Summary: Refactoring Python: Why and how to restructure your code

Refactoring is basic activity that all developers do. This post summarizes interesting talk, learn how to do them more effectively

Refactoring is often not budgeted. But as Brett Slatkin mentions in his PyCon 2016 talk it is one of the most important thing experienced developers get into habbit of doing. Following is summary of the talk

• What is refactoring?

  • Repeatedly reorganizing and rewriting code until its obvious* to new reader.
  • When you’re refactoring is optimizing for readability

• When do you refactor?

  • In advance
  • For testing
  • “Don’t repeat yourself”
  • Brittleness

  • Complexity

• What’s the difference between good and great programmer?

  • Spend almost equal time in Writing and Testing code, Refactoring and Styling & Documentaton
  • Half-time in refactoring

• How do you refactor?

  • Identify bad code
  • Improve it
  • Run tests
  • Fix and improve tests
  • Repeat

• How do you refactor in practice?

  • Rename, split and move
  • Simplify
  • Redraw boundaries

• Book Recommendations

  • Refactoring by Martin Fowler
  • Refactoring Ruby Edition by Jay Fields
  • Effective Python

• Strategies

  • Prerequisites
  • Thorough Tests
  • Quick test (Should be less than 5 mins)
  • Source control
  • Willing to make mistakes
  • Extract Variables & Extract Functions
  • Simplify expressions in if/else blocks to more readable names (i.e. Refactor them into variables)
  • One step furthure: Extract them into functions
  • Extracting functions is good, but over time it will become complicated and hard to understand i.e. what it is doing and how to test it
  • If that is a case, Extract a Class from it and use magic method def __bool__(self). Now replace functional version with class version (Aka Non-zero)
  • Reason to do this is for testing. Makes it easy to test thing out
  • Things to remember:
  • Extract variables and functions to improve readability
  • Extract variables into classes to improve testablity
  • Use __boo__ to indicate a class is a paper trail
  • Extract Class & Move Fields
  • If you have one class with complex logic and does everything in one single class that is a sign it needs refactoring

  • How do you redraw boundaries?

    • Add an improved interface
    • Maintain backwards compatability
    • Issue warning for old usage
    • import warnings
    • warnings.warn("something")
    • python -W error your_code.py will convert warnings to errors
  • Refactor Pet class to Animal

  • Raise TypeError for mixed usages (old and new one)

    • Migrate old usage to new usage
    • Run tests to verify the new correctness
    • Fix and improve broken tests
    • Remove code from interface

    • Things to Remember:

      • Split classes using optional arguments to __init__
      • Use @property to move methods and fields between classes
      • Issue warnings in old code paths to find their occurance
    • Need defensive property tombstones