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

[Christian Heimes]

On 2016-05-18 15:20, Daniel Holth 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?

Python has a two important states related to threads. The
PyInterpreterState contains the state of an interpreter instance: sys
module, loaded modules and a couple of additional settings. Usually
there is just one interpreter state in a Python process. Additional
interpreter states are used to implement subinterpreters.

Each C thread, that wants to run Python code, must have a PyThreadState.
The thread state contains a reference to a PyInterpreterState. Each
PyThreadState has a PyObject *dict member. You can stick Python objects
into the dict. The interpreter cleans up the dict when it reaps a thread.

How performance critical is your code? Does the interpreter have to
check the value of the thread local frequently? In that case you should
add a new member to typedef struct _ts PyThreadState in pystate.h right
before /* XXX signal handlers should also be here */.

Otherwise you can simply use PyThreadState_GetDict(). It returns a
Python dict object that is local to the current thread. You can simply
use a fixed key like in Modules/_decimal/_decimal.c.

Christian