FAQ: Unit Testing - Assert Methods III: Exception and Warning Methods

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1

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This community-built FAQ covers the “Assert Methods III: Exception and Warning Methods” exercise from the lesson “Unit Testing”.

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This exercise can be found in the following Codecademy content:

Learn Intermediate Python 3

FAQs on the exercise Assert Methods III: Exception and Warning Methods

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2

Hello! I’ve recently started to learn Python. Could anybody explain why this code (with argument of function in parentheses exactly) isn’t working? Thank you.

import unittest
import alerts
class SystemAlertTests(unittest.TestCase):
    def test_power_outage_alert(self):
        self.assertRaises(alerts.PowerError, alerts.power_outage_detected(True))

unittest.main()

alerts.py file is below:

class PowerError(Exception):
    pass

def power_outage_detected(outage_detected):
    if outage_detected:
        raise PowerError('A power outage has been detected somewhere in the system')
    else:
        print('All systems receiving power')
3

That full stop is possibly the problem. Parameters are separated by commas.

4

The problem is in this line:

`self.assertRaises(alerts.PowerError, alerts.power_outage_detected(True))`

Although it works in such a way:

`self.assertRaises(alerts.PowerError, alerts.power_outage_detected, True)`

image

3 Likes
5

Apologies for a delayed reply but that is not a valid alternative. The function(functionArguments) call is evaluated before the .assertRaises method is called so the there is uncaught exception instead of the actual test.

In the unittest framework it will handle the error (so other tests can still be run) but you will not get the expected pass/fail results for that test. The correct syntax is the one given above.

From the documentation these are the two valid call signatures-

assertRaises ( exception , callable , *args , **kwds )
assertRaises ( exception , * , msg=None )

3 Likes
6

I should probably add that second call signature would be used in the following way-

...
    with self.assertRaises(specificException):
        function(functionArrguments)

In this way you can directly call your function with the relevant arguments (some people prefer that style) and wait for an exception where the context manager handles it instead. In this way your tests follow the standard pass/fail steps again (no loose exceptions).

As per the docs on unittest.TestCase.assertRaises this might be useful if you wanted to get more testing done on the exception itself as you can temporarily name the exception thrown-

with self.assertRaises(specificException) as error:
    function(functionArrguments)

# then use something like
self.assertSomething(error.attribute)
# docs have a better example, worth a look
1 Like
7

Agreed, but someone should remove this from the exercise hints, since it pretty clearly implies that the OPs syntax is also acceptable, but is apparently not true:

image

2 Likes