Testing python descriptors revisited

Question

I just learned about python's descriptors, and I think they might be just the thing to solve a problem I have. I would like to write tests for the descriptor class I wrote and I found another post with some example code, but I don't really understand it.

import unittest


class MockClass(object):
    # add any methods to assert access here


class DescriptorTests(unittest.TestCase):
    def _make_one(self, *args, **kw):
        from original_module import DescriptorClass
        return DescriptorClass(*args, **kw)

    def test_class_access(self):
        # only a type is passed in, no instance
        desc = self._make_one()
        res = desc.__get__(None, MockClass)
        self.assertEqual(res.someattribute, 'somevalue')

    # etc.  


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

Two questions. First, what's the purpose of MockClass? I simply wrote

class MockClass(object):
    pass

and the tests seem to work. Does MockClass need to have any functionality?

Second, what's the purpose of the DescriptorTests._make_one method? Why not import DescriptorClass right after unittest and then create an instance of the descriptor within DescriptorTests.test_class_access like so:

import unittest
from original_module import DescriptorClass

class DescriptorTests(unittest.TestCase):
    def test_class_access(self):
        desc = DescriptorClass()
        # other code

Thanks in advance.

Background, FWIW

I'm writing a program that models a physical phenomenon and so I am writing classes that require several public data attributes that behave like physical properties. For example, my class needs a temperature attribute that store's the object's temperature in Kelvin. Negative values of temperature in Kelvin make no sense. Other physical properties have similar constraints (negative values of mass make no sense, etc.). Instead of writing a bunch of essentially the same @propertys, I have abstracted this functionality into a descriptor class.

I have written a PhysicalProperty descriptor class that is supposed to take a lower bound when instantiated and raise an exception if the user tries to set the value less than the lower bound. I want to test PhysicalProperty to make sure it throws the exception appropriately.

score 0 · Answer 1 · edited May 23 '17 at 10:26

I don't think descriptors are applicable here and I think that you should consider using a mocking framework instead for this type of testing. For Python, there's the Mox library. To see how to mock out properties with Mox, see this question.

This technique is known as dependency injection (see Wikipedia, SO, and Martin Fowler for more info).

I have written a PhysicalProperty descriptor class that is supposed to take a lower bound when instantiated and raise an exception if the user tries to set the value less than the lower bound. I want to test PhysicalProperty to make sure it throws the exception appropriately.

You can verify that an exception is thrown via assertRaises() or assertRaisesRegexp() methods provided by subclassing unittest.TestCase, as you're already doing in your code.

Testing python descriptors revisited

Background, FWIW

1 Answers1