threads, mutual exclusion, and lists

On Aug 15, 1:36 pm, Scott <smba...@gmail.comwrote:

I have two threads that share a python list. One thread adds to the
list with append(), the other thread removes items with pop().

My question is -- are python list operations atomic? If they are not,
then I assume I need to put some mutual exclusion around the append()
and pop() calls ?

Thanks,
Scott

You might want to consider using a Queue instead. It is designed to be
thread-safe.

I have two threads that share a python list. One thread adds to the

list with append(), the other thread removes items with pop().

My question is -- are python list operations atomic?

Yes, they are in the current implementation of CPython (all versions).
Notice that not *all* operations are atomic (e.g. .sort() is not),
but both append and pop happen to be atomic (which is primarily because
they don't need to call back to user-defined functions, unlike sort).

It's not a language property, though; things may be different in Jython
or IronPython.

Regards,
Martin

My question is -- are python list operations atomic? If they are not,

then I assume I need to put some mutual exclusion around the append()
and pop() calls ?

They are not, but there is one included in the standard library:
http://docs.python.org/dev/lib/module-Queue.html

Matt

>My question is -- are python list operations atomic? If they are not,

>then I assume I need to put some mutual exclusion around the append()
and pop() calls ?

They are not, but there is one included in the standard library:
http://docs.python.org/dev/lib/module-Queue.html

Why do you think they are not?

Regards,
Martin

Why do you think they are not?

Because they aren't. You even mentioned that a few operations that
aren't atomic. If operations are atomic it isn't necessarily because
of the design of the list, but the design of CPython. More
specifically the GIL. I don't mean to imply that you can't get a
multi-threaded app to communicate using lists, but the Queue is
explicitly built for it and better suited.

Matt

>Why do you think they are not?

>
Because they aren't. You even mentioned that a few operations that
aren't atomic.

OTOH, the OP specifically asked for .append() and .pop(), which are
atomic.

Regards,
Martin