[Python-Dev] Adding a threadlocal to the Python interpreter

[Nick Coghlan]

On 18 May 2016 at 23:20, Daniel Holth <dholth at gmail.com> wrote:
> I would like to take another stab at adding a threadlocal "str(bytes) raises
> an exception" to the Python interpreter, but I had a very hard time
> understanding both how to add a threadlocal value to either the interpreter
> state or the threadlocal dict that is part of that state, and then how to
> access the same value from both Python and CPython code. The structs were
> there but it was just hard to understand. Can someone explain it to me?

Christian covered the C aspects of the API, while the general purpose
Python aspects live in the threading module.

However, the Python level thread-local API doesn't provide direct
access to the thread state dict. Instead, it provides access to
subdicts stored under per-object keys in that dict, keyed as
"thread.local.<id>":

* Key definition in local_new:
https://hg.python.org/cpython/file/tip/Modules/_threadmodule.c#l705
* Subdict creation in _ldict:
https://hg.python.org/cpython/file/tip/Modules/_threadmodule.c#l810

Getting access to state stored that way from C is *possible*, but
significantly less convenient than access the thread state directly.

What that means is that any time we want to expose thread local state
to both C and Python code, it will generally be the responsibility of
the C code to both manage the key in the thread state dict (or the
field in the thread state struct), and also to provide a Python API
for access that state.

Cheers,
Nick.

-- 
Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia