So what's the deal with Python these days? Do people still write for 2.7 which is the default "python" in most (all?) Linux distributions, or is it safe to write for python3 now?
Yes.
> which is the default "python" in most (all?) Linux distributions
Most distributions ship both 2.x and 3.x at the same time. Ubuntu ported most (all?) of its tools to 3.x already.
> is it safe to write for python3 now?
What do you mean by safe? It's been stable for a long time now. As long as you verify all your dependencies work, you should be fine. The only annoying thing you may run into is a situation where only one of the versions is provided: for example if you want to drive ufw (3.x only) from salt (2.x only), you'll have to resort to some kind of IPC instead.
There was a poll about the current state a month or two ago on HN.
Google code shows download counts so you can get an idea of popularity of each release for Windows developers:
https://code.google.com/p/apsw/downloads/list?can=1&q=&colsp...
I've since moved the project to github which doesn't show the download counts for releases.
Before you start writing in Python 3, make sure your dependencies will run on Python 3.
Mercurial is an application you use to version control your source. I'm not sure I understand why any python developer would care if Mercurial was written in python 2 as long as it worked. Maybe if someone was writing a Mercurial plugin or patching Mercurial they would care.
Hopefully these start disappearing, as Ubuntu and Arch have already moved to 3.x.
Luckily, a Twisted Reactor can be built on top of asyncio, and this will likely happen as people port more parts of Twisted to Python 3 (it's a long-running project with fairly little interest from developers willing to put work into it).
Alternatively, people could start to build new libraries that implement all the abstractions and protocol implementations that Twisted has to offer. I think it'd be preferred that even if each abstraction/protocol were a separate repo and installable package, they'd all be under the same "banner", so that everything interoperates correctly and using the same set of abstractions.
And you did say that some people - people who write plugins or develop Mercurial for living - do care later in your sentence. That's some developers. And the parent was asking whether Python 3 was safe to use. It's nice to know that some major Python projects are not ready for Python 3.
The problem is rarely the major libraries, it's that one obscure minor library that almost no one (except you in your Very Import Application) uses.