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

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()