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All Things Pythonic
Python 3000 and You
by Guido van van Rossum
March 18, 2008
I've posted the slides from my PyCon 2008 keynote on Here are the URLs, plus a warning against the temptation of changing your APIs at the same time as porting to Py3k. This is really important!


The slides of my keynote are now up on There's both a PowerPoint and a PDF file.

I'd like to take this opportunity to remind you of a really important issue that I neglected to mention in the talk: Don't change your APIs incompatibly when porting to Py3k.

Yes, you heard that right: even though Python 3.0 is changing incompatibly, I implore you (especially if you're maintaining a library that's used by others) not to make incompatible changes to your API. If you have make API changes, do them before you port to 3.0 -- release a version with the new API for Python 2.5, or 2.6 if you must. (Or do it later, after you've released a port to 3.0 without adding new features.)

Why? Think of your users. Suppose Ima Lumberjack has implemented a web 2.0 app for managing his sawmill. Ima is a happy user of your most excellent web 2.0 framework. Now Ima wants to upgrade his app to Py3k. He waits until you have ported your framework to Py3k. He does everything by the books, runs his source code through the 2to3 tool, and starts testing. Imagine his despair when the tests fail: how is he going to tell whether the breakage is due to your API changes or due to his own code not being Py3k-ready?

On the other hand, if port your web 2.0 framework to Py3k without making API changes, Ima's task is much more focused: the bugs he is left with after running 2to3 are definitely in his own code, which (presumably :-) he knows how to debug and fix.

The same recommendation applies even more strongly if your library is a dependency for other libraries -- due to the fan-out the pain caused to others multiplies. If one of those packages gives up (even temporarily) its Py3k porting effort, this may prevent many other libraries and apps from being ported at all!

So, once more for emphasis: Don't change your APIs at the same time as porting to Py3k!

Update: Martijn Faassen borrowed the time machine and wrote the same blog entry two weeks ago.

PS. The 3.0final release is now scheduled for September 3, 2008. See PEP 361.

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About the Blogger

Guido van Rossum is the creator of Python, one of the major programming languages on and off the web. The Python community refers to him as the BDFL (Benevolent Dictator For Life), a title straight from a Monty Python skit. He moved from the Netherlands to the USA in 1995, where he met his wife. Until July 2003 they lived in the northern Virginia suburbs of Washington, DC with their son Orlijn, who was born in 2001. They then moved to Silicon Valley where Guido now works for Google (spending 50% of his time on Python!).

This weblog entry is Copyright © 2008 Guido van van Rossum. All rights reserved.

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