Needed: Gentle transition path [LWN.net]

Needed: Gentle transition path

Posted Oct 21, 2009 22:33 UTC (Wed) by dwheeler (guest, #1216)
In reply to: Proposal: Moratorium on Python language changes by sergey
Parent article: Proposal: Moratorium on Python language changes

I agree, a freeze is neither necessary, nor solves the real problem. The real problem is that there there's no gentle transition path for applications that use libraries. If I write a Python app that uses 20 libraries, which transitively include 100 more, then all 120 libraries have to switch to Python 3 before *I* can use Python 3. Nuts. That's a "flag day" on an unworkable scale.

What we need is a Python implementation that can take BOTH the old AND new syntax/semantics, and allow each library to say if they're using the old or new one. Then, each library can switch on different timelines.

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Needed: Gentle transition path

Posted Oct 22, 2009 11:04 UTC (Thu) by epa (subscriber, #39769) [Link]

Isn't the idea that you port your code to Python 2.6, which will give you warnings when you use Python-3-unsafe constructs? Once it runs cleanly under 2.6 you should be able to switch over to 3 when you want. (I'm not a Python hacker so please correct me if I got the wrong impression.)

Needed: Gentle transition path

Posted Oct 22, 2009 14:20 UTC (Thu) by foom (subscriber, #14868) [Link]

That's pretty much the idea. But there's a couple problems:

Once you're on 2.6, without warnings, you need to run your code through the "2to3" converter.. It's a great idea, but it's not *perfect*. There's still a lot of manual work. Especially because mixed in with all the syntax and other removals/deprecations, the fundamental string model changed.

As the Python 3 "What's New" document says: "Everything you thought you knew about binary data and Unicode has changed." So, basically every usage of python2 "str" (8bit) strings needs to be checked by a human to see if it was intended to be a raw bytestring, or a textual string.

Many projects want to stay compatible with some version of Python < 2.6 for a year or so more, since 2.6 just came out recently, and most users don't have it installed yet. E.g. RHEL 5 comes with python 2.4, and the last Debian release comes with 2.5.

Python 2.6 doesn't do it (yet)

Posted Oct 22, 2009 16:50 UTC (Thu) by dwheeler (guest, #1216) [Link]

Yes, in theory, you can use Python 2.6 and "import from __future" to get a Python 3.0-like language, and use a Python subset so that 2to3 can catch the rest. But it isn't easy. And that's the advantage of Python: Usually, stuff is easy.