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How to Unit Test Functions that Rely on CLI Input Using argparse

A comprehensive guide on unit testing functions that utilize command line interface (CLI) input with argparse in Python. Learn strategies to effectively control CLI inputs during unit tests. --- This video is based on the question https://stackoverflow.com/q/72706396/ asked by the user 'Nick M' ( https://stackoverflow.com/u/5532352/ ) and on the answer https://stackoverflow.com/a/72721621/ provided by the user 'Jason Harrison' ( https://stackoverflow.com/u/54745/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions. Visit these links for original content and any more details, such as alternate solutions, latest updates/developments on topic, comments, revision history etc. For example, the original title of the Question was: How do you unit test functions that rely on data inputted on CLI? Also, Content (except music) licensed under CC BY-SA https://meta.stackexchange.com/help/l... The original Question post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license, and the original Answer post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license. If anything seems off to you, please feel free to write me at vlogize [AT] gmail [DOT] com. --- How to Unit Test Functions that Rely on CLI Input Using argparse When developing Python applications, you may want to capture user input through the Command Line Interface (CLI). This can often complicate things when you're trying to unit test your functions, especially if they rely on inputs provided via command line arguments. If you have encountered challenges while unit testing a Python function that leverages argparse, you’re in the right place. In this post, we will break down how to properly set up your unit tests to effectively handle CLI input. Understanding the Problem You might have a function like this that processes data based on flags given via the command line: [[See Video to Reveal this Text or Code Snippet]] The function create_filename_to_watch determines what filename to watch based on a date flag, either “Previous month” ('P') or “Current month” ('C'). The challenge is how to effectively simulate this input in your unit tests. Setting Up the Unit Test To achieve this, you will have to follow some specific steps. Here’s how you can go about it: Step 1: Use Monkey Patching for CLI Arguments Monkey patching is a technique used to modify or extend the behavior of libraries or modules at runtime. You'll likely want to manipulate sys.argv, the list that contains command line arguments passed to your script. Here’s a simple example of how to patch the command line arguments within your test: [[See Video to Reveal this Text or Code Snippet]] Step 2: Handle Global Variables As suggested, you need to be cautious with global variables like args.date_flag. Instead of fetching the value directly from args, modify the function to accept parameters which improves testability. Here’s a modified version of your function: [[See Video to Reveal this Text or Code Snippet]] Step 3: Adapt Your Tests With the modified function, your tests will now look like this: [[See Video to Reveal this Text or Code Snippet]] Conclusion Unit testing functions that rely on CLI inputs can be daunting, but breaking down the problem into manageable steps helps demystify the process. By using monkey patching to simulate command line arguments and adjusting function parameters for better testability, you can modify your approach significantly. Remember, embracing good coding practices, like minimizing global dependencies, will not only make unit testing easier but also enhance the maintainability of your code. Implement these strategies in your next project, and you'll find that mocking and testing CLI inputs becomes a smooth process!