It's good practice to, as much as possible, write tests for your code. If you're working with Python, pytest makes the process of writing and running tests much smoother. I wrote a few posts some time back on getting started with testing with pytest, so if you're completely new to it, you might want to take a look at them:

• Python TDD with Pytest -- Getting Started
• Asserting Exceptions with Pytest
• Capturing print statements while debugging
• Skipping tests

For testing CLI apps, Click provides a convenient module: click.testing which has some useful functions (notably CliRunner()) to help us invoke commands and check their behavior.

We'll go ahead and test each part of our app - creating, reading, updating and deleting.

Installing pytest and writing the first test

pytest can be installed via pip:

(env) $ pip install pytest

After installing pytest, create a tests folder in the root directory and add the first test file:

(env) $ mkdir tests && cd tests

(env) $ touch test_app.py

In the test_app file, add the following code for a start:

def test_add():
    pass

To run the test, run pytest on the terminal:

(env) $ pytest
================== test session starts ====================
platform linux -- Python 3.7.3, pytest-5.1.0, py-1.8.0, pluggy-0.12.0
rootdir: /home/wangonya/code/contacts-cli
collected 1 item

tests/test_app.py .                              [100%]

================== 1 passed in 0.04s =======================

Testing the add command

Let's edit the test_app file to add a test to see if the add command adds a new contact:

from click.testing import CliRunner

from app import add

runner = CliRunner()

def test_add():
    response = runner.invoke(add, ["test-user", "-m", "0"])
    assert response.exit_code == 0
    assert "Contact test-user added!" in response.output
    assert "{'mobile': '0'}" in response.output

First, we invoke the command as we would on the terminal, passing in the required arguments and options: response = runner.invoke(add, ["test-user", "-m", "0"]).

We then check that the command executes successfully: assert response.exit_code == 0.

If the command executes successfully, we expect a success message should be returned in the response with the values we added:

assert "Contact test-user added!" in response.output
assert "{'mobile': '0'}" in response.output

The rest of the tests will pretty much follow the same format.

Testing the list command

def test_list():
    response = runner.invoke(list)
    assert response.exit_code == 0
    assert "Here\'s a list of all your contacts:" in response.output
    assert "'test-user': {'mobile': '0'}" in response.output

The list command doesn't take any arguments or options so we just call it directly: response = runner.invoke(list).

Testing the view command

def test_view():
    response = runner.invoke(view, "test-user")
    assert response.exit_code == 0
    assert "{'mobile': '0'}" in response.output

Testing the update command

def test_update():
    response = runner.invoke(update, ["test-user", "-m", "12345"])
    assert response.exit_code == 0
    assert "Contact updated!" in response.output
    assert "{'mobile': '12345'}" in response.output

Testing the delete command

def test_delete():
    response = runner.invoke(delete, "test-user")
    assert response.exit_code == 0
    assert "Contact deleted!" in response.output

    # call view on test-user to confirm it doesn't exist
    response = runner.invoke(view, "test-user")
    assert response.exit_code == 0
    assert "The contact you searched for doesn't exist" in response.output

Improvements

As your application grows, you may want to consider using fixtures and set up things like runner in a conftest.py file. We got away with it here because our tests were simple and all in a single file. Once multiple test files are introduced, following the approach we used here would lead to a lot of unnecessarily duplicated code.

Also, we made direct calls to our API in the tests. This operation should ideally be mocked.