This Old Pony: Working with Legacy Django Projects

DjangoCon 2016: "Legacy" is a word that produces fear in many software developers. However it means nothing more than software that already exists, and you're probably working with it already, admit it or not. We'll take a tour through what legacy Django projects look like, how to approach them, and how to work on them while keeping your wits.

Transcript

1. This Old Pony
   DjangoCon US 2016- Ben Lopatin
   Working with legacy Django projects
   

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2. Partner and developer
   // Wellfire Interactive
   @bennylope
   ben@wellfire.co
   

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3. Legacy software?
   

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7. What is “legacy”?
   

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8. Code someone else
   wrote
   

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10. Code that has no tests
    

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12. Software that’s in
    production
    

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14. Written software
    is an investment
    

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15. Ugly, old systems
    deliver value
    

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16. The world runs on
    legacy software
    

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17. What is legacy
    Django?
    

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19. 0 tests run in 0 seconds
    

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20. Not radically unique
    

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21. Some assumptions
    

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22. Goal: improve code in
    discrete steps
    

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24. This is the right way.
    

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25. This is the right way.
    This is some good ideas.
    

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27. Code Review
    

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28. Why?
    

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29. Step 0:
    Ask questions
    

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30. Talk to people
    

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31. What’s it supposed to do?
    What does it do?
    Known bugs?
    Planned features?
    

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32. Reading the code
    

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33. Confusing areas
    Architecture
    Code smell
    Style
    &c.
    

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34. Tools
    

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35. flake8
    PyCharm
    pylint
    

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36. Logging
    

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37. Understand
    production errors
    

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38. try:
    do_some_stuff()
    make_some_calls()
    except:
    pass # TODO: handle this! (Chris)
    

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39. try:
    do_some_stuff()
    make_some_calls()
    except:
    logger.exception("Chris's error")
    

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40. Testing
    

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41. Why?
    

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43. Fix bugs
    Guide development
    Deployment confidence
    

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44. Tests provide
    information about
    code quality.
    

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45. Test suite scenarios
    

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46. Great tests!
    No tests!
    Bad tests
    Slow tests!
    

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47. Great tests!
    

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48. No tests!
    

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49. Bad tests!
    

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50. Slow tests!
    

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51. Prioritizing new tests
    

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52. Bugs
    

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53. Smoke tests
    View-centric integration tests
    

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54. New features
    

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55. Refactoring
    

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56. Upgrading
    

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57. Backwards
    incompatibilities
    

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58. Superpower:
    tox
    

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59. tox
    tox tox
    tox
    tox
    tox
    tox
    tox
    tox
    tox
    tox tox
    tox
    

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60. [tox]
    envlist =
    flake8,
    deployed,
    py27-django{19,18,17}
    [testenv]
    setenv =
    PYTHONPATH = {toxinidir}
    commands = python manage.py test
    deps =
    django17: Django>=1.7,<1.8
    django18: Django>=1.8,<1.9
    django19: Django>=1.9,<1.10
    -r{toxinidir}/requirements/base.txt
    

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61. Goal: LTS
    

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62. Dependencies
    

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63. Tight integration
    with obsolete libraries
    

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64. Unnecessary version
    [over]-specification
    

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65. Version support
    mismatches
    

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67. {
    

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68. {
    Version
    Compatibility
    

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70. Upgrade!
    

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72. Solutions
    

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73. Patch and pray
    (upstream)
    

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74. Fork
    

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75. Extract what you used
    

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76. Remove/replace
    

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77. pur
    pip-tools
    

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78. $ pur -r requirements.txt
    Updated flask: 0.9 -> 0.10.1
    Updated sqlalchemy: 0.9.10 -> 1.0.12
    Updated alembic: 0.8.4 -> 0.8.6
    All requirements up-to-date.
    

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79. Issues
    

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80. Formatting
    

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82. autopep8
    PyCharm
    

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83. Settings
    

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84. Secrets!
    

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85. bandit
    

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86. One big app
    

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88. Solution:
    Break up apps
    

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89. Big views
    (lots o’ logic)
    

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90. “Fat models”
    

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91. Humongous managers
    

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92. Expansive forms
    

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93. Not just views
    

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94. Seek to minimize scope
    of changes, continually.
    

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95. Refactor as you go
    

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96. New feature?
    New app!
    

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97. Feature uses old code?
    Refactor old code.
    

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98. What didn’t we talk
    about?
    

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99. Continuous integration
    Automated deployment
    Configuration management
    Documentation
    Security
    

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100. The End
     

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101. I am ready for your questions
     

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