pytest-mock Basics
==================

Last Thursday I learned about at a local python meetup. The presenter showed how he uses for his work, and it was kinda eye opening. I knew what mocking was,...

Date: March 14, 2022

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.

``` python
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.

``` python
# 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
```