Python CLI program unit testing

Question

I am working on a python Command-Line-Interface program, and I find it boring when doing testings, for example, here is the help information of the program:

usage: pyconv [-h] [-f ENCODING] [-t ENCODING] [-o file_path] file_path

Convert text file from one encoding to another.

positional arguments:
  file_path

optional arguments:
  -h, --help            show this help message and exit
  -f ENCODING, --from ENCODING
                        Encoding of source file
  -t ENCODING, --to ENCODING
                        Encoding you want
  -o file_path, --output file_path
                        Output file path

When I made changes on the program and want to test something, I must open a terminal, type the command(with options and arguments), type enter, and see if any error occurs while running. If error really occurs, I must go back to the editor and check the code from top to end, guessing where the bug positions, make small changes, write print lines, return to the terminal, run command again...

Recursively.

So my question is, what is the best way to do testing with CLI program, can it be as easy as unit testing with normal python scripts?

Answer 1

Given that you are explicitly asking about testing for a command line application, I believe that you are aware of unit-testing tools in python and that you are actually looking for a tool to automate end-to-end tests of a command line tool. There are a couple of tools out there that are specifically designed for that. If you are looking for something that's pip-installable, I would recommend cram. It integrates well with the rest of the python environment (e.g. through a pytest extension) and it's quite easy to use:

Simply write the commands you want to run prepended with $ and the expected output prepended with . For example, the following would be a valid cram test:

  $ echo Hello
    Hello

By having four spaces in front of expected output and two in front of the test, you can actually use these tests to also write documentation. More on that on the website.

Answer 2

For Python 3.5+, you can use the simpler subprocess.run to call your CLI command from your test.

Using pytest:

import subprocess

def test_command__works_properly():
    try:
        result = subprocess.run(['command', '--argument', 'value'], check=True, capture_output=True, text=True)
    except subprocess.CalledProcessError as error:
        print(error.stdout)
        print(error.stderr)
        raise error

The output can be accessed via result.stdout, result.stderr, and result.returncode if needed.

The check parameter causes an exception to be raised if an error occurs. Note Python 3.7+ is required for the capture_output and text parameters, which simplify capturing and reading stdout/stderr.

Answer 3

Maybe too little too late, but you can always use

import os.system
result =  os.system(<'Insert your command with options here'>
assert(0 == result)

In that way, you can run your program as if it was from command line, and evaluate the exit code.

(Update after I studied pytest) You can also use capsys. (from running pytest --fixtures)

capsys Enable text capturing of writes to sys.stdout and sys.stderr.

The captured output is made available via ``capsys.readouterr()`` method
calls, which return a ``(out, err)`` namedtuple.
``out`` and ``err`` will be ``text`` objects.

Answer 4

pytest-console-scripts is a Pytest plugin for testing python scripts installed via console_scripts entry point of setup.py.

Answer 5

Start from the user interface with functional tests and work down towards unit tests. It can feel difficult, especially when you use the argparse module or the click package, which take control of the application entry point.

The cli-test-helpers Python package has examples and helper functions (context managers) for a holistic approach on writing tests for your CLI. It's a simple idea, and one that works perfectly with TDD:

1. Start with functional tests (to ensure your user interface definition) and
2. Work towards unit tests (to ensure your implementation contracts)

Functional tests

NOTE: I assume you develop code that is deployed with a setup.py file or is run as a module (-m).

• Is the entrypoint script installed? (tests the configuration in your setup.py)
• Can this package be run as a Python module? (i.e. without having to be installed)
• Is command XYZ available? etc. Cover your entire CLI usage here!

Those tests are simplistic: They run the shell command you would enter in the terminal, e.g.

def test_entrypoint():
    exit_status = os.system('foobar --help')
    assert exit_status == 0

Note the trick to use a non-destructive operation (e.g. --help or --version) as we can't mock anything with this approach.

Towards unit tests

To test single aspects inside the application you will need to mimic things like command line arguments and maybe environment variables. You will also need to catch the exiting of your script to avoid the tests to fail for SystemExit exceptions.

Example with ArgvContext to mimic command line arguments:

@patch('foobar.command.baz')
def test_cli_command(mock_command):
    """Is the correct code called when invoked via the CLI?"""
    with ArgvContext('foobar', 'baz'), pytest.raises(SystemExit):
        foobar.cli.main()

    assert mock_command.called

Note that we mock the function that we want our CLI framework (click in this example) to call, and that we catch SystemExit that the framework naturally raises. The context managers are provided by cli-test-helpers and pytest.

Unit tests

The rest is business as usual. With the above two strategies we've overcome the control a CLI framework may have taken away from us. The rest is usual unit testing. TDD-style hopefully.

Disclosure: I am the author of the cli-test-helpers Python package.

Answer 6

I think it's perfectly fine to test functionally on a whole-program level. It's still possible to test one aspect/option per test. This way you can be sure that the program really works as a whole. Writing unit-tests usually means that you get to execute your tests quicker and that failures are usually easier to interpret/understand. But unit-tests are typically more tied to the program structure, requiring more refactoring effort when you internally change things.

Anyway, using py.test, here is a little example for testing a latin1 to utf8 conversion for pyconv::

# content of test_pyconv.py

import pytest

# we reuse a bit of pytest's own testing machinery, this should eventually come
# from a separatedly installable pytest-cli plugin. 
pytest_plugins = ["pytester"]

@pytest.fixture
def run(testdir):
    def do_run(*args):
        args = ["pyconv"] + list(args)
        return testdir._run(*args)
    return do_run

def test_pyconv_latin1_to_utf8(tmpdir, run):
    input = tmpdir.join("example.txt")
    content = unicode("\xc3\xa4\xc3\xb6", "latin1")
    with input.open("wb") as f:
        f.write(content.encode("latin1"))
    output = tmpdir.join("example.txt.utf8")
    result = run("-flatin1", "-tutf8", input, "-o", output)
    assert result.ret == 0
    with output.open("rb") as f:
        newcontent = f.read()
    assert content.encode("utf8") == newcontent

After installing pytest ("pip install pytest") you can run it like this::

$ py.test test_pyconv.py
=========================== test session starts ============================
platform linux2 -- Python 2.7.3 -- pytest-2.4.5dev1
collected 1 items

test_pyconv.py .

========================= 1 passed in 0.40 seconds =========================

The example reuses some internal machinery of pytest's own testing by leveraging pytest's fixture mechanism, see http://pytest.org/latest/fixture.html. If you forget about the details for a moment, you can just work from the fact that "run" and "tmpdir" are provided for helping you to prepare and run tests. If you want to play, you can try to insert a failing assert-statement or simply "assert 0" and then look at the traceback or issue "py.test --pdb" to enter a python prompt.

Answer 7

I would not test the program as a whole this is not a good test strategy and may not actually catch the actual spot of the error. The CLI interface is just front end to an API. You test the API via your unit tests and then when you make a change to a specific part you have a test case to exercise that change.

So, restructure your application so that you test the API and not the application it self. But, you can have a functional test that actually does run the full application and checks that the output is correct.

In short, yes testing the code is the same as testing any other code, but you must test the individual parts rather than their combination as a whole to ensure that your changes do not break everything.

Answer 8

You can use standard unittest module:

# python -m unittest <test module>

or use nose as a testing framework. Just write classic unittest files in separate directory and run:

# nosetests <test modules directory>

Writing unittests is easy. Just follow online manual for unittesting

Answer 9

So my question is, what is the best way to do testing with CLI program, can it be as easy as unit testing with normal python scripts?

The only difference is that when you run Python module as a script, its __name__ attribute is set to '__main__'. So generally, if you intend to run your script from command line it should have following form:

import sys

# function and class definitions, etc.
# ...
def foo(arg):
    pass

def main():
    """Entry point to the script"""

    # Do parsing of command line arguments and other stuff here. And then
    # make calls to whatever functions and classes that are defined in your
    # module. For example:
    foo(sys.argv[1])


if __name__ == '__main__':
    main()

Now there is no difference, how you would use it: as a script or as a module. So inside your unit-testing code you can just import foo function, call it and make any assertions you want.

Answer 10

The short answer is yes, you can use unit tests, and should. If your code is well structured, it should be quite easy to test each component separately, and if you need to to can always mock sys.argv to simulate running it with different arguments.

Answer 11

This isn't for Python specifically, but what I do to test command-line scripts is to run them with various predetermined inputs and options and store the correct output in a file. Then, to test them when I make changes, I simply run the new script and pipe the output into diff correct_output -. If the files are the same, it outputs nothing. If they're different, it shows you where. This will only work if you are on Linux or OS X; on Windows, you will have to get MSYS.

Example:

python mycliprogram --someoption "some input" | diff correct_output -

To make it even easier, you can add all these test runs to your 'make test' Makefile target, which I assume you already have. ;)

If you are running many of these at once, you could make it a little more obvious where each one ends by adding a fail tag:

python mycliprogram --someoption "some input" | diff correct_output - || tput setaf 1 && echo "FAILED"