about anything more than the type of the exception that’s raised, you can check as follows:

>>> from testfixtures import ShouldRaise
>>> with ShouldRaise(ValueError):
...     the_thrower()

If you’re feeling slack and just want to check that an exception is raised, but don’t care about the type of that exception, the following will suffice:

>>> from testfixtures import ShouldRaise
>>> with ShouldRaise():
...     the_thrower()

If no exception is raised by the code under test, ShouldRaise will raise an AssertionError to indicate this:

>>> from testfixtures import ShouldRaise
>>> with ShouldRaise():
...     the_thrower(throw=False)
Traceback (most recent call last):
AssertionError: No exception raised!

ShouldRaise has been implemented such that it can be successfully used to test if code raises both SystemExit and KeyboardInterrupt exceptions.

To help with SystemExit and other exceptions that are tricky to construct yourself, ShouldRaise instances have a raised attribute. This will contain the actual exception raised and can be used to inspect parts of it:

>>> import sys
>>> from testfixtures import ShouldRaise
>>> with ShouldRaise() as s:
...     sys.exit(42)
>>> s.raised.code

The should_raise() decorator

If you are working in a traditional unittest environment and want to check that a particular test function raises an exception, you may find the decorator suits your needs better:

from testfixtures import should_raise

@should_raise(ValueError('Not good!'))
def test_function():

This decorator behaves exactly as the ShouldRaise context manager described in the documentation above.


It is slightly recommended that you use the context manager rather than the decorator in most cases. With the decorator, all exceptions raised within the decorated function will be checked, which can hinder test development. With the context manager, you can make assertions about only the exact lines of code that you expect to raise the exception.

Exceptions that are conditionally raised

Some exceptions are only raised in certain versions of Python. For example, in Python 2, bytes() will turn both bytes and strings into bytes, while in Python 3, it will raise an exception when presented with a string. If you wish to make assertions that this behaviour is expected, you can use the unless option to ShouldRaise as follows:

import sys
from testfixtures import ShouldRaise

PY2 = sys.version_info[:2] <  (3, 0)

with ShouldRaise(TypeError, unless=PY2):


Do not abuse this functionality to make sloppy assertions. It is always better have two different tests that cover a case when an exception should be raised and a case where an exception should not be raised rather than using it above functionality. It is only provided to help in cases where something in the environment that cannot be mocked out or controlled influences whether or not an exception is raised.