OOP was not a silver bullet. How to write reusable code? Through hard work its not free or its magic. Required forethought and attention to detail. Caveat: Not everything needs to be library

Code Review arGreg Ward gave a great PyCon 2015 talk on how to make reusable code. We liked it, basically lays out 9 point guidelines that helps programmers make informed decisions about their design. Most of it might seem obvious but putting into use is where all the effort lies. Its a great investment of about 25mins of your time.


  • OOP was not a silver bullet
  • How to write reusable code?

    • Through hard work its not free or its magic.
    • Required foretought and attention to detail
  • Caveat:

    • Not everything needs to be library
  • Principles of Reusable Code:

    • Be a better programmer
    • Fewer classes, more functions
    • Funtions != procedures (beware of side effects)
    • Fewer frameworks, more libraries
    • Don’t go overboard
    • Don’t solve everything
    • Test, test, test
    • Document, document, document
    • Extensibility != reusability

Be a better programmer

  • Read more code
  • Write more code
  • Read more books
  • Do not learn to program from Google
  • Book reccommendations:

    • The Practice of Programming
    • The Pragmatic Programmer
    • Beautiful Code
  • Review and Refactor your code mercilessly

Fewer classes, more functions

  • Don’t write classes when you need function
  • A collection of related functions with shared state: that’s a class make one!
  • Anti Pattern 1

    • a class with a single public method
    • especially a class with single method
  • Anti Pattern 2

    • Opposite of Anti Pattern 1
    • If set of functions use same arguments and use similar data structure need to be in a Class
class ThingLoader:
    def __init__(self, host, id):
        self.host = host
        self.id = id

    def load(self):

Functions != Procedures

  • Pascal’s best idea: functions compute stuff, procedures do stuff
  • mainstream imperative languages (C, Java, Python) don’t care
  • You should care about distinction
  • Rule of thumb: Every function should either return a value or have a side effect. Never both!
  • Anti Pattern 1

    • C programmer are notorious for this sort of thing
    • Python programmers also do it
    • Compute and Modify original data-structure is bad

Fewer frameworks, more libraries

  • Library: Code that you call (you are in control)
  • Framework: Code that calls you (frameworks are in control)
  • You can’t pick and choose which bits of framework you want
  • It’s typically all or nothing
  • This hampers reuse

Don’t go overboard

  • OOP is a useful trick have in the toolbox
  • Thus, calsses are not evil
  • A computer is just a big box of numbers (with some wires going and some wires going out)
  • Side effects are unavoidable: they are not evil
  • Certain problem domains call out for constraints
  • Frameworks are not evil

Don’t solve everything

  • Avoid the temptation to handle every case: you will miss something
  • Who can supply the functionality you left out? programmers, the very people who use your library
  • Who can remove that bit of ill-advised code you put in that’s ruining my dat? NOBODY

Test, test, test

  • Automated test are necessary but not sufficient
  • Untested code is useless
  • Tested code might be useful
  • No controversy here: You must write tests

Document, document, document

  • Documentation is necessary but not sufficient
  • Undocumented code is useless
  • Documented code might be useful
  • Inline reference docs (docstring, java, etc) is bare minimum
  • When you document every public method, you will find gaps
  • This will also help write tests

Extensibility != Reusability

  • Another part of the early 90s snake-oil promise: extensible, reusable software components
  • Just because you type “class Foo” does not make your code reusable or extensible
  • These are largely independent properties of software


  • There is no such thing as free lunch
  • Good stuff require efforts
  • That includes reusable and/or extensible code
  • Beware of pompous windbags, clueless pontifications and other purveyors of snake oil