An extreme talk about the Zen of Python


live-noted by Zbigniew Siciarz (zsiciarz) and submitted as a pull request

  • by Daniel Greenfeld


In the Python community we are taught from the outset of learning the language that the Zen of Python serves as a guide for how we should construct our codebases and projects. Rather than go into the zen-like meanings of each statement, this talk will explore how individual koans are implemented via detailed displays of sophisticated code examples.


No easy code examples, Daniel’s gonna show some twisted, complicated code.

Daniel’s grandparents come from Dynów, Poland. (Applause)

He was a Python programmer at NASA and later started consulting work.

Met @audreyr at PyCon 2010.

Runs some opencomparison sites - djangopackages etc.

The Zen of Python

Written by Tim Peters, author of timsort algorithm - a really smart guy according to Daniel.

The Opening

Beautiful is better than ugly. Explicit is better than implicit. Simple is better than complex. Complex is better than complicated. Flat is better than nested. Sparse is better than dense. Readability counts.

Demonstrated using super().

Geometrical figures, Ring derives from Circle. Obvious what super will do. But what if it wasn’t so simple?

super does things automatically and can lead to ambiguity.

In the face of ambiguity, refuse the temptation to guess.

Absolutely inheriting __init__ from base class: explicit, simpler, more readable.

Explicit > Implicit

Circle.__init__ > super()

Django CBVs

Composition, inheritance, polymorphism and other funny words.

What’s the ancestor chain to UpdateView? Answer: 8 ancestors. Impossible to memorize what each of them does.

form_valid(), but which one?

OMG! OMG! OMG! Even more mixins. Let’s print the MRO. A screenful of <class '...'> follows.

Filter the list on classes which have form_valid() method -> only 5 classes (I was lucky).

MRO is simple, but simple is relative.

Moving on

Django controversy

  • WSGI (fixed)
  • configuration and setup (working on it)
  • CBVs (working on it)
  • not MVC-compliant (and this is fine)


Is MVC applicable on the web?

The Zen of Python doesn’t mention MVC.

Separation of presentation from content

Django is fine. CBVs are not along the lines of Zen.

Controversy: web2py

Implicit > explicit.

Follows it’s own design patterns.

Where are the imports? No imports neccessary. WAT?


Nobody expected Spanish Inqusition!

web2py breaks 3 koans of the Zen. Implicit behaviors, ambiguity, namespaces.

Although practicality beats purity.

Easy to install, easy to learn, less boilerplate. Web2py <3 Windows.

Similar to Django in terms of contention and trackage.

Exceptions in exception handling

Django Packages

Updates metadata from PyPI, Github, Bitbucket. PyPI unstable, APIs change, projects get deleted etc.

First: concatenating some string with error messages from exception handlers.

Traceback wanted. Type of the error, message, location.

Code is silent - for no good reason apart from laziness.

Solution: added logging in __init__ in a custom Exception subclass.

Code is not silent anymore. Errors are noisy.

Cleaner code

Even more controversy. (Unless you’re Dutch).


Decorators are easy to explain!

Wrapper function running code before/after the decorated function.

Getting harder to explain… closures etc.

Now let’s talk about decorators with arguments. general laughter

Danny is evil, uses confusing names: multiplier, multiple


Don’t forget functools.wraps. The decorator code in the slides is growing like a tumor.

It’s not easy to (explain how to) write decorators.

But decorators are awesome! Using them is like Zen, writing is not.

The last section

Getting it done vs technical dept

Tests & docs take time. Do we have to do them? Maybe not. But it brings a lot of risks.

Must-have docs

  • installation/deploy
  • coding standards
  • how to run tests
  • version information

Test patterns

Test harness must at least run even without tests.

Use tests, not shell/repl.

Use coverage, reject code that drops coverage.

Don’t use doctests.


Powerful, useful, precise.

import * makes development faster. IMPORT ALL THE THINGS!

Confusing imports, same names in os and re. Subtle trouble!

Replaces things from in builtins ( breaks open)

The open() story needs different arguments than open. You’re screwed if you confuse these calls.


import * is for people who know what to do.

Remember __all__.


The Zen of Python (repeated)

One more thing

Capoeira moves!