Last Thursday I learned about pytest-mock at a local python meetup. The
presenter showed how he uses pytest-mock for his work, and it was kinda eye
opening. I knew what mocking was, but I had not seen it in this context.
Discovery
Watching him use pytest-mock I realized that mocking was not as hard as I had
made it out to be. You can install pytest-mock, use the mocker fixture, and
patch objects methods with what you want them to be.
install
pytest-mock is out on pypi and can be installed with pip.
python -m pip install pytest-mock
What I actually did
Sometimes I fall victim to making these posts nice and easy to follow. It takes more steps than just pip install, you need a place to practice in a nice sandbox. Here is how I make my sandboxes.
mkdir ~/git/learn-pytest-mock
cd ~/git/learn-pytest-mock
# well actually open a new tmux session there
echo pytest-mock > requirements.txt
# I copied in my .envrc, and ran direnv allow, which actually just made me a virtual env as follows
python3 -m venv .venv --prompt $(basename $PWD)
source .venv/bin/activate
# now install pytest-mock
pip install -r requirements.txt
# make some tests to mock
mkdir tests
nvim tests/test_me.py
create a tests/test_me.py
I just wanted to do something that was worth mocking, the first thing that came
to mind was to do something that made a network call. Here I made a method
that uses requests to go get the content on my homepage, but changes it's
return behavior based on the status_code of the request.
I want to mock out requests to ensure that GoGetter can handle both 200
(http success) and 404 (http not found) status codes.
# tests/test_me.py
import requests
class GoGetter:
"""
The thing I am testing, this is usually imported into the test file, but
defined here for simplicity.
"""
def get(self):
"""
Get the content of `https://waylonwalker.com` and return it as a string
if successfull, or False if it's not found.
"""
r = requests.get("https://waylonwalker.com")
if r.status_code == 200:
return r.content
if r.status_code == 404:
return False
class DummyRequester:
def __init__(self, content, status_code):
"""
mock out content and status_code
"""
self.content = content
self.status_code = status_code
def __call__(self, url):
"""
The way I set this up GoGetter is going to call an instance of this
class, so the easiest way to make it work was to implement __call__.
"""
self.url = url
return self
def test_success_get(mocker):
"""
Show that the GoGetter can handle successful calls.
"""
go_getter = GoGetter()
# Use the mocker fixture to change how requests.get works while inside of test_success_get
mocker.patch.object(requests, "get", DummyRequester("waylonwalker", 200))
assert "waylon" in go_getter.get()
def test_failed_get(mocker):
"""
Show that the GoGetter can handle failed calls.
"""
go_getter = GoGetter()
# Use the mocker fixture to change how requests.get works while inside of test_failed_get
mocker.patch.object(requests, "get", DummyRequester("waylonwalker", 404))
assert go_getter.get() is False