Sfpython Unit Testing With Mock 20170614

Unit Testing with Mock

This talk is about unit testing.

Speaker: Brian Weber, SRE at Twitter

Lets execute tests in your code safely

Tests execute your code.

For example, a command line function that deletes hard drives...

How do we do it? `mock`

What is a `mock`?

unitteset.mock is a library for Testing in Python. It allows you to replace parts of your system under test with mock objects... (is this text from the unittest documentation?)

Why use `mock`?

Stop the state-changing parts so you can actually run tests.
Write better code
_____ (missed it!)

Patch decorator

Mocks are plastic

Everything the mock does returns another mock

spec - flexible but not safe
spec_set - safer, somewhat immutable
autospec - match function signatures of class instances

Mocks have a bunch of introspection features

called, call_count, call_args, call_args list, method_calls, ... (more)
assert_called, assert_called_once, ... (more)

Example: API Requests

Two options

Patch locally defined function

Replace portion of function being tested - side-effect.

Patch external library

Which would you pick? USE BOTH! It's really easy to write, don't worry about it and write both.

Another thing: Mock "Factory"

Kind of a factory but not really.

DO NOT mock

The filesystem - too many surfaces and edges. Lots of debate but don't do it
use the temp dir module or whatever it is called. Very simple
Acceptance and Integration tests

Additional info

Write mocks as PyTest fixtures. PyTest and mock go hand-in-hand.
How do you mock a for loop that hits an API a certain number of times?
- Mock the parent library and setup some sort of a "factory" module
- One instantiation setup that mocks all the behavior
- For loop means some expected behavior inside for loop and some expected behavior outside for loop.
Usually you will patch directly in the namespace you are testing.
- Direct accuracy over what you are replacing.