Adventures in Machine Learning

Testing Techniques for Python CLI Applications: A Comprehensive Guide

& Overview

Python is a popular programming language used in many industries and applications, including developing command line interface (CLI) programs. Testing these CLI applications becomes crucial in ensuring their functionality and reliability.

Therefore, this article aims to highlight the importance of testing and the techniques used to test Python CLI applications.

Importance of Testing

Testing is an essential part of the software development process that ensures that the application functions as per the requirements and specifications. However, many developers tend to underestimate the importance of testing.

Testing helps improve the functionality, reliability, and maintainability of the application. Verifying code with tests can also increase confidence in making code changes and reduce the risk of defects.

Moreover, it acts as a documentation process for developers. Developers can create tests for requirements, which documents how to use the codebase correctly.

Therefore, investing in testing helps to identify potential problems early in the development process and saves resources in the long run.

Four Techniques Overview

There are four main techniques used to test Python CLI applications:

1. “Lo-Fi” Debugging With Print


Systematic Unit Testing


Integration Testing


End-To-End Testing

Each technique is unique and serves a different purpose, covering various levels of function within the application. Therefore, we will discuss each technique in detail.

“Lo-Fi” Debugging With Print()

Print() is a simple yet powerful technique for debugging CLI Python applications. It is often the first stage of debugging since it requires minimal setup and is effortless to use.

When a developer is unsure of what’s happening, they can add print statements to the code to see what’s going on when that line executes. It helps identify issues such as values of variables and paths through the code.

Here’s an example code showing the use of print():


def add_nums(a, b):

print(f”adding {a} and {b}”)

return a + b

result = add_nums(2, 3)

print(f”The result is {result}”)


The output of this code would be:


adding 2 and 3

The result is 5


Print statements can also be nested within loops and conditionals to help locate where the code stops executing as expected. However, using too many print statements can result in cluttered output, which becomes challenging to analyze.

Therefore, pprint() is an alternative option to print() that presents the output in a more organized and readable manner.

Systematic Unit Testing

Unit testing is a technique that tests the smallest components of an application called units. Units comprise functions or methods that execute specific tasks within the application.

The objective of unit testing is to ensure that each function meets the expected output when provided with specific inputs. Developers write test cases for each unit to test its correctness.

The test cases define inputs and the expected output for the unit. If the function matches the expected output, the test case passes, and if not, it fails.

The benefits of systematic unit testing are that it helps ensure that every part of the code works correctly before integrating it with other parts. It helps locate bugs early in the development process and serves as documentation for the codebase.

Integration Testing

Integration testing involves testing how different units work together to form a more significant function. The purpose of integration testing is to identify any issues in the interaction between different units as a group.

Integration tests help detect problems such as communication errors in APIs, components, and modules working as expected. The developer must understand the application’s dependencies and external systems it interacts with to perform integration testing fully.

End-To-End Testing

End-to-end testing is a form of testing that examines the application’s whole function from start to finish. The goal of end-to-end testing is to ensure that the application meets the user’s needs and requirements.

End-to-end testing is an essential technique to verify that the application’s integrations and dependencies work correctly, and the user interface behaves as expected. It involves testing various scenarios with actual user inputs.

End-to-end testing may include user acceptance testing, which ensures that the application meets the users’ needs.


In conclusion, testing Python CLI applications is crucial in ensuring their functionality and reliability. There are four main techniques used to test Python CLI applications: “Lo-Fi” Debugging With Print, systematic unit testing, integration testing, and end-to-end testing.

Each technique is unique and serves a different purpose, covering various levels of function within the application. Print() is the most basic debugging technique, while end-to-end testing tests the application’s whole function.

Investing in testing helps to identify potential problems early in the development process and saves resources in the long run.

3) Using a Debugger

Developers encounter errors regularly while developing software applications. A debugger is a powerful tool that helps developers identify and locate bugs in an application’s source code.

This tool allows developers to execute code step-by-step and view its behavior during execution.

Setting Breakpoints & Watches

Breakpoints are the trigger points that stop the execution of an application’s source code while debugging.

Developers usually set them at specific code lines, where they want to investigate further. Once the code execution reaches the breakpoint, the debugger stops it, and the developer can inspect the variable values, change them, or resume the execution from that point.

Watches are expressions that developers consider important while debugging. They allow the developer to evaluate any expression of the program’s variables and continue to monitor these expressions as the program executes.

The debugger monitors variables during the execution, and the developer can review their current value whenever they want.

Steps (Step Over, Step In, Step Out)

The steps are the sequence of debugger functions available to developers to navigate the code execution.

The three most essential steps for CLI Python applications are:

1. Step over: This function executes the next line of code and stops if it encounters a breakpoint.

If there are no breakpoints, it proceeds with code execution for the next line. 2.

Step in: This function goes inside a sub-function for debugging. It permits inspection of the inside code of a function, with the ability to execute one line of code at a time.

3. Step out: This function continues execution until it reaches the end of the current function, then stops.

Using a debugger to narrow down the potential problem is an efficient way to resolve issues. By pausing the application at problematic points, it is easier to examine and resolve the issue.

4) Unit Testing with Pytest and Mocks

Unit testing checks individual units of application code, such as functions or classes, to ensure they meet the requirements and work as expected. They isolate and examine individual code operations to ensure their reliability before integrating them into the overall application.

Pytest Basics

Pytest is a Python testing framework that makes unit testing easier and provides powerful and flexible functionalities. It is highly recommended because it automates test discovery and allows developers to write less boilerplate code.

It also has an easy-to-use syntax, which means developers write tests without having to learn entirely new keywords and methods. Pytest discovers tests automatically, so developers do not need to specify tests’ names manually.

Unit Test Fixtures

Pytest fixtures are methods that set things up before running the tests. They are functions that prepare the test’s starting state before running it.

They help write complete test cases by automating the setup and teardown processes. We can think of fixtures as pre-test code that ensures everything is ready for a test run.

For example:


import pytest


def initiate_app():

return App()

def test_app_start(initiate_app):

assert initiate_app.start() == “Application started.”


In this example, the initiate_app fixture creates a new instance of the App class before we create a test. The test_app_start function expects the start function of the App class to return a string “Application started.”

During the test run, Pytest invokes the initiate_app fixture to create the App instance, making it available as the argument for the test_app_start function.

Mocking in Unit Tests

Mocking in unit tests allows developers to isolate specific parts of the application and replace them with mock functions. This process helps test a function without relying on other functions.

It also permits a developer to imitate specific conditions, such as incorrect or strange data, or the failure of a dependent function to return the expected output.

Mocking obtains the required results without having to execute the entire code of an application.

In unit testing, fixtures help test “mock” objects or “replacement” code that the main code relies on for functionality. By replacing the code with a mock version, developers can test the primary code’s assumptions.

For example:


from unittest.mock import MagicMock

def test_mock():

mock_obj = MagicMock(return_value=”Mocked function called.”)

result = mock_obj()

assert result == “Mocked function called.”


In this example, we create a mock object for the MagicMock class, which returns the value “Mocked function called.” A test calls the mock object, and we assert that the return value is equal to the expected value.


Debuggers and unit testing frameworks are powerful tools that developers use to ensure the reliability and functionality of their application. Unit testing helps to isolate and examine individual parts of the code, while debuggers help locate and diagnose application errors more efficiently.

It is essential to implement these testing processes before launching applications into production to prevent problems before they occur. 5)

Integration Testing

Overview of

Integration Testing

Integration testing is a testing methodology that involves testing how different components of an application interact with each other.

In software development, Applications are typically split into smaller modules and functions that work together to create the overall application. Integration testing checks that these different modules work together as expected.

There are two main integration testing approaches: top-down and bottom-up. Top-down or “non-incremental” integration testing verifies that high-level modules work correctly before integrating lower-level modules.

Bottom-up or “incremental” integration testing, starts with testing low-level units and continually integrates higher-level units until the entire application is fully tested.

Integration testing should identify and report errors, ensure that recently coded software is tested before further development and deliver the software.

In addition, developers can catch errors early in the development stage by integrating modules as soon as they are complete.

Testing CLI Applications

Command-line applications directly interface with an operating system, performing a wide range of tasks ranging from basic file manipulations to more complex server-side deployments. CLI Python applications are executable and are compatible with any system, making them ideal for testing in different environments.

Testing CLI applications involves many of the same processes as other applications with some significant differences:

1. Command-line applications interface with the operating system.

CLI applications may need to interact with an application programming interface (API) or an operating system service. 2.

Testing must check that the application performs properly. 3.

CLI applications may involve more complex data models and require specific testing of remote server interactions. Thus, CLI testing requires an additional level of testing beyond application testing.

Integration testing must be designed to verify the communication between the command-line elements and the operating system.

6) Putting It All Together

Recap of Testing Techniques

Software testing is a critical component of the software development process. Developers use various testing techniques to ensure that the application is reliable and functions as intended.

“Lo-Fi” Debugging With Print is a simple and elegant debugging technique that developers use to diagnose problems in their code. Print statements allow developers to see the program’s execution, variable state, and control flow.

Systematic Unit Testing is the practice of writing and running tests for code units or small parts of an application individually. This ensures that units perform as expected.

Integration testing examines modules of the application and ensures that they work well together. This prevents bugs from creeping in when the modules are combined into an application.

End-to-end testing involves exercising the entire application or feature from beginning to end.

Pytest Basics involves automating unit tests using the Pytest framework, which makes it easy to detect errors and write less boilerplate code.

Mocking in Unit Tests helps test code that depends on third-party services or libraries without relying on these dependencies.

Importance of Comprehensive Testing

Comprehensive testing is critical as it ensures code reliability and efficacy, translates to better user satisfaction with the product and cost savings in terms of maintenance and re-work. The main goal of testing is to deliver products that meet clients’ quality requirements and provide critical feedback to developers of future products.

In conclusion, the combination of using the appropriate testing techniques and tools will improve software development processes and ultimately produce more reliable, cost-effective software. CLI applications have unique testing requirements due to the direct interaction with operating systems and services, necessitating proper integration testing to ensure their reliability.

The software must also be tested as a whole to ensure the individual components are working together as expected. In summary, this article emphasizes the importance of testing Python CLI applications and introduces four testing techniques: “Lo-Fi” Debugging With Print,

Systematic Unit Testing,

Integration Testing, and End-to-End Testing.

Pytest Basics and Mocking are also discussed as useful tools for unit testing. Integration testing is essential for CLI applications due to their distinctive interaction with operating systems.

Comprehensive testing is essential to ensure code reliability, user satisfaction, and cost savings. By implementing the appropriate testing techniques and tools, developers can produce more reliable, cost-effective software with fewer bugs.

Proper testing and debugging lead to fewer issues in production, satisfied end-users, and successful delivery of software that meets clients’ quality requirements.

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