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.
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()
>>> with ShouldRaise(ValueError('Is good!')): ... the_thrower() Traceback (most recent call last): ... AssertionError: ValueError('Is good!',) (expected) != ValueError('Not good!',) (raised)
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()
>>> from testfixtures import ShouldRaise >>> with ShouldRaise(): ... the_thrower(throw=False) Traceback (most recent call last): ... AssertionError: No exception raised!
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 42
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(): the_thrower()
This decorator behaves exactly as the
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
import sys from testfixtures import ShouldRaise PY2 = sys.version_info[:2] < (3, 0) with ShouldRaise(TypeError, unless=PY2): bytes('something')
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.