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
2. Partner and developer // Wellfire Interactive @bennylope ben@wellfire.co
3. Legacy software?
7. What is “legacy”?
8. Code someone else wrote
10. Code that has no tests
12. Software that’s in production
14. Written software is an investment
15. Ugly, old systems deliver value
16. The world runs on legacy software
17. What is legacy Django?
19. 0 tests run in 0 seconds
20. Not radically unique
21. Some assumptions
22. Goal: improve code in discrete steps
24. This is the right way.
25. This is the right way. This is some good ideas.
27. Code Review
28. Why?
29. Step 0: Ask questions
30. Talk to people
31. What’s it supposed to do? What does it do? Known bugs? Planned features?
32. Reading the code
33. Confusing areas Architecture Code smell Style &c.
34. Tools
35. flake8 PyCharm pylint
36. Logging
37. Understand production errors
38. try: do_some_stuff() make_some_calls() except: pass # TODO: handle this! (Chris)
39. try: do_some_stuff() make_some_calls() except: logger.exception("Chris's error")
40. Testing
41. Why?
43. Fix bugs Guide development Deployment confidence
44. Tests provide information about code quality.
45. Test suite scenarios
46. Great tests! No tests! Bad tests Slow tests!
47. Great tests!
48. No tests!
49. Bad tests!
50. Slow tests!
51. Prioritizing new tests
52. Bugs
53. Smoke tests View-centric integration tests
54. New features
55. Refactoring
56. Upgrading
57. Backwards incompatibilities
58. Superpower: tox
59. tox tox tox tox tox tox tox tox tox tox tox tox tox
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
61. Goal: LTS
62. Dependencies
63. Tight integration with obsolete libraries
64. Unnecessary version [over]-specification
65. Version support mismatches
67. {
68. { Version Compatibility
70. Upgrade!
72. Solutions
73. Patch and pray (upstream)
74. Fork
75. Extract what you used
76. Remove/replace
77. pur pip-tools
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.
79. Issues
80. Formatting
82. autopep8 PyCharm
83. Settings
84. Secrets!
85. bandit
86. One big app
88. Solution: Break up apps
89. Big views (lots o’ logic)
90. “Fat models”
91. Humongous managers
92. Expansive forms
93. Not just views
94. Seek to minimize scope of changes, continually.
95. Refactor as you go
96. New feature? New app!
97. Feature uses old code? Refactor old code.
98. What didn’t we talk about?
99. Continuous integration Automated deployment Configuration management Documentation Security
100. The End
101. I am ready for your questions