Скачать или смотреть How to Call a Program with Command Line Parameters Using Pytest

Learn how to effectively test command line programs in Python using Pytest's `monkeypatch` fixture. This guide breaks down the process step-by-step for clarity.
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Testing Command Line Programs in Python with Pytest

When you're new to testing with Pytest, especially when it comes to passing command line parameters to your Python programs, things can feel a bit overwhelming. In this guide, we'll address a common challenge faced by developers: how to test a program that accepts command line parameters.

The Problem at Hand

You have a Python program that you normally run using command line arguments. For example:

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Inside this program, which potentially uses modules like argparse, you want to test the output against expected values. Yet, most Pytest resources you find focus on testing functions directly. So, how do you utilize Pytest to call your program and validate its behavior? Do you need to resort to using os.system()?

Fear not! The answer lies within the capabilities of Pytest, specifically the monkeypatch fixture that allows you to simulate and test your program's input and output without modifying the actual argv in your environment. Let’s break down the solution step-by-step.

Solution Overview

Step 1: Program Under Test

First, let's consider a simple program that adds 2 to a number read from a file, and outputs the result to another file. The program looks something like this:

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Step 2: Configuring Your Test

Next, create a test file named test_myprog.py. In this file, you'll set up a test function that utilizes the monkeypatch fixture to simulate command line argument passing:

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Step 3: Preparing Input File

For the function to behave correctly, you also need to create the input file myprog_input.txt containing the value to be read:

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Step 4: Running the Program Manually

You can run your program manually from the command line to see how it works:

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Expected output when running manually:

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Step 5: Testing with Pytest

Finally, execute your tests using Pytest. This will run the test function you defined and validate that the output is as expected:

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The output will confirm if the test passed successfully:

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Conclusion

Testing Python programs that require command line parameters is straightforward with Pytest, especially using the monkeypatch fixture to simulate command-line arguments. By following the outlined steps, you can effectively ensure your program's functionality without altering its actual operation. Happy testing!