Test Driven Development on Google App Engine

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

Adewale Oshineye Developer Advocate @ Google

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

TDD Is Hard

Especially On App Engine

What makes TDD on App Engine hard?

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

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

What should we do?

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

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

What did I do?

Exploration New to App Engine New App Engine services

Exploration Mock-up the UI Write handlers Get it working

Exploration Realisation Regret Refactor

Exploration Add tests Extract testable classes Add more tests

What should you do?

2 books

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

Exploitation Write functional tests at the WSGI level

Exploitation Sketch out handlers

Exploitation Write unit tests Write domain classes

Exploitation Passing unit tests Passing functional tests

Exploitation Red-Green-Refactor

Exploitation UI does the right thing Repeat for next feature

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

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

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

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

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/

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Test Driven Development on Google App Engine

by Ade Oshineye

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