Stubs in Python: A Practical Approach to Test Dummies


Welcome to a comprehensive guide on Stubs in Python! Here, we will provide a practical approach to understanding and using Stubs for testing purposes.

Definition of Stubs in Python

Stubs are a type of software testing technique used by developers to test code components that have not yet been developed or are not available for testing. It is an effective tool in creating test dummies that can simulate the behavior of an actual function or module.

In simple terms, a stub is a small piece of code that represents another module or function. It takes the place of the real version while still allowing for the testing of other parts of the application which depend on it.

Importance of Stubs in Software Development

The importance of stubs cannot be overemphasized as they are crucial to software development and testing. When building large-scale applications, different developers may be responsible for different modules or functions; hence, some components may not be available at certain development stages. In such cases, stubs come in handy as they allow developers to simulate these components during testing and integration phases.

Stubs also enable developers to identify defects early in the development process when they can be fixed with minimal effort and cost. This saves time and resources that could have been spent later fixing more complex issues discovered during system integration or acceptance tests.

Overview of The Article

The article is structured into several sections covering all aspects related to using stubs effectively for software development:

  • We will begin with explaining what stubs are, their types, and why you should use them.
  • You will learn how to implement stubs in Python with our step-by-step guide
  • We will cover the unittest.mock module and its usefulness when working with stubs.
  • We will provide some tips for using stubs effectively and avoiding common pitfalls.

By the end of this article, you will be well-equipped to use stubs for your software development projects!

What are Stubs?

In software development, stubs refer to pieces of code that are used for testing purposes. They are small and simple functions that act as placeholders for more complex routines or objects.

The main objective of using stubs is to isolate and test specific parts of a program without having to run the entire application. This makes it easier and faster to identify and fix errors in the code.

Definition and Explanation

A stub is a type of test double, which is used in place of a real object or function during testing. It replaces the functionality of the original object with a simpler implementation that can be easily controlled and manipulated by the tester. Stubs are often used when the original code or module is not yet available or ready for testing.

Types of Stubs

There are several types of stubs that can be used in software development:

Dummy Stub

A dummy stub does not have any logic implemented within it. It simply returns predefined values that can be used for testing purposes. For example, if you have a function that requires an input parameter, but you do not want to create an actual object for this parameter during testing, you can use a dummy stub to simulate the input.

Fake Stub

A fake stub is similar to a dummy stub but has some basic logic implemented within it. This allows it to return more realistic values based on certain conditions or inputs provided during testing. For example, if you have a function that returns different results based on whether an input value is true or false, you can use a fake stub to replicate this behavior.

Spy Stub

A spy stub monitors calls made to another object during testing without affecting its behavior. This allows the tester to observe how the object is being used without interfering with its functionality. For example, if you have a function that calls another function within it, you can use a spy stub to monitor the input and output values of the inner function.

Mock Stub

A mock stub is similar to a spy stub but has more advanced functionality. It can simulate the behavior of an entire object or module during testing, allowing for more comprehensive testing of complex functions and applications.

For example, if you have a function that relies on multiple external modules, you can use a mock stub to simulate the behavior of these modules during testing. By using these different types of stubs in Python programming, developers can easily test their code in isolation and identify any issues before they become larger problems.

Why Use Stubs?

In software development, automated testing is an essential component of a successful development cycle. As the complexity of software systems increases, so does the need for comprehensive testing.

However, writing tests for complex systems can be time-consuming and challenging. This is where stubs come in handy.

Stubs are simplified versions of a function or method that stand in for the real implementation during testing. They can be used to isolate the code being tested, making it easier to verify that specific functionality works as expected.

Benefits of Using Stubs in Software Development

Using stubs can provide a myriad of benefits for software developers:

  • Faster Test Execution: stubs reduce the dependencies between functions and methods, allowing tests to run faster.
  • Simpler Test Code: by isolating functionality with stubs, test code becomes simpler and more straightforward to write.
  • Increased Confidence in Code: by ensuring specific functions work correctly with appropriate test data, stubs help increase confidence in code quality and minimize defects.

Real-World Examples

The benefits of using stubs can be seen across many real-world examples. Two such examples are testing database connections and APIs. Testing Database Connections:

The ability to connect to a database is critical to many applications’ functionality; however, it can also slow down test execution time due to network latency issues or database constraints. By using stubs that simulate the database connection rather than connecting physically, we can shorten our feedback loop while still verifying expected behaviors without waiting on any lengthy queries being executed on production databases during development cycles. Testing APIs:

APIs are often external to the test environment, and testing their behavior typically involves long wait times while endpoints are reached. Additionally, API responses could change over time, requiring developers to update their tests continually.

Stubs can be used to simulate API behavior and reduce dependencies on any external services or third-party APIs. This way, developers can test specific functionality in isolation without waiting for an API response from a remote or external source.

How to Implement Stubs in Python

If you are a developer, you might be familiar with the importance of writing tests for your code. Tests help ensure that your code is working as intended and they prevent bugs that could affect the user experience.

One of the most useful tools for testing is stubs. In this article we will explore how to implement stubs in your Python projects.

Step-by-Step Guide for Creating a Dummy Stub

One type of stub is a dummy stub. It’s used when you need a function to be called but you don’t actually care about its output.

Here’s an example:

def add_numbers(a, b): return a + b

def do_math(): result = add_numbers(1, 2)

return result

In this case, `do_math` calls `add_numbers`, which returns the sum of `a` and `b`.

But let’s say that we want to test whether `do_math` is calling `add_numbers` at all, and we don’t care about what it returns. This is where a dummy stub comes in handy.

Defining the Function to be Tested

The first step is defining the function that will call our dummy stub, which in this case is `do_math`. We will write our test cases around this function.

def do_math():

result = add_numbers(1, 2) return result

Creating the Dummy Function

Next, we need to create our dummy function. The purpose of the dummy function is simply to replace the original function (`add_numbers`) during testing.

python def do_nothing(*args):


Here we define a new function called `do_nothing`, which takes any number of arguments and simply does nothing.

Defining the Arguments and Return Values

Now that we have our dummy function, we need to define its arguments and return values. Since `add_numbers` takes two arguments (`a` and `b`), our dummy function should take two arguments as well.

python def do_nothing(a, b):


We don’t need to define a return value for our dummy function since we’re not going to use it.

Writing Test Cases for the Dummy Function

We can write test cases that use the dummy function. Here’s an example of how we can use our dummy stub in a test case:

def test_do_math_calls_add_numbers(): with patch('__main__.add_numbers', side_effect=do_nothing) as add_mock:

do_math() add_mock.assert_called_once_with(1, 2)

In this test case, we’re using the `patch` method from Python’s built-in `unittest.mock` library. We’re telling it to replace `add_numbers` with our dummy function (`do_nothing`) during testing.

Then we call `do_math`. We check whether `add_numbers` was called once with the correct arguments.

Implementing stubs in Python might seem daunting at first but is actually straightforward once you get used to it. The benefits you will reap from having well-tested code will make it all worth it in the end!

Mocking with Python’s unittest.mock Module

What is unittest.mock module?

Python’s unittest.mock module is a powerful tool for testing your code. It gives you the ability to simulate specific behaviors and responses to function calls, whether it be to return a certain value or raise a specific exception. This can be incredibly useful when you need to test functions that have dependencies on other parts of your application or external services that may not be available during testing.

The unittest.mock module offers a variety of tools for mocking objects including the MagicMock, which mimics an object’s interface with methods and properties that can be added dynamically during testing, as well as the Mock class which returns values based on how the object was configured. In addition, using the mock library in Python allows you to use ‘assert_called_with’ method which confirms that the method being tested was called with specific arguments.

How to use unittest.mock module for testing?

Mocking with Python’s unittest.mock module can be used in several ways, but one common way is by patching. Patching replaces an object in your code with a mock object for the duration of a test case or test run.

Here’s an example:

import requests

from unittest import TestCase from unittest.mock import patch

class TestRequestsWithPatch(TestCase): @patch('requests.get')

def test_request_status_code_with_patch(self, mock_get): # Configure mock_get response

mock_response = Mock() mock_response.status_code = 200

mock_get.return_value = mock_response # Call function under test

result = my_module.get_page_data() # Make assertions against expected behavior

self.assertEqual(result.status_code, 200)

In this example, we are patching requests.get() so that it returns our mocked response instead of making a real HTTP request.

Then, we call the function under test and make assertions against the expected behavior. Another way to use unittest.mock is by creating a custom mock object using MagicMock or Mock classes.

This can be useful when you need to create complex mock objects that cannot be created using patching. Overall, mocking with Python’s unittest.mock module is a powerful testing tool that can make it easier to test your code by isolating dependencies, speeding up testing time, and making it easier to debug issues.

Tips for Effective Use of Stubs in Python

Avoiding Overuse: When to Stop Using Stubs

Stubs are extremely useful in simplifying tests that involve complex dependencies. However, it is important to remember that they are not a replacement for actual testing of the code. Overusing stubs can lead to incomplete tests and can create a false sense of security in the quality of the code.

If a function or its dependencies change significantly, use real objects instead of stubs to ensure that the tests accurately reflect the current state of the code. Stubs should also be used sparingly when testing public interfaces as these need to be tested using actual dependencies.

Overuse may lead to missed issues with integration and overall system performance. Keep in mind that not all code needs stubbing; sometimes it is better to test everything integrally if possible.

Testing Edge Cases: Don’t Forget About Extremes

Testing edge cases is crucial as they help expose hidden flaws in your software’s logic and can help identify bugs that may remain undiscovered otherwise. When using stubs, it is important not only to test for typical scenarios but also consider unusual cases where inputs are at their extreme limit values or out-of-bounds inputs.

For example, when testing user input validation functions, you should consider what would happen if an input was empty or too large, rather than focusing solely on valid inputs (like alphanumeric strings). Testing edge cases will give confidence that your program is protected from unexpected behavior from users who may try entering unusual data sets.

Maintainability: Updates and Refactoring Code With Ease

To maintain robustness while implementing software development life cycles (SDLC), refactoring should be done gracefully without leading issues such as breaking existing functionality or introducing new bugs into production environments. Updating your test suite quickly and efficiently is important for the overall success of the development process. Stubs can help simplify this process because they can be easily modified to accommodate changes in your code.

This makes refactoring easier, as the stubs and their dependencies can quickly be updated to reflect changes in the system. Additionally, by using stubs with well-defined interfaces, modifications made to dependencies will have a lower impact on other parts of the system.


Stubs are a powerful tool for simplifying tests involving complex dependencies. The use of these dummies allows developers to test code more efficiently and effectively while also improving maintainability during an SDLC. As with any tool, it is important not to overuse stubs to ensure tests cover all cases effectively and avoid false positives that could lead to issues in real-world scenarios.

When used judiciously and with care towards edge cases testing, stubs can significantly reduce testing time while increasing confidence in your team’s software quality. So whether you’re building a small app or managing large enterprise systems, implementing stubs into your testing workflow may prove very effective for you!

Related Articles