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All Things Pythonic
Python 3000 FAQ
by Guido van van Rossum
July 28, 2007
Some questions that people often ask about Python 3000 (and answers). (First in a series of posts.)


Q. I want to learn Python. Should I learn Python 2.6 or Python 3.0?

A. Definitely learn Python 2.x (the latest version out is 2.5). I expect it'll be two years before you'll need to learn Python 3.0, and the differences aren't that great from a beginner's perspective: most of what you'll learn about 2.x will still hold about 3.0.

Q. If you're killing reduce(), why are you keeping map() and filter()?

A. I'm not killing reduce() because I hate functional programming; I'm killing it because almost all code using reduce() is less readable than the same thing written out using a for loop and an accumulator variable. On the other hand, map() and filter() are often useful and when used with a pre-existing function (e.g. a built-in) they are clearer than a list comprehension or generator expression. (Don't use these with a lambda though; then a list comprehension is clearer and faster.)

Q. Multi-core processors will be standard even on laptops in the near future. Is Python 3.0 going to get rid of the GIL (Global Interpreter Lock) in order to be able to benefit from this feature?

A. No. We're not changing the CPython implementation much. Getting rid of the GIL would be a massive rewrite of the interpreter because all the internal data structures (and the reference counting operations) would have to be made thread-safe. This was tried once before (in the late '90s by Greg Stein) and the resulting interpreter ran twice as slow. If you have multiple CPUs and you want to use them all, fork off as many processes as you have CPUs. (You write your web application to be easily scalable, don't you? So if you can run several copies on different boxes it should be trivial to run several copies on the same box as well.) If you really want "true" multi-threading for Python, use Jython or IronPython; the JVM and the CLR do support multi-CPU threads. Of course, be prepared for deadlocks, live-locks, race conditions, and all the other nuisances that come with multi-threaded code.

Q. I prefer to use the same source code for 2.x and 3.0; I really don't want to have to use the 2to3 source conversion tool. Why can't you make that work?

A. Suit yourself. The intersection of 2.6 and 3.0 is large, but there are several things you can't do: you can't use Unicode literals (2.6 only), nor bytes literals (3.0 only). The only print syntax that works the same in both versions is print(x). When you catch exceptions you can't inspect their values, because the 2.6 syntax uses , while 3.0 uses as. You can't use .iterkeys(), but .keys() works differently in 2.6 and 3.0. You can't use xrange(), but range() works differently. You can't use metaclasses, as the syntax for specifying a metaclass is completely changed in 3.0. And so on. Restricting yourself to the intersection of the two versions is very painful and limited. We're not introducing backwards compatibility syntax in 3.0, because that would defeat the purpose (we've had backwards compatibility forever in 2.x, and the whole point of 3.0 is to clean up the mess).

P.S. For another batch of Q/A pairs, see the sequel post.

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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 © 2007 Guido van van Rossum. All rights reserved.

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