How I test my code: pytest and fixtures (part 2)

June 21, 2016

This is part 2 in my series on "How I test code". Part 1 discusses testing habits and how to motivate yourself to write them. This post goes into more Python specific tools and conventions around testing.

Python has a number of test runners to extend and simplify writing (unit) tests. My personal preference is pytest which is super robust and feature rich. It let’s you write tests as simple "asserts", has a brilliant plugin ecosystem that "just works" after pip install pytest-[somePlugin], and let's you leverage powerful fixtures to keep things DRY.

A small flavor of what tests look like with pytest:

import pytest
from mypackage import best_movie, perform_division

def test_best_movie():
 movie = best_movie(director='P.T. Anderson')
 assert movie == 'There Will Be Blood'

def test_perform_division():
 with pytest.raises(ValueError):
  # call with parameters that should yield error
  perform_division(12, 0)

Running your tests is as easy as:

py.test --verbose

Organizing tests

pytest does a great job of detecting tests. All you need to do is name test modules with a prefix: test_*. Each test function should similarly be named def test_*:.

Furthermore, I like to organize test files to reflect my source code. The following package:

|-- tools

... would result in the following test structure:

|-- tools

You notice that I'm "repeating" the term "tools" for the "docker"-test module. This is because pytest requires globally unique test module names!

Test fixtures

I think this is the key concept to start mastering tests. Fixtures are pluggable components that can be shared across many tests to setup pre-conditions like:

  • setup a database connection
  • read in lines form a file

They each have their own setup and tear down blocks and you control if they are reset on a function/module/session basis.

Let's add a few items to our setup:

|-- fixtures                    # store static files here
|-- tools
|--                 # write fixture functions here

Inside you can add fixture functions that will be exposed to your tests. You mark a function as a fixture with a decorator. If you don't need setup/tear down you can use a simple @pytest.fixture. Otherwise it's easiest to use @pytest.yield_fixture.

import pytest
from myPackage import DatabaseAPI

def db_connection():
 _db_connection = DatabaseAPI(uri=':memory:')
 yield _db_connection
def test_add_row(db_connection):
 name = 'Paul T. Anderson'
 add_row(name=name, age=34)
 assert db_connection.get_row(name=name).age == 34

When pytest runs the above function it will look for a fixture called db_connection and run it. Whatever is yielded (or returned) will be passed along to the test function. We set the "scope" of the fixture to "function" so as soon as the test is complete, the block after the yield statement will run. You can pass as many fixtures as you want to a test.

Tip: test fixtures accept parameter-dependencies the same way as test functions. It's perfectly possible to combine several test fixtures.

Additional fixtures can be installed through plugins and pytest itself comes with a few built in. For example there's the handy tmpdir fixture that provides unique temporary folders where you can test various side effects.

from mypackage import touch

def test_write_file(tmpdir):
 # GIVEN an empty dir
 assert len(tmpdir.listdir()) == 0
 # WHEN touching a new file
 new_path = tmpdir.join('newfile.txt')
 # THEN there should be a new file created
 assert len(tmpdir.listdir()) == 1


I've only touched on some of the features that make pytest so powerful. I would highly recommend reading up on the framework and picking out other features that might benefit you.

Part 3 is coming up and will show how you can automate your test workflow; both locally and remotely. I will also cover how to measure test coverage.