[Python-Dev] Threading and callbacks - missing documentation (original) (raw)

Martin v. Loewis martin@v.loewis.de
12 Apr 2002 09:03:54 +0200

Jack Jansen <Jack.Jansen@oratrix.com> writes:

As I'm currently very interested in this for two separate reasons (one work-related, and another one is that I want to make the Mac toolbox modules release the GIL when appropriate) I could write something up, but only if (a) it isn't documented somewhere already and (b) someone can tell me how it actually works:-).

The issue in a callback is that you need to obtain a thread state before getting the interpreter lock back. There are two ways to do this:

  1. the _tkinter approach: You can guarantee that the call-back occurs in the context of the same thread that originally released the GIL. In that case, save the thread state in a global variable, and restore it inside the callback (see ENTER_TCL/ENTER_PYTHON).

  2. the "free threading" approach: You assume that callback may occur in the context of an arbitrary thread, even in a thread that was not originally created by Python. In that case, inside the call-back, use PyThreadState_New to create a fresh thread state, then obtain the GIL for this tread state. When the callback is done, delete the thread state. You need to preserve a pointer to the interpreter somehow in a global variable (or provide other means to locate the interpreter).

The advantage of 1 is that tracebacks go right back into the main module, even from the callback; in approach 2, tracebacks go only back to the entry in the callback (the callback appears to come out of nowhere). The advantage of 2 is that it is more general.

HTH, Martin