This document introduces Pretenders, a tool for testing applications that rely on external network services. Pretenders allows setting up mock HTTP and SMTP servers to test how applications handle different response types from external APIs and services. It works by pre-programming responses, replaying them during tests, and verifying requests. Tests can point applications to Pretenders' mock servers instead of real services for integration testing without patching code. Pretenders runs mock servers separately from the main server for configuration and results collection. Sample client test code is shown to define presets, run an application using the mock, and verify requests.

1. Pretenders
(fake servers for testing)
Don't GET (nor POST) me wrong
Carles Barrobés - @technomilk

2. Motivation
We want to test applications that rely on
external network services.
Are we sending the right stuff?
Do we deal properly with different types of
responses?

3. Main Use Cases
HTTP:
Applications that call external HTTP APIs
AJAX client code
SMTP:
Applications that send e-mail

4. Design Decisions
Preset/Replay/Verify mocking pattern
Mock at the "wire protocol" level
Simplicity of client test code
Configuration and checks via a ReST API
• Usable in polyglot environments

5. Preset/Replay/Verify
We pre-program our pretend server with pre-
canned responses (presets)
We let the test run, and the mock server
replays the presets
Finally, we verify what requests/data were sent
to the server

6. Mock at wire protocol level
Instead of "patch-style" mocking
• Usable for testing non-Python applications
• Usable in integration tests (where you
cannot patch running code)
Just make your application point to the mock
server:port instead of the real service

7. How it all works together
Start the main boss server (i.e. manually)
Each time you create a Mock, it:
• Tells the boss to instantiate a new mock server
(pretender)
• Returns the port of the new server
• Your mock acts as a proxy to the remote server for
configuration and collection of results
The boss takes care of cleaning up stale pretenders

8. Sample client test code
from pretenders.http.client import HttpMock
mock = HttpMock('localhost', 8000) #boss host and port
# Define presets
mock.when('GET /hello').reply('Hello')
mock.when('(POST|PUT) /somewhere').reply(status=400)
mock.reply('{"temperature": 23}', headers={'Content-Type':
'application/json'})
# Set your app to the fake URL, and exercise it
set_service_url(mock.pretend_access_point)
… run stuff
# Verify requests your code made
r = mock.get_request(0)
assert_equal(r.method, 'GET')
assert_equal(r.url, '/weather?city=barcelona')

9. THANK YOU!
Pretenders was written by:
Alex Couper and Carles Barrobés
$ pip install pretenders
Docs: http://pretenders.readthedocs.org
Code: https://github.com/txels/pretenders