[Python-Dev] Re: Thread safety of deques (original) (raw)

Phillip J. Eby pje at telecommunity.com
Tue Jul 20 00:53:16 CEST 2004

At 11:01 PM 7/19/04 +0100, Paul Moore wrote:

>> To contrast, list.append() does not claim thread safety. > > List operations like most C functions are also atomic.

Thanks for that clarification. A note somewhere in the python-level documentation, stating that operations implemented in C are typically atomic, and so thread-safe, might be useful.

Perhaps it would be useful to point out that "safe" here means, "doesn't corrupt the dictionary or crash Python". It doesn't mean "actually safe". :)

It would probably be "safest" not to describe anything as "thread safe", but rather point out that in general, Python won't crash because a data structure was accessed or altered by more than one thread, and that most operations on most built-in types are atomic.

However, anything that calls back into Python code isn't necessarily atomic, and in general, almost anything can call back into Python code.

For example, the seemingly harmless dictionary item assignment can call back into Python for a custom type's hash and eq methods, so could it be that a thread switch could actually occur during this statement:

 foo[bar] = baz

if 'bar' has such custom methods? Maybe I'm wrong, and the GIL is still held in such a case, but I'm not sure how.

More information about the Python-Dev mailing list