Python assertIsInstance()
Summary: in this tutorial, you’ll learn how to use Python assertIsInstance() method to test if an object is an instance of a class.
Introduction to the Python assertIsInstance() method
The assertIsInstance()TestCase class of the unittest module. The assertIsInstance()
The following shows the syntax of the assertIsInstance() method:
assertIsInstance(obj, cls, msg=None)Code language: Python (python)
In this syntax:
objis the object to test.clsis a class or a tuple of classes.msgis an optional string that will be displayed if theobjis not an instance of theclsclass.
Internally, the assertIsInstance() uses the isinstance() function to check if the object is an instance of the cls class.
If the cls is not the class of the obj but the base class of the class of the obj, the test will also pass.
Since the object class is the base class of all classes, the assertIsInstance(obj, object) will always pass.
Python assertIsInstance() method examples
Let’s create a shape.py module with two classes Shape and Square. The Shape class is the base class of the Square class:
class Shape:
passclass Square(Shape):
pass
Code language: Python (python)
To make it simple, the Shape and Square classes have no implementation.
1) Using the Python assertIsInstance() method example
The following example uses the assertIsInstance() method to test if the square object is an instance of the Square class:
import unittestfrom shape import Shape, Square
class TestShape(unittest.TestCase):
def setUp(self):
self.square = Square()
def test_is_instance(self):
self.assertIsInstance(self.square, Square)
Code language: Python (python)
Run the test:
python -m unittest -vCode language: Python (python)
Output:
test_is_instance (test_shape.TestShape) ... ok----------------------------------------------------------------------
Ran 1 test in 0.001s
OK
Code language: plaintext (plaintext)
Because the square instance variable is an object of the Square class, the test passes.
2) Using the Python assertIsInstance() method with a base class example
The following example uses the assertIsInstance() method to test if the square is an instance of the Shape class:
import unittestfrom shape import Shape, Square
class TestShape(unittest.TestCase):
def setUp(self):
self.square = Square()
def test_is_instance(self):
self.assertIsInstance(self.square, Square)
def test_is_instance_of_parent_class(self):
self.assertIsInstance(self.square, Shape)
Code language: Python (python)
Run the test:
python -m unittest -vCode language: Python (python)
Output:
test_is_instance (test_shape.TestShape) ... ok
test_is_instance_of_parent_class (test_shape.TestShape) ... ok----------------------------------------------------------------------
Ran 2 tests in 0.001s
OK
Code language: plaintext (plaintext)
3) Using the Python assertIsInstance() method to test if an object is an instance of the object class
The following example uses assertIsInstance() method to test if the square instance variable is an instance of the object class:
import unittestfrom shape import Shape, Square
class TestShape(unittest.TestCase):
def setUp(self):
self.square = Square()
def test_is_instance(self):
self.assertIsInstance(self.square, Square)
def test_is_instance_of_parent_class(self):
self.assertIsInstance(self.square, Shape)
def test_is_instance_of_object(self):
self.assertIsInstance(self.square, object)
Code language: Python (python)
Run the test:
python -m unittest -vCode language: Python (python)
Output:
test_is_instance (test_shape.TestShape) ... ok
test_is_instance_of_object (test_shape.TestShape) ... ok
test_is_instance_of_parent_class (test_shape.TestShape) ... ok----------------------------------------------------------------------
Ran 3 tests in 0.001s
OK
Code language: plaintext (plaintext)
Python assertIsNotInstance() method
The assertIsNotInstance() is the opposite of the assertIsInstance() method. It tests if an object is not an instance of a class:
assertNotIsInstance(obj, cls, msg=None)Code language: Python (python)
For example:
import unittestfrom shape import Shape, Square
class TestShape(unittest.TestCase):
def setUp(self):
self.square = Square()
def test_is_instance(self):
self.assertIsInstance(self.square, Square)
def test_is_instance_of_parent_class(self):
self.assertIsInstance(self.square, Shape)
def test_is_instance_of_object(self):
self.assertIsInstance(self.square, object)
def test_is_not_instance(self):
shape = Shape()
self.assertNotIsInstance(shape, Square)
Code language: Python (python)
In this example, the test_is_not_instance() method uses the assertIsNotInstance() method to test if a Shape object is an instance of the Square class.
Run the test:
python -m unittest -vCode language: Python (python)
Output:
test_is_instance (test_shape.TestShape) ... ok
test_is_instance_of_object (test_shape.TestShape) ... ok
test_is_instance_of_parent_class (test_shape.TestShape) ... ok
test_is_not_instance (test_shape.TestShape) ... ok----------------------------------------------------------------------
Ran 4 tests in 0.002s
OK
Code language: plaintext (plaintext)
Summary
- Use the
assertIsInstance()method to test if an object is an instance of a class. - Use the
assertIsNotInstance()method to test if an object is not an instance of a class.