Related
How does one write a unit test that fails only if a function doesn't throw an expected exception?
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)
Since Python 2.7 you can use context manager to get ahold of the actual Exception object thrown:
import unittest
def broken_function():
raise Exception('This is broken')
class MyTestCase(unittest.TestCase):
def test(self):
with self.assertRaises(Exception) as context:
broken_function()
self.assertTrue('This is broken' in context.exception)
if __name__ == '__main__':
unittest.main()
assertRaises
In Python 3.5, you have to wrap context.exception in str, otherwise you'll get a TypeError
self.assertTrue('This is broken' in str(context.exception))
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)
How do you test that a Python function throws an exception?
How does one write a test that fails only if a function doesn't throw
an expected exception?
Short Answer:
Use the self.assertRaises method as a context manager:
def test_1_cannot_add_int_and_str(self):
with self.assertRaises(TypeError):
1 + '1'
Demonstration
The best practice approach is fairly easy to demonstrate in a Python shell.
The unittest library
In Python 2.7 or 3:
import unittest
In Python 2.6, you can install a backport of 2.7's unittest library, called unittest2, and just alias that as unittest:
import unittest2 as unittest
Example tests
Now, paste into your Python shell the following test of Python's type-safety:
class MyTestCase(unittest.TestCase):
def test_1_cannot_add_int_and_str(self):
with self.assertRaises(TypeError):
1 + '1'
def test_2_cannot_add_int_and_str(self):
import operator
self.assertRaises(TypeError, operator.add, 1, '1')
Test one uses assertRaises as a context manager, which ensures that the error is properly caught and cleaned up, while recorded.
We could also write it without the context manager, see test two. The first argument would be the error type you expect to raise, the second argument, the function you are testing, and the remaining args and keyword args will be passed to that function.
I think it's far more simple, readable, and maintainable to just to use the context manager.
Running the tests
To run the tests:
unittest.main(exit=False)
In Python 2.6, you'll probably need the following:
unittest.TextTestRunner().run(unittest.TestLoader().loadTestsFromTestCase(MyTestCase))
And your terminal should output the following:
..
----------------------------------------------------------------------
Ran 2 tests in 0.007s
OK
<unittest2.runner.TextTestResult run=2 errors=0 failures=0>
And we see that as we expect, attempting to add a 1 and a '1' result in a TypeError.
For more verbose output, try this:
unittest.TextTestRunner(verbosity=2).run(unittest.TestLoader().loadTestsFromTestCase(MyTestCase))
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.
From http://www.lengrand.fr/2011/12/pythonunittest-assertraises-raises-error/:
First, here is the corresponding (still dum :p) function in file dum_function.py:
def square_value(a):
"""
Returns the square value of a.
"""
try:
out = a*a
except TypeError:
raise TypeError("Input should be a string:")
return out
Here is the test to be performed (only this test is inserted):
import dum_function as df # Import function module
import unittest
class Test(unittest.TestCase):
"""
The class inherits from unittest
"""
def setUp(self):
"""
This method is called before each test
"""
self.false_int = "A"
def tearDown(self):
"""
This method is called after each test
"""
pass
#---
## TESTS
def test_square_value(self):
# assertRaises(excClass, callableObj) prototype
self.assertRaises(TypeError, df.square_value(self.false_int))
if __name__ == "__main__":
unittest.main()
We are now ready to test our function! Here is what happens when trying to run the test:
======================================================================
ERROR: test_square_value (__main__.Test)
----------------------------------------------------------------------
Traceback (most recent call last):
File "test_dum_function.py", line 22, in test_square_value
self.assertRaises(TypeError, df.square_value(self.false_int))
File "/home/jlengrand/Desktop/function.py", line 8, in square_value
raise TypeError("Input should be a string:")
TypeError: Input should be a string:
----------------------------------------------------------------------
Ran 1 test in 0.000s
FAILED (errors=1)
The TypeError is actually raised, and generates a test failure. The problem is that this is exactly the behavior we wanted :s.
To avoid this error, simply run the function using lambda in the test call:
self.assertRaises(TypeError, lambda: df.square_value(self.false_int))
The final output:
----------------------------------------------------------------------
Ran 1 test in 0.000s
OK
Perfect!
... and for me is perfect too!!
Thanks a lot, Mr. Julien Lengrand-Lambert.
This test assert actually returns a false positive. That happens because the lambda inside the 'assertRaises' is the unit that raises type error and not the tested function.
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])
If you are using pytest you can use pytest.raises(Exception):
Example:
def test_div_zero():
with pytest.raises(ZeroDivisionError):
1/0
And the result:
$ py.test
================= test session starts =================
platform linux2 -- Python 2.6.6 -- py-1.4.20 -- pytest-2.5.2 -- /usr/bin/python
collected 1 items
tests/test_div_zero.py:6: test_div_zero PASSED
Or you can build your own contextmanager to check if the exception was raised.
import contextlib
#contextlib.contextmanager
def raises(exception):
try:
yield
except exception as e:
assert True
else:
assert False
And then you can use raises like this:
with raises(Exception):
print "Hola" # Calls assert False
with raises(Exception):
raise Exception # Calls assert True
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()
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
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()
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)
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()
If you want to check the exception message (see the "assertRaisesRegex with context manager" option below to check only part of it):
def test_raises(self):
with self.assertRaises(RuntimeError) as error:
raise RuntimeError("your exception message")
self.assertEqual(str(error.exception), "your exception message")
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. Handy when you want check only part of the 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. Handy when you want check only part of the 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_1b_raises_context_manager_and_error_message(self):
with self.assertRaises(RuntimeError) as error:
your_function()
self.assertEqual(str(error.exception), "your exception message")
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.
You can use assertRaises from the unittest module:
import unittest
class TestClass():
def raises_exception(self):
raise Exception("test")
class MyTestCase(unittest.TestCase):
def test_if_method_raises_correct_exception(self):
test_class = TestClass()
# Note that you don’t use () when passing the method to assertRaises
self.assertRaises(Exception, test_class.raises_exception)
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()
For await/async aiounittest there is a slightly different pattern:
https://aiounittest.readthedocs.io/en/latest/asynctestcase.html#aiounittest.AsyncTestCase
async def test_await_async_fail(self):
with self.assertRaises(Exception) as e:
await async_one()
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
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)
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
How does one write a unit test that fails only if a function doesn't throw an expected exception?
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)
Since Python 2.7 you can use context manager to get ahold of the actual Exception object thrown:
import unittest
def broken_function():
raise Exception('This is broken')
class MyTestCase(unittest.TestCase):
def test(self):
with self.assertRaises(Exception) as context:
broken_function()
self.assertTrue('This is broken' in context.exception)
if __name__ == '__main__':
unittest.main()
assertRaises
In Python 3.5, you have to wrap context.exception in str, otherwise you'll get a TypeError
self.assertTrue('This is broken' in str(context.exception))
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)
How do you test that a Python function throws an exception?
How does one write a test that fails only if a function doesn't throw
an expected exception?
Short Answer:
Use the self.assertRaises method as a context manager:
def test_1_cannot_add_int_and_str(self):
with self.assertRaises(TypeError):
1 + '1'
Demonstration
The best practice approach is fairly easy to demonstrate in a Python shell.
The unittest library
In Python 2.7 or 3:
import unittest
In Python 2.6, you can install a backport of 2.7's unittest library, called unittest2, and just alias that as unittest:
import unittest2 as unittest
Example tests
Now, paste into your Python shell the following test of Python's type-safety:
class MyTestCase(unittest.TestCase):
def test_1_cannot_add_int_and_str(self):
with self.assertRaises(TypeError):
1 + '1'
def test_2_cannot_add_int_and_str(self):
import operator
self.assertRaises(TypeError, operator.add, 1, '1')
Test one uses assertRaises as a context manager, which ensures that the error is properly caught and cleaned up, while recorded.
We could also write it without the context manager, see test two. The first argument would be the error type you expect to raise, the second argument, the function you are testing, and the remaining args and keyword args will be passed to that function.
I think it's far more simple, readable, and maintainable to just to use the context manager.
Running the tests
To run the tests:
unittest.main(exit=False)
In Python 2.6, you'll probably need the following:
unittest.TextTestRunner().run(unittest.TestLoader().loadTestsFromTestCase(MyTestCase))
And your terminal should output the following:
..
----------------------------------------------------------------------
Ran 2 tests in 0.007s
OK
<unittest2.runner.TextTestResult run=2 errors=0 failures=0>
And we see that as we expect, attempting to add a 1 and a '1' result in a TypeError.
For more verbose output, try this:
unittest.TextTestRunner(verbosity=2).run(unittest.TestLoader().loadTestsFromTestCase(MyTestCase))
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.
From http://www.lengrand.fr/2011/12/pythonunittest-assertraises-raises-error/:
First, here is the corresponding (still dum :p) function in file dum_function.py:
def square_value(a):
"""
Returns the square value of a.
"""
try:
out = a*a
except TypeError:
raise TypeError("Input should be a string:")
return out
Here is the test to be performed (only this test is inserted):
import dum_function as df # Import function module
import unittest
class Test(unittest.TestCase):
"""
The class inherits from unittest
"""
def setUp(self):
"""
This method is called before each test
"""
self.false_int = "A"
def tearDown(self):
"""
This method is called after each test
"""
pass
#---
## TESTS
def test_square_value(self):
# assertRaises(excClass, callableObj) prototype
self.assertRaises(TypeError, df.square_value(self.false_int))
if __name__ == "__main__":
unittest.main()
We are now ready to test our function! Here is what happens when trying to run the test:
======================================================================
ERROR: test_square_value (__main__.Test)
----------------------------------------------------------------------
Traceback (most recent call last):
File "test_dum_function.py", line 22, in test_square_value
self.assertRaises(TypeError, df.square_value(self.false_int))
File "/home/jlengrand/Desktop/function.py", line 8, in square_value
raise TypeError("Input should be a string:")
TypeError: Input should be a string:
----------------------------------------------------------------------
Ran 1 test in 0.000s
FAILED (errors=1)
The TypeError is actually raised, and generates a test failure. The problem is that this is exactly the behavior we wanted :s.
To avoid this error, simply run the function using lambda in the test call:
self.assertRaises(TypeError, lambda: df.square_value(self.false_int))
The final output:
----------------------------------------------------------------------
Ran 1 test in 0.000s
OK
Perfect!
... and for me is perfect too!!
Thanks a lot, Mr. Julien Lengrand-Lambert.
This test assert actually returns a false positive. That happens because the lambda inside the 'assertRaises' is the unit that raises type error and not the tested function.
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])
If you are using pytest you can use pytest.raises(Exception):
Example:
def test_div_zero():
with pytest.raises(ZeroDivisionError):
1/0
And the result:
$ py.test
================= test session starts =================
platform linux2 -- Python 2.6.6 -- py-1.4.20 -- pytest-2.5.2 -- /usr/bin/python
collected 1 items
tests/test_div_zero.py:6: test_div_zero PASSED
Or you can build your own contextmanager to check if the exception was raised.
import contextlib
#contextlib.contextmanager
def raises(exception):
try:
yield
except exception as e:
assert True
else:
assert False
And then you can use raises like this:
with raises(Exception):
print "Hola" # Calls assert False
with raises(Exception):
raise Exception # Calls assert True
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()
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
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()
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)
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()
If you want to check the exception message (see the "assertRaisesRegex with context manager" option below to check only part of it):
def test_raises(self):
with self.assertRaises(RuntimeError) as error:
raise RuntimeError("your exception message")
self.assertEqual(str(error.exception), "your exception message")
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. Handy when you want check only part of the 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. Handy when you want check only part of the 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_1b_raises_context_manager_and_error_message(self):
with self.assertRaises(RuntimeError) as error:
your_function()
self.assertEqual(str(error.exception), "your exception message")
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.
You can use assertRaises from the unittest module:
import unittest
class TestClass():
def raises_exception(self):
raise Exception("test")
class MyTestCase(unittest.TestCase):
def test_if_method_raises_correct_exception(self):
test_class = TestClass()
# Note that you don’t use () when passing the method to assertRaises
self.assertRaises(Exception, test_class.raises_exception)
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()
For await/async aiounittest there is a slightly different pattern:
https://aiounittest.readthedocs.io/en/latest/asynctestcase.html#aiounittest.AsyncTestCase
async def test_await_async_fail(self):
with self.assertRaises(Exception) as e:
await async_one()
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
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)
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
There are ways how programmer can make programming and refactoring easier and more simple, python is very good in this area.
I'm curious whether is there a more elegant way to solve my problem than brute-force writing the same code multiple times again and again.
Situation:
I'm writing a code. There are many equal methods calling with different arguments sequentially.
For example - I have this code:
...
...
my_method(1)
my_method(2)
my_method(3)
my_method(4)
...
my_method(10)
...
So I have this code written, everything works fine but suddenly I find out that I need to make a log file so I have to put try-except on everyone of this methods so the code will look like this:
...
...
try:
my_method(3)
except Exception as e:
print_to_file(log.txt,str(e))
...
...
try:
my_method(8)
except Exception as e:
print_to_file(log.txt,str(e))
...
...
Do I have a better option than changing every my_method(x) calling and putting it into try-except clause? I know that it is a mistake of the programmer who had to think about it at the beginning but these situations happens.
EDIT: According to the answer - the code above is the simple example. In real code there are no int arguments given but dates where there is no logic there so I can't put it into the loop. Assume that the arguments can't be generated.
If you're using the logger supplied by python, you can redirect exception output to the log as opposed to have to put a ton of try blocks everywhere:
import os, sys
import logging
logger = logging.getLogger(__name__)
handler = logging.StreamHandler(stream=sys.stdout)
logger.addHandler(handler)
def handle_exception(exc_type, exc_value, exc_traceback):
if issubclass(exc_type, KeyboardInterrupt):
sys.__excepthook__(exc_type, exc_value, exc_traceback)
return
logger.error("Uncaught exception", exc_info=(exc_type, exc_value, exc_traceback))
sys.excepthook = handle_exception
if __name__ == "__main__":
raise RuntimeError("Test unhandled")
Now is an exception is thrown, you won't need a try block, it will be written to the log regardless
ref
You can take advantage of the fact that a function, in python, is totally an object, and write a function that takes in another function, runs it, and logs any exceptions
def sloppyRun(func, *args, **kwargs):
"""Runs a function, catching all exceptions
and writing them to a log file."""
try:
return func(*args, **kwargs) #running function here
except:
logging.exception(func.__name__ + str(args) + str(kwargs))
#incidentally, the logging module is wonderful. I'd recommend using it.
#It'll even write the traceback to a file.
And then you can write something like
sloppyRun(my_method, 8) #note the lack of parens for my_method
You could have like a context manager or a decorator to log what you need, when you need to. if you intend to always log an exception when you use that function, I would suggest going the simple decorator rule or even a try and except inside that function. If it is functions not in your code or you dont want them to always log,then I would used a context manager (called as with ..:)
A context manager example code
import functools
class LoggerContext():
def __enter__(self):
# function that is called on enter of the with context
# we dont need this
pass
def __exit__(self, type, value, traceback):
# If there was an exception, it will be passed to the
# exit function.
# type = type of exception
# value = the string arg of the exception
# traceback object for you to extract the traceback if you need to
if traceback:
# do something with exception like log it etc
print(type, value, traceback)
# If the return value of the exit function is not True, python
# interpreter re-raises the exception. We dont want to re-raise
# the exception
return True
def __call__(self, f):
# this is just to make a context manager a decorator
# so that you could use the #context on a function
#functools.wraps(f)
def decorated(*args, **kwds):
with self:
return f(*args, **kwds)
return decorated
#LoggerContext()
def myMethod(test):
raise FileNotFoundError(test)
def myMethod2(test):
raise TypeError(test)
myMethod('asdf')
with LoggerContext():
myMethod2('asdf')
A simple decorator example:
import functools
def LoggerDecorator(f):
#functools.wraps(f)
def decorated(*args, **kwds):
try:
return f(*args, **kwds)
except Exception as e:
# do something with exception
print('Exception:', e)
return decorated
#LoggerDecorator
def myMethod3(test):
raise IOError(test)
myMethod3('asdf')
I was wondering, is there a simple magic method in python that allows customization of the behaviour of an exception-derived object when it is raised? I'm looking for something like __raise__ if that exists. If no such magic methods exist, is there any way I could do something like the following (it's just an example to prove my point):
class SpecialException(Exception):
def __raise__(self):
print('Error!')
raise SpecialException() #this is the part of the code that must stay
Is it possible?
I don't know about such magic method but even if it existed it is just some piece of code that gets executed before actually raising the exception object. Assuming that its a good practice to raise exception objects that are instantiated in-place you can put such code into the __init__ of the exception. Another workaround: instead of raising your exception directly you call an error handling method/function that executes special code and then finally raises an exception.
import time
from functools import wraps
def capture_exception(callback=None, *c_args, **c_kwargs):
"""捕获到异常后执行回调函数"""
assert callable(callback), "callback 必须是可执行对象"
def _out(func):
#wraps(func)
def _inner(*args, **kwargs):
try:
res = func(*args, **kwargs)
return res
except Exception as e:
callback(*c_args, **c_kwargs)
raise e
return _inner
return _out
def send_warning():
print("warning message..............")
class A(object):
#capture_exception(callback=send_warning)
def run(self):
print('run')
raise SystemError("测试异常捕获回调功能")
time.sleep(0.2)
if __name__ == '__main__':
a = A()
a.run()
How does one write a unit test that fails only if a function doesn't throw an expected exception?
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)
Since Python 2.7 you can use context manager to get ahold of the actual Exception object thrown:
import unittest
def broken_function():
raise Exception('This is broken')
class MyTestCase(unittest.TestCase):
def test(self):
with self.assertRaises(Exception) as context:
broken_function()
self.assertTrue('This is broken' in context.exception)
if __name__ == '__main__':
unittest.main()
assertRaises
In Python 3.5, you have to wrap context.exception in str, otherwise you'll get a TypeError
self.assertTrue('This is broken' in str(context.exception))
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)
How do you test that a Python function throws an exception?
How does one write a test that fails only if a function doesn't throw
an expected exception?
Short Answer:
Use the self.assertRaises method as a context manager:
def test_1_cannot_add_int_and_str(self):
with self.assertRaises(TypeError):
1 + '1'
Demonstration
The best practice approach is fairly easy to demonstrate in a Python shell.
The unittest library
In Python 2.7 or 3:
import unittest
In Python 2.6, you can install a backport of 2.7's unittest library, called unittest2, and just alias that as unittest:
import unittest2 as unittest
Example tests
Now, paste into your Python shell the following test of Python's type-safety:
class MyTestCase(unittest.TestCase):
def test_1_cannot_add_int_and_str(self):
with self.assertRaises(TypeError):
1 + '1'
def test_2_cannot_add_int_and_str(self):
import operator
self.assertRaises(TypeError, operator.add, 1, '1')
Test one uses assertRaises as a context manager, which ensures that the error is properly caught and cleaned up, while recorded.
We could also write it without the context manager, see test two. The first argument would be the error type you expect to raise, the second argument, the function you are testing, and the remaining args and keyword args will be passed to that function.
I think it's far more simple, readable, and maintainable to just to use the context manager.
Running the tests
To run the tests:
unittest.main(exit=False)
In Python 2.6, you'll probably need the following:
unittest.TextTestRunner().run(unittest.TestLoader().loadTestsFromTestCase(MyTestCase))
And your terminal should output the following:
..
----------------------------------------------------------------------
Ran 2 tests in 0.007s
OK
<unittest2.runner.TextTestResult run=2 errors=0 failures=0>
And we see that as we expect, attempting to add a 1 and a '1' result in a TypeError.
For more verbose output, try this:
unittest.TextTestRunner(verbosity=2).run(unittest.TestLoader().loadTestsFromTestCase(MyTestCase))
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.
From http://www.lengrand.fr/2011/12/pythonunittest-assertraises-raises-error/:
First, here is the corresponding (still dum :p) function in file dum_function.py:
def square_value(a):
"""
Returns the square value of a.
"""
try:
out = a*a
except TypeError:
raise TypeError("Input should be a string:")
return out
Here is the test to be performed (only this test is inserted):
import dum_function as df # Import function module
import unittest
class Test(unittest.TestCase):
"""
The class inherits from unittest
"""
def setUp(self):
"""
This method is called before each test
"""
self.false_int = "A"
def tearDown(self):
"""
This method is called after each test
"""
pass
#---
## TESTS
def test_square_value(self):
# assertRaises(excClass, callableObj) prototype
self.assertRaises(TypeError, df.square_value(self.false_int))
if __name__ == "__main__":
unittest.main()
We are now ready to test our function! Here is what happens when trying to run the test:
======================================================================
ERROR: test_square_value (__main__.Test)
----------------------------------------------------------------------
Traceback (most recent call last):
File "test_dum_function.py", line 22, in test_square_value
self.assertRaises(TypeError, df.square_value(self.false_int))
File "/home/jlengrand/Desktop/function.py", line 8, in square_value
raise TypeError("Input should be a string:")
TypeError: Input should be a string:
----------------------------------------------------------------------
Ran 1 test in 0.000s
FAILED (errors=1)
The TypeError is actually raised, and generates a test failure. The problem is that this is exactly the behavior we wanted :s.
To avoid this error, simply run the function using lambda in the test call:
self.assertRaises(TypeError, lambda: df.square_value(self.false_int))
The final output:
----------------------------------------------------------------------
Ran 1 test in 0.000s
OK
Perfect!
... and for me is perfect too!!
Thanks a lot, Mr. Julien Lengrand-Lambert.
This test assert actually returns a false positive. That happens because the lambda inside the 'assertRaises' is the unit that raises type error and not the tested function.
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])
If you are using pytest you can use pytest.raises(Exception):
Example:
def test_div_zero():
with pytest.raises(ZeroDivisionError):
1/0
And the result:
$ py.test
================= test session starts =================
platform linux2 -- Python 2.6.6 -- py-1.4.20 -- pytest-2.5.2 -- /usr/bin/python
collected 1 items
tests/test_div_zero.py:6: test_div_zero PASSED
Or you can build your own contextmanager to check if the exception was raised.
import contextlib
#contextlib.contextmanager
def raises(exception):
try:
yield
except exception as e:
assert True
else:
assert False
And then you can use raises like this:
with raises(Exception):
print "Hola" # Calls assert False
with raises(Exception):
raise Exception # Calls assert True
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()
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
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()
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)
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()
If you want to check the exception message (see the "assertRaisesRegex with context manager" option below to check only part of it):
def test_raises(self):
with self.assertRaises(RuntimeError) as error:
raise RuntimeError("your exception message")
self.assertEqual(str(error.exception), "your exception message")
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. Handy when you want check only part of the 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. Handy when you want check only part of the 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_1b_raises_context_manager_and_error_message(self):
with self.assertRaises(RuntimeError) as error:
your_function()
self.assertEqual(str(error.exception), "your exception message")
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.
You can use assertRaises from the unittest module:
import unittest
class TestClass():
def raises_exception(self):
raise Exception("test")
class MyTestCase(unittest.TestCase):
def test_if_method_raises_correct_exception(self):
test_class = TestClass()
# Note that you don’t use () when passing the method to assertRaises
self.assertRaises(Exception, test_class.raises_exception)
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()
For await/async aiounittest there is a slightly different pattern:
https://aiounittest.readthedocs.io/en/latest/asynctestcase.html#aiounittest.AsyncTestCase
async def test_await_async_fail(self):
with self.assertRaises(Exception) as e:
await async_one()
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
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)
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