Introduction to TDD and First Tests

Length: 00:14:22

Lesson Summary:

With our project structured, we’re finally ready to start implementing the logic to create database backups. We’re going to tackle this project using “Test Driven Development”, so let’s learn the basics of TDD now.

Documentation For This Video

Installing pytest

For this course, we’re using pytest as our testing framework. It’s a simple tool, and although there is a unit testing framework built into Python, I think that pytest is a little easier to understand. Before we can use it though, we need to install it. We’ll use pipenv and specify that this is a “dev” dependency:

(pgbackup-E7nj_BsO) $ pipenv install --dev pytest
...
Adding pytest to Pipfile's [dev-packages]…
Locking [dev-packages] dependencies…
Locking [packages] dependencies…
Updated Pipfile.lock (5c8539)!

Now the line that we wrote in our Makefile that utilized the pytest, CLI will work.

Writing Our First Tests

The first step of TDD is writing a failing test. In our case, we’re going to go ahead and write a few failing tests. Using pytest, our tests will be functions with names that start with test_. As long as we name the functions properly, the test runner should find and run them.

We’re going to write three tests to start:

  1. A test that shows that the CLI fails if no driver is specified.
  2. A test that shows that the CLI fails if there is no destination value given.
  3. A test that shows, given a driver and a destination, that the CLI’s returned Namespace has the proper values set.

At this point, we don’t even have any source code files, but that doesn’t mean that we can’t write code that demonstrates how we would like our modules to work. The module that we want is called cli, and it should have a create_parser function that returns an ArgumentParser configured for our desired use.

Let’s write some tests that exercise cli.create_parser and ensure that our ArgumentParser works as expected. The name of our test file is important; make sure that the file starts with test_. This file will be called test_cli.py.

~/code/pgbackup/tests/test_cli.py

import pytest

from pgbackup import cli

url = "postgres://bob:password@example.com:5432/db_one"

def test_parser_without_driver():
    """
    Without a specified driver the parser will exit
    """
    with pytest.raises(SystemExit):
        parser = cli.create_parser()
        parser.parse_args([url])

def test_parser_with_driver():
    """
    The parser will exit if it receives a driver
    without a destination
    """
    parser = cli.create_parser()
    with pytest.raises(SystemExit):
        parser.parse_args([url, "--driver", "local"])

def test_parser_with_driver_and_destination():
    """
    The parser will not exit if it receives a driver
    with a destination
    """
    parser = cli.create_parser()

    args = parser.parse_args([url, "--driver", "local", "/some/path"])

    assert args.driver == "local"
    assert args.destination == "/some/path"

Running Tests

Now that we’ve written a few tests, it’s time to run them. We’ve created our Makefile already, so let’s make sure our virtualenv is active and run them:

$ pipenv shell
(pgbackup-E7nj_BsO) $ make
PYTHONPATH=./src pytest
======================================= test session starts =======================================
platform linux -- Python 3.6.4, pytest-3.3.2, py-1.5.2, pluggy-0.6.0
rootdir: /home/user/code/pgbackup, inifile:
collected 0 items / 1 errors

============================================= ERRORS ==============================================
_______________________________ ERROR collecting tests/test_cli.py ________________________________
ImportError while importing test module '/home/user/code/pgbackup/tests/test_cli.py'.
Hint: make sure your test modules/packages have valid Python names.
Traceback:
tests/test_cli.py:3: in 
    from pgbackup import cli
    E   ImportError: cannot import name 'cli'
    !!!!!!!!!!!!!!!!!!!!!!!!!!!!! Interrupted: 1 errors during collection !!!!!!!!!!!!!!!!!!!!!!!!!!!!!
    ===================================== 1 error in 0.11 seconds =====================================
    make: *** [test] Error 2

We get an ImportError from our test file because there is no module in pgbackup named cli. This is awesome because it tells us what our next step is. We need to create that file.


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