A comment on the previous entry about how builtins are atomic asked a really good question:

Is there actually a guarantee in CPython that they're atomic, or is that a side-effect of implementation?

There are two answers, at least in my opinion. The legalistic answer is that there is no guarantee at all, because there's nothing in the documentation. But this is because the CPython documentation doesn't talk about concurrency issues at all; even the thread module documentation doesn't really say anything. In theory you can go from 'the documentation doesn't promise anything' to 'the documentation allows an implementation to do anything if you don't explicitly lock'. However, this is robot logic, not human logic; the real truth is that threading is not really a core Python feature (even today it is in the 'optional operating system' modules section), and the documentation reflects that.

I maintain that the practical answer is that it is guaranteed, at least for CPython, despite the lack of explicit documentation to that effect. Whether legalistic purists like it or not, in practice if you do not document something and then expose a single implementation to people, that implementation's long standing behavior becomes implicit documentation. This is especially so if the behavior is quite useful and the alternatives painful.

Or in short: it's guaranteed because there's no documentation about it either way and that's how CPython has always behaved. And it's really handy.

And as a pragmatic matter, anything that wants to be compatible with existing CPython threaded code is going to have to mimic this behavior (given Python 3K, this is less reassuring than it seems). However, IronPython, Jython, and any other similar projects out there may be not all that interested in supporting existing CPython threaded code; I suspect that they feel that they have better native threading models that you should use instead.

(Although for what it is worth IronPython does not document any differences in what is and isn't atomic for basic types in their list of differences between IronPython and CPython.)

(This is one of those entries that is going to get me pilloried.)