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Mon Jun 28 14:00:41 EDT 2004 · https://mail.python.org/pipermail/python-dev/2004-June/045696.html

[Phillip J. Eby]
> What's wrong with using (e.g. setting attributes on)
> 'threading.currentThread()'?  It's already implemented and available in 2.3
> (and numerous prior versions).

Jim's interest here is speed.  currentThread() is a Python-level
function call, which invokes the platform spelling for getting a
thread id, which is then used in a dict lookup to get the associated
Thread object.  The C API's PyThreadState_GetDict() has none of those
expenses:  a pointer to a thread-local dict is sitting in the current
thread state struct, available cheaply from C but currently not
available at all from Python.

If you need to access thread-local values frequently, the overhead of
the currentThread() approach can swamp the time spent doing useful
work.  Jim sees this in Zope3 machinery now.  I expect we're also
seeing it in the sandbox's Decimal.py (which needs to access
thread-local context on every operation, indirects thru yet another
layer of Python-level call to get at it).

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[Python-Dev] Proposal: thread.get_dict