How to write a custom `.assertFoo()` method in Python?

Question

I'm writing some test cases for my application using Python's unittest. Now I need to compare a list of objects with a list of another objects to check if the objects from the first list are what I'm expecting.

How can I write a custom .assertFoo() method? What should it do? Should it raise an exception on failure? If yes, which exception? And how to pass the error message? Should the error message be a unicode string or a bytestring?

Unfortunately, the official documentation doesn't explain how to write custom assertion methods.

If you need a real-world example for this, continue reading.

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The code I'm writing is somewhat like this:

def assert_object_list(self, objs, expected):     for index, (item, values) in enumerate(zip(objs, expected)):         self.assertEqual(             item.foo, values[0],             'Item {0}: {1} != {2}'.format(index, item.foo, values[0])         )         self.assertEqual(             item.bar, values[1],             'Item {0}: {1} != {2}'.format(index, item.bar, values[1])         )  def test_foobar(self):     objs = [...]  # Some processing here     expected = [         # Expected values for ".foo" and ".bar" for each object         (1, 'something'),         (2, 'nothing'),     ]     self.assert_object_list(objs, expected) 

This approach makes it extremely easy to describe the expected values of each object in a very compact way, and without needing to actually create full objects.

However... When one object fails the assertion, no further objects are compared, and this makes debugging a bit more difficult. I would like to write a custom method that would unconditionally compare all objects, and then would display all objects that failed, instead of just the first one.

Answer 1

I use the multiple inheritance in these cases. For example:

First. I define a class with methods that will incorporate.

import os  class CustomAssertions:     def assertFileExists(self, path):         if not os.path.lexists(path):             raise AssertionError('File not exists in path "' + path + '".') 

Now I define a class that inherits from unittest.TestCase and CustomAssertion

import unittest  class MyTest(unittest.TestCase, CustomAssertions):     def test_file_exists(self):         self.assertFileExists('any/file/path')  if __name__ == '__main__':     unittest.main()