assertIsInstance() in Python is a unittest library function that is used in unit testing to check whether an object is an instance of a given class or not. This function will take three parameters as input and return a boolean value depending upon the assert condition. If the object is an instance ofthe given class it will return true else it returns false.

Syntax: assertIsInstance(object, className, message)

Parameters: assertIsInstance() accept three parameters which are listed below with explanation:

• object:  Object which is checked as an instance of the given class
• className: Class name to be compared for object instance
• message: a string sentence as a message which got displayed when the test case got failed.

Listed below are two different examples illustrating the positive and negative test case for given assert function:

Example 1: Negative Test case

# test suite
import unittest

# test class


class Myclass:
    x = 5


class Myclass2:
    x = 6


class TestClass(unittest.TestCase):
    # test function to test whether obj is instance of class
    def test_negative(self):
        objectName = Myclass()
        # error message in case if test case got failed
        message = "given object is not instance of Myclass."
        # assertIsInstance() to check if obj is instance of class
        self.assertIsInstance(objectName, Myclass2, message)


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

Output:

F
======================================================================
FAIL: test_negative (__main__.TestStringMethods)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/a2e9f97d79f7d8c1fbd00c4df704d402.py", line 22, in test_negative
    self.assertIsInstance(objectName, Myclass2, message)
AssertionError: <__main__.Myclass object at 0x7f1e9bead0b8> is not an instance of <class '__main__.Myclass2'> : given object is not instance of Myclass.

----------------------------------------------------------------------
Ran 1 test in 0.001s

FAILED (failures=1)

Example 2: Positive Test case

# test suite
import unittest

# test class
class Myclass:
    x = 5


class TestClass(unittest.TestCase):
    # test function to test whether obj is instance of class
    def test_positive(self):
        objectName = Myclass()
        # error message in case if test case got failed
        message = "given object is not instance of Myclass."
        # assertIsInstance() to check if obj is instance of class
        self.assertIsInstance(objectName, Myclass, message)


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

Output:

.
----------------------------------------------------------------------
Ran 1 test in 0.000s

OK

Reference: https://docs.python.org/3/library/unittest.html