This document discusses the challenges of test-driven development (TDD) on Google App Engine. Some difficulties include models having a lifecycle between transient and persistent states, tests requiring the datastore, and keys being auto-generated. The author advocates for exploring features by writing mock user interfaces first before adding tests. They also recommend exploiting features using a "vertical slice" approach of mocking interfaces, writing functional tests at the WSGI level, and iterating with unit and functional tests using refactoring. Heuristics discussed include separating domain objects from models, using factories for models, and remembering that if something is hard to test it may be flawed.

1. Real-world TDD on App
Engine
Mistakes Made and Lessons Learned

2. Adewale Oshineye
Developer Advocate @ Google

3. Projects and Examples
JaikuEngine: http://code.google.com/p/jaikuengine/
FeedParser: http://code.google.com/p/feedparser/
Streamer: http://github.com/adewale/streamer

4. TDD Is Hard

5. Especially On App Engine

6. What makes TDD on App
Engine hard?

8. Models are Active Records
Models have a lifecycle (transient to persistent)
Tests need datastore
Keys are usually auto-generated

9. Platform and services
System services are opaque
Development environment isn’t an exact mirror of
production environment

10. What should we do?

11. What should we do?
http://en.wikipedia.org/wiki/File:John_Major,_October_2007.jpg

12. What should we do?
“We will do precisely what
the British nation has
done all through its history
when it had its back to the
wall — turn round and
fight for the things it
believes in, and that is
what I shall do.”
http://en.wikipedia.org/wiki/File:John_Major,_October_2007.jpg

13. What did I do?

14. Exploration
New to App Engine
New App Engine services

15. Exploration
Mock-up the UI
Write handlers
Get it working

16. Exploration
Realisation
Regret
Refactor

17. Exploration
Add tests
Extract testable classes
Add more tests

18. What should you do?

19. 2 books

20. Exploitation
Vertical slice for one feature
Mock-up the UI
Refine

22. Exploitation
Write functional tests at the WSGI level

23. Exploitation
Sketch out handlers

24. Exploitation
Write unit tests
Write domain classes

25. Exploitation
Passing unit tests
Passing functional tests

26. Exploitation
Red-Green-Refactor

27. Exploitation
UI does the right thing
Repeat for next feature

29. Ways and means
Find a good testing framework
nosegae and gae-testbed
learn how to use stubs for the various platform
services
learn how to cycle between functional and unit tests

30. Heuristics
Domain objects don’t have to be model objects
Use simple factories for model objects

31. Heuristics
If in doubt, write a test
Testability requires work
If it’s hard to test it’s probably broken

32. Heuristics
Some things can only be understood in production
End-to-end is further than you think
even if you take this into account

34. References
gae-testbed http://github.com/jgeewax/gaetestbed/
nose-gae http://code.google.com/p/nose-gae/
nose http://code.google.com/p/python-nose/
Webtest http://pythonpaste.org/webtest/