basic python threading question

Aahz Maruch aahz at
Fri Sep 29 20:46:50 CEST 2000

In article <Cc4B5.55776$dZ2.18164522 at>,
Tom <nospam at> wrote:
>"Aahz Maruch" <aahz at> wrote in message
>news:8r2hdl$6rr$1 at
>> In article <q_IA5.42927$dZ2.15588642 at>,
>> Tom <nospam at> wrote:
>>>This would be a problem for threading, unless all atomic objects are
>>>thead-safe.  Now, all atomic objects appear to me to be immutable.  Are
>>>immutable objects inherently thread-safe in Python (even if the ref count
>> Yes, immutable objects are thread-safe.  Remember that assignment in
>> Python is a binding operation, not a modification operation.
>But, even an immutable object has its reference count modified by a binding
>operation.  And this sounds like a problem to me, since (quoting from
>section 8.1 of  API Ref):
>"when two threads simultaneously increment the reference count of the same
>object, the reference count could end up being incremented only once instead
>of twice. "

This is a different problem that occurs only when writing C extensions
that interact with Python objects *and* the extension releases the
Python global interpreter lock.  Let us know if this is your situation,
and we can talk about ways to resolve it (the short answer is to acquire
the GIL before updating Python objects).

>I'm finding threading on Python to be more complicated than threading in C.

I'm betting that if you use a Pythonic threading system (i.e. by
subclassing threading.Thread), you'll find threads in Python to be
extremely simple.

>It also seems that threading in any interpreted environment must have a
>fairly heavy overhead, and so is best avoided.

Yes and no.  You might as well remove your comment about threading and
it'll be just as accurate -- interpreted environments like Python do
indeed exact an overhead no matter what you're doing.  But just try
writing your own dictionary type in C and see how much fun you have.
                      --- Aahz (Copyright 2000 by aahz at

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