How do you test that a Python function throws an exception?

Daryl_Spitzer_4766 0
Viewed 733618

How does one write a unittest that fails only if a function doesn't throw an expected exception?

unit-testing exception python
19 Answers

Use TestCase.assertRaises (or TestCase.failUnlessRaises) from the unittest module, for example:

import mymod

class MyTestCase(unittest.TestCase):
    def test1(self):
        self.assertRaises(SomeCoolException, mymod.myfunc)
Moe_30510

The code in my previous answer can be simplified to:

def test_afunction_throws_exception(self):
    self.assertRaises(ExpectedException, afunction)

And if a function takes arguments, just pass them into assertRaises like this:

def test_afunction_throws_exception(self):
    self.assertRaises(ExpectedException, afunction, arg1, arg2)
Daryl_Spitzer_47660

Your code should follow this pattern (this is a unittest module style test):

def test_afunction_throws_exception(self):
    try:
        afunction()
    except ExpectedException:
        pass
    except Exception:
       self.fail('unexpected exception raised')
    else:
       self.fail('ExpectedException not raised')

On Python < 2.7 this construct is useful for checking for specific values in the expected exception. The unittest function assertRaises only checks if an exception was raised.

Daryl_Spitzer_47660

As I haven't seen any detailed explanation on how to check if we got a specific exception among a list of accepted one using context manager, or other exception details I will add mine (checked on python 3.8).

If I just want to check that function is raising for instance TypeError, I would write:

with self.assertRaises(TypeError):
    function_raising_some_exception(parameters)

If I want to check that function is raising either TypeError or IndexError, I would write:

with self.assertRaises((TypeError,IndexError)):
    function_raising_some_exception(parameters)

And if I want even more details about the Exception raised I could catch it in a context like this:

# Here I catch any exception    
with self.assertRaises(Exception) as e:
    function_raising_some_exception(parameters)

# Here I check actual exception type (but I could
# check anything else about that specific exception,
# like it's actual message or values stored in the exception)
self.assertTrue(type(e.exception) in [TypeError,MatrixIsSingular])
user_1684650

If you are using Python 3, in order to assert an exception along with its message, you can use assertRaises in context manager and pass the message as a msg keyword argument like so:

import unittest

def your_function():
    raise RuntimeError('your exception message')

class YourTestCase(unittest.TestCase):
    def test(self):
        with self.assertRaises(RuntimeError, msg='your exception message'):
            your_function()


if __name__ == '__main__':
    unittest.main()
user_40886750

I use doctest[1] almost everywhere because I like the fact that I document and test my functions at the same time.

Have a look at this code:

def throw_up(something, gowrong=False):
    """
    >>> throw_up('Fish n Chips')
    Traceback (most recent call last):
    ...
    Exception: Fish n Chips

    >>> throw_up('Fish n Chips', gowrong=True)
    'I feel fine!'
    """
    if gowrong:
        return "I feel fine!"
    raise Exception(something)

if __name__ == '__main__':
    import doctest
    doctest.testmod()

If you put this example in a module and run it from the command line both test cases are evaluated and checked.

[1] Python documentation: 23.2 doctest -- Test interactive Python examples

pi_152740

There are a lot of answers here. The code shows how we can create an Exception, how we can use that exception in our methods, and finally, how you can verify in a unit test, the correct exceptions being raised.

import unittest

class DeviceException(Exception):
    def __init__(self, msg, code):
        self.msg = msg
        self.code = code
    def __str__(self):
        return repr("Error {}: {}".format(self.code, self.msg))

class MyDevice(object):
    def __init__(self):
        self.name = 'DefaultName'

    def setParameter(self, param, value):
        if isinstance(value, str):
            setattr(self, param , value)
        else:
            raise DeviceException('Incorrect type of argument passed. Name expects a string', 100001)

    def getParameter(self, param):
        return getattr(self, param)

class TestMyDevice(unittest.TestCase):

    def setUp(self):
        self.dev1 = MyDevice()

    def tearDown(self):
        del self.dev1

    def test_name(self):
        """ Test for valid input for name parameter """

        self.dev1.setParameter('name', 'MyDevice')
        name = self.dev1.getParameter('name')
        self.assertEqual(name, 'MyDevice')

    def test_invalid_name(self):
        """ Test to check if error is raised if invalid type of input is provided """

        self.assertRaises(DeviceException, self.dev1.setParameter, 'name', 1234)

    def test_exception_message(self):
        """ Test to check if correct exception message and code is raised when incorrect value is passed """

        with self.assertRaises(DeviceException) as cm:
            self.dev1.setParameter('name', 1234)
        self.assertEqual(cm.exception.msg, 'Incorrect type of argument passed. Name expects a string', 'mismatch in expected error message')
        self.assertEqual(cm.exception.code, 100001, 'mismatch in expected error code')


if __name__ == '__main__':
    unittest.main()
user_10769650

I just discovered that the Mock library provides an assertRaisesWithMessage() method (in its unittest.TestCase subclass), which will check not only that the expected exception is raised, but also that it is raised with the expected message:

from testcase import TestCase

import mymod

class MyTestCase(TestCase):
    def test1(self):
        self.assertRaisesWithMessage(SomeCoolException,
                                     'expected message',
                                     mymod.myfunc)
Daryl_Spitzer_47660

For those on Django, you can use context manager to run the faulty function and assert it raises the exception with a certain message using assertRaisesMessage

with self.assertRaisesMessage(SomeException,'Some error message e.g 404 Not Found'):
    faulty_funtion()
user_81731670
user_4117180

This will raise TypeError if setting stock_id to an Integer in this class will throw the error, the test will pass if this happens and fails otherwise

def set_string(prop, value):
   if not isinstance(value, str):
      raise TypeError("i told you i take strings only ")
   return value

class BuyVolume(ndb.Model):
    stock_id = ndb.StringProperty(validator=set_string)

from pytest import raises
buy_volume_instance: BuyVolume = BuyVolume()
with raises(TypeError):
  buy_volume_instance.stock_id = 25
user_52877340

There are 4 options (you'll find full example in the end):

assertRaises with context manager

def test_raises(self):
    with self.assertRaises(RuntimeError):
        raise RuntimeError()

assertRaises one-liner

Pay attention: instead of function call, here you use your function as callable (without round brackets).

def test_raises(self):
    self.assertRaises(RuntimeError, your_function)

assertRaisesRegex with context manager

Second parameter is regex expression and is mandatory. This option you should use if you want check exception message.

def test_raises_regex(self):
    with self.assertRaisesRegex(RuntimeError, r'.* exception message'):
        raise RuntimeError('your exception message')

assertRaisesRegex one-liner

Second parameter is regex expression and is mandatory. This option you should use if you want check exception message.

Pay attention: instead of function call, here you use your function as callable (without round brackets).

def test_raises_regex(self):
    self.assertRaisesRegex(RuntimeError, r'.* exception message', your_function)

Full code example:

import unittest

def your_function():
    raise RuntimeError('your exception message')

class YourTestCase(unittest.TestCase):

    def test_1_raises_context_manager(self):
        with self.assertRaises(RuntimeError):
            your_function()

    def test_2_raises_oneliner(self):
        self.assertRaises(RuntimeError, your_function)

    def test_3_raises_regex_context_manager(self):
        with self.assertRaisesRegex(RuntimeError, r'.* exception message'):
            your_function()

    def test_4_raises_regex_oneliner(self):
        self.assertRaisesRegex(RuntimeError, r'.* exception message', your_function)

if __name__ == '__main__':
    unittest.main()

Although it's up to developer which style to follow I prefer both methods using context manager.

user_77441060

Unit testing with unittest would be preferred, but if you would like a quick fix, we can catch the exception, assign it to a variable, and see if that variable is an instance of that exception class.

Lets assume our bad function throws a ValueError.

    try:
      bad_function()
    except ValueError as e:
      assert isinstance(e, ValueError)
user_155730160

While all the answers are perfectly fine, I was looking for a way to test if a function raised an exception without relying on unit testing frameworks and having to write test classes.

I ended up writing the following:

def assert_error(e, x):
    try:
        e(x)
    except:
        return
    raise AssertionError()

def failing_function(x):
    raise ValueError()

def dummy_function(x):
    return x

if __name__=="__main__":
    assert_error(failing_function, 0)
    assert_error(dummy_function, 0)

And it fails on the right line :

Traceback (most recent call last):
  File "assert_error.py", line 16, in <module>
    assert_error(dummy_function, 0)
  File "assert_error.py", line 6, in assert_error
    raise AssertionError()
AssertionError
user_47914080
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