How to test command line applications in Python?

Question

I know Python unittest. I have some experience using it for testing Python subprograms.

Now I need to add testing my command line application (not just a Python function) written in Python. I want to call it with certain arguments and certain input in stdin and test output in stdout.

How to integrate testing a command line tool with other unittest test cases?

Or what to use instead of unittest?

Answer 1

You can still use the standard unittest format and test the whole application as a standard function. Make a wrapper that makes the script entry point a simple wrapper, like:

if __name__ == "__main__":
    sys.exit(main(sys.argv))

As long as you don't abuse global variables for keeping the state, you only need to test the main() function.

If you want to test scripts which you don't have control over, you can still use unittest and subprocess by writing a test like:

def test_process(self):
    result = subprocess.run(['your_script', 'your_args', ...], capture_output=True)
    self.assertIn('expected out', result.stdout)

def test_process_failure(self):
    result = subprocess.run(['your_script', 'your_args', ...], capture_output=True)
    self.assertEqual(result.returncode, 1)