79. Mocking in Tests with unittest.mock

Here are Python code examples for simulating objects and behaviors in unit tests using unittest.mock:

1. Mocking a Function Return Value

In this example, the mock object is used to simulate a function's return value.

import unittest
from unittest.mock import Mock

def fetch_data():
    return "Real Data"

class TestMocking(unittest.TestCase):
    def test_mock_function(self):
        mock = Mock(return_value="Mocked Data")
        result = mock()
        self.assertEqual(result, "Mocked Data")

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

---

2. Mocking an Object Method

Here, we mock an object’s method to simulate specific behavior.

import unittest
from unittest.mock import Mock

class Database:
    def connect(self):
        return "Real Connection"

class TestMocking(unittest.TestCase):
    def test_mock_method(self):
        db = Mock(spec=Database)
        db.connect.return_value = "Mocked Connection"
        result = db.connect()
        self.assertEqual(result, "Mocked Connection")

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

---

3. Mocking a Class Constructor

This snippet demonstrates mocking the constructor of a class to return a mock object.

import unittest
from unittest.mock import Mock, patch

class Database:
    def __init__(self):
        self.connection = "Real Connection"

    def get_data(self):
        return "Real Data"

class TestMocking(unittest.TestCase):
    @patch('__main__.Database')
    def test_mock_constructor(self, MockDatabase):
        mock_db = MockDatabase.return_value
        mock_db.get_data.return_value = "Mocked Data"
        result = mock_db.get_data()
        self.assertEqual(result, "Mocked Data")

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

---

4. Mocking a File Read Operation

Here we mock the behavior of reading a file to simulate file operations.

import unittest
from unittest.mock import mock_open, patch

class TestMocking(unittest.TestCase):
    @patch('builtins.open', mock_open(read_data="Mocked file content"))
    def test_mock_file_read(self):
        with open('fake_file.txt', 'r') as f:
            content = f.read()
        self.assertEqual(content, "Mocked file content")

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

---

5. Mocking a Property

In this example, we mock a class property to simulate its behavior.

import unittest
from unittest.mock import Mock

class Database:
    @property
    def status(self):
        return "Real Status"

class TestMocking(unittest.TestCase):
    def test_mock_property(self):
        db = Mock(spec=Database)
        db.status = "Mocked Status"
        self.assertEqual(db.status, "Mocked Status")

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

---

6. Mocking an External API Request

This demonstrates mocking an external API call to avoid hitting the real API in unit tests.

import unittest
from unittest.mock import patch
import requests

def fetch_api_data(url):
    response = requests.get(url)
    return response.json()

class TestMocking(unittest.TestCase):
    @patch('requests.get')
    def test_mock_api_request(self, mock_get):
        mock_get.return_value.json.return_value = {"key": "value"}
        result = fetch_api_data('https://fakeapi.com')
        self.assertEqual(result, {"key": "value"})

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

---

7. Mocking Side Effects

In this example, a side effect is used to simulate an exception being raised during method calls.

import unittest
from unittest.mock import Mock

class TestMocking(unittest.TestCase):
    def test_mock_side_effect(self):
        mock = Mock()
        mock.side_effect = ValueError("Mocked Exception")
        
        with self.assertRaises(ValueError) as context:
            mock()
        self.assertEqual(str(context.exception), "Mocked Exception")

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

---

8. Mocking Multiple Return Values

This example demonstrates how to simulate different return values for consecutive calls to a mocked method.

import unittest
from unittest.mock import Mock

class TestMocking(unittest.TestCase):
    def test_mock_multiple_return_values(self):
        mock = Mock()
        mock.side_effect = [1, 2, 3]
        
        result1 = mock()
        result2 = mock()
        result3 = mock()
        
        self.assertEqual(result1, 1)
        self.assertEqual(result2, 2)
        self.assertEqual(result3, 3)

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

---

9. Mocking a Method with Arguments

This snippet shows how to mock methods that take arguments and return a value based on those arguments.

import unittest
from unittest.mock import Mock

class TestMocking(unittest.TestCase):
    def test_mock_method_with_arguments(self):
        mock = Mock()
        mock.return_value = 10
        mock.side_effect = lambda x: x * 2
        
        result = mock(5)
        self.assertEqual(result, 10)

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

---

10. Mocking an Iterable Object

In this example, the mock object is used to simulate an iterable object.

import unittest
from unittest.mock import Mock

class TestMocking(unittest.TestCase):
    def test_mock_iterable(self):
        mock = Mock()
        mock.__iter__.return_value = iter([1, 2, 3])
        
        result = list(mock)
        self.assertEqual(result, [1, 2, 3])

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

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These examples demonstrate various ways to mock functions, methods, classes, and external APIs using unittest.mock. Mocking allows you to simulate complex behaviors, test edge cases, and isolate components, ensuring your tests are faster and more reliable.