Neil Chazin - Strategies for testing Async code

Testing code is important. Testing, primarily unit-testing async code requires heading off the the standard roadway of unit testing in python. This talk will provide a map to help you along the new path towards testing async code.

Topics include:

a brief intro to asyncio and challengs in testing with it
running coroutines (and other awaitables) under test
mocking coroutines
testing "main" asyncio loops

https://us.pycon.org/2019/schedule/presentation/215/

Transcript

1. Strategies For Testing Async Code
   Neil Chazin
   

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2. Who am I?
   — Programming in Python since the early 2000s
   — Believe that testing software is important
   — @neilathotep
   neil chazin - @neilathotep
   

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3. Work
   — Currently Senior Software
   Engineer/Tech Lead at Agari
   — Agari secures digital
   communications to ensure
   humanity prevails over evil
   neil chazin - @neilathotep
   

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4. Brief Outline
   1. Why?
   2. Quick intro to async & asyncio
   3. Challenges of testing
   — Some solutions I've found
   4. Wrap up
   neil chazin - @neilathotep
   

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5. Why?
   neil chazin - @neilathotep
   

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6. — Testing is important
   — Performance is often important
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8. Async in Python
   A very brief intro
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9. General Async Basics
   — Concurrency through cooperation
   — yield control when 'waiting' - asynchronous results
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10. asyncio
    — framework for asynchronous computing
    — available in stdlib as of 3.4
    — Improved syntax in 3.5
    — Incremental changes in 3.6 and 3.7
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11. Two primary concepts
    — Coroutines: perform asynchronous work
    — Event loops: schedule asynchronous work
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12. Key syntax
    — async - define a coroutine
    — await - 'call' a coroutine
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13. Example use case
    We need something simple...
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19. Challenge
    Testing a coroutine
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20. Call as a function
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21. Call as a function
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22. Tests pass but...
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23. Tests pass but...
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24. Await the coroutine
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25. SyntaxError
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26. Run in an event loop
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27. Run in an event loop
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28. Success
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29. Use pytest-asyncio
    — pip install pytest-asyncio
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34. Python 3.7
    New function asyncio.run() simplifies things
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36. Challenge
    Mocks and patching
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37. Mocks o!en relied upon
    — Unittest.mock.Mock won't mock a coroutine
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43. Challenge
    Context managers
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45. Challenge
    More complex use cases
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46. — Previous patterns all work great but some short
    comings I've found:
    — test coroutines that do not run in the MainThread
    when executing
    — test syncronous functions that call into event
    loops ()
    — functional tests
    — Event Loop variants (e.g. uvloop)
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51. Calling across threads
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58. Wrap up
    — Try out asyncio programming
    — Keep testing
    — Find sample code and presentation at:
    h!ps://bit.ly/nchazin-pycon2019
    neil chazin - @neilathotep
    

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