如何使用pytest中的fixture测试类__init__方法引发异常

Question

I am trying to get some more skills with pytest (unit testing).

I try to test if an exception is raised when a class is instantiated without a must have parameter.

I tried to create a fixture for that but this leads to a problem that when the fixture is called it tries to create the class inside it with missing parameter and raises my exception before pytest actually asserts that the exception was raised.

I managed to overcome this by not using fixture and just instantiating the class inside the test function but I wonder if there is a more elegant way of using a fixture for that.

Example class:

class MyClass(object):

    def __init__(self, must_have_parameter=None):

    if not must_have_parameter:

        raise ValueError("must_have_parameter must be set.")

When trying to use this fixture in a test I naturally get an error.

@pytest.fixture()
def bad_class_instantiation():

    _bad_instance = MyClass()

    return _bad_instance

Then there is a test:

def test_bad_instantiation(bad_class_instantiation):

    with pytest.raises(ValueError, message="must_have_parameter must be set."):

        bad_class_instantiation()

This test fails as the class gets instantiated before the test case runs (That is my interpretation)? It still shows that a ValueError occured and the custom message gets displayed..

If I change the test case to:

def test_bad_instantiation():

    with pytest.raises(ValueError, message="must_have_parameter must be set."):

        bad_instance = MyClass()

Then the test passes.

Is there a way of using a fixture for this or I should just call the class inside the test function and call it a day ?

Thanks for your time.

Tomasz

Answer 1

I don't see any benefit to the fixture in this situation. I would just create the object in the test method.

Having an optional parameter with a default value and then raising an exception when the parameter is missing seems very strange. Unless you really need a custom error message, consider removing the default value.