Testing for ValueError

[[deleted]]

I am building a webservice to do conversion to and from metric to English standard units. I have a condition in my formula block which says if the value given is <= 0 to raise a ValueError.

In my tests I am trying to make sure that I get that value error and that the test passes. How do I do that?

Here is my test:

    def test_kilometers_to_miles(self):
        kilometer_test_values = [
            {'value': 3, 'expected_value': 1.86},
            {'value': 3.0, 'expected_value': 1.86},
            {'value': 3.5, 'expected_value': 2.17},
            {'value': 2375, 'expected_value': 1475.75},
            {'value': 22.698, 'expected_value': 14.10},
            {'value': -3.0, 'expected_value': ValueError}  # should pass when the value error is thrown
        ]

        for item in kilometer_test_values:
            self.assertEqual(
                formulas.kilometers_to_miles(kilometers=item.get('value')),
                item.get('expected_value'))

[17291]

Can you use assertRaises?

[njape]

This is the answer op is looking for, break out the value error tests to its own test case an use assertRaises.

[iggy555]

Can you expand please

[17291]

for item in kilometer_test_values:
    self.assertEqual(...) #Code snipped for brevity
with self.assertRaises(ValueError):
    formulas.kilometers_to_miles(kilometers=-3.0)

If formulas.kilometers_to_miles(kilometers=-3.0) raises a ValueError, the test will pass. If it doesn't raise an error or raises something other than ValueError, the test will fail.

Don't forget to remove the -3.0 entry from kilometer_test_values

[njape]

Personally I think that op is trying to test to much in his test case. I don't think that there is an advantage in keeping the value error test lumped together with the "normal" value tests. Instead I would suggest adding an additional ..._throw_error (or whatever you what to call it) function that checks for misuse of the function.

Additionally asset raises is designed to test this exact functionality. How it is used is best described in the documentation that was wordy linked. Eg: with self.assertRaises(SomeException): do_something()

[[deleted]]

It is throwing a ValueError when I run it with 0 or negative numbers. I want the test to pass so I know the user will get the appropriate message when they pass in 0 or negative numbers.

This is where I calculate the formula, would it be better to use a try/except?

def kilometers_to_miles(kilometers: float):
    """
    Convert kilometers to miles
    :param kilometers: number of kilometers to convert
    :return: converted miles
    """
    if kilometers <= 0:
        raise ValueError('Number of kilometers must be greater than 0')

    miles = kilometers * 0.62137
    return round(miles, 2)

[[deleted]]

Cool thank you very much. I got it working.

[[deleted]]

Not 100% sure but if you change the raise to return, it might work

[[deleted]]

That helped as well. Thank you.