An Extreme Talk about the Zen of Python


live-noted by Audrey Roy (audreyr) and submitted as a pull request

Daniel Greenfeld

  • Family on both sides from Poland
  • Principal at Cartwheel Web, PSF & DSF member
  • Fiancee is @audreyr!
  • Co-lead on OpenComparison which powers and more

Zen of Python

  • By Tim Peters, author of Timsort which is used everywhere
  • Extreme examples of each follow


  • Built-in method
  • Walkthrough of circle-ellipse problem
  • Can create ambiguity
  • Hard to remember syntax for super()
  • Circle.__init__(self, outer) is more explicit and simpler
  • Explicit is better than implicit

Django CBVs

  • Quiz: What is the ancestor chain for django.views.generic.edit.UpdateView?

    • Answer: There are 8 things. Hard to remember what each ancestor does.
  • In super(ActionUpdateView, self).form_valid(form), which form_valid() is being called?

  • If not careful, super() can cause MRO problems

  • Possible mitigations:

    • Hope maintainers aren’t angry
    • Convert to functional view
    • Explore better patterns


  • Special cases aren’t special enough to break the rules, although practicality beats purity
  • WSGI is fixed
  • Config & installation - working on it
  • CBVs - working on it
  • Not MVC. Model-Template-View. Web not necessarily appropriate for MVC.


  • Where did the response object come from?

  • 3 koans broken:

    • Explicit is better than implicit
    • In the name of ambiguity, refuse the temptation to guess
    • Namespaces are good…
  • In their case, it’s a design decision. “Practicality beats purity”

    • Easier for beginners
    • Easy to learn their namespace pattern
    • For experienced devs, saves boilerplate
  • Web2py’s easy installation process is where they shine


  • Example showing a general exception
  • By printing (e), you don’t get the error type or stack trace
  • Fixed code with a custom exception that gets raised and prints additional info


  • Awesome to use

  • Easy to explain what they do

  • He did a walkthrough of a sample decorator without arguments. Then one that accepts an argument.

    • 3 nested functions
    • Hard to read
  • He corrected himself because he didn’t use @functools.wraps, which is the better way to define decorators. More complexity.

  • Hard to explain implementation

  • If the implementation is hard to explain, it’s a bad idea. If the implementation is easy to explain, it may be a good idea.

Getting it done vs. technical debt

  • Tests and docs take time. Skipping them risks:

    • Multiple coding standards
    • Deploying broken code
    • Problems upgrading dependencies
    • Forgetting install/deploy
  • Must document:

    • Installation/deployment
    • Coding standards
    • How to run tests
    • Config
  • Easy test patterns:

    • Always make sure test harness can run
    • Use tests instead of shell/repl
    • After 1st deadline, reject incoming code that drops coverage
    • Use


  • Powerful, useful, precise
  • Dangerous to use import *
>>> from re import *
>>> from os import *

>>> re.error == os.error

Breaking built-ins

Continued from above:

>>> compare_builtins(re)
  • Breaks compile() built-in
>>> compare_builtins(os)
  • Breaks open() built-in
  • Bad shortcut pattern to teach beginners. Technical debt.


  • Our community is built off of the Zen of Python
  • Thank you: Richard Jones, Raymond Hettiger, Matt Harrison, Kenneth Love, PyCon Poland, others