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Python and Boehm-Demers GC, I have code.

Tim Peters tim_one at email.msn.com
Sat Jul 17 03:13:05 EDT 1999 
[Tim]
> ... Did the "collection" phase of BDW find any trash at all?

[Neil Schemenauer]
> It doesn't unless you create some.  The patch creates a version
> of Python that uses GC only to clean up reference cycles.

Just as it appeared <wink>.  So it's really no surprise that the overall
timing didn't change much in pybench, right?

> ...
> BTW, _tkinter is not working perfectly yet.  I think lambdas
> passed as callbacks are getting collected.

Such as to key bindings?  Harmonic convergence (I was just looking at that
in IDLE tonight!).

When you pass a lambda (or bound method object, or any callable object -- CO
for short) as a binding, Tkinter.py eventually wraps the CO in an instance
of Tkinter.CallWrapper, and the .__call__() method of that instance is
passed on to _tkinter.c's Tkapp_CreateCommand via Tkinter.Misc._register().

The interesting thing here is that nothing in Tkinter.py holds on to
anything from which the CO can be reached.  Instead _tkinter.c's
Tkapp_CreateCommand bumps the CO's refcount, stuffs a pointer to the CO in a
heap-allocated new instance of a PythonCmd_ClientData struct, and passes a
pointer to the latter on to Tcl via Tcl_CreateCommand.  _tkinter.c doesn't
hold on to a reference to this either:  when Tkapp_CreateCommand returns,
Tcl the sole owner of the CO reference that was added, and Tcl is the only
one who knows the address of the PythonCmd_ClientData struct.

So if the CO is an embedded lambda expression, there is only one reference
to it in total, and only Tcl can reach it (which it does when the binding
triggers, and then it passes a pointer to the PythonCmd_ClientData struct
back to _tkinter.c's PythonCmd).  So if BDW isn't crawling over Tcl/Tk's
memory too, the lambda will look like trash, and the PythonCmd_ClientData
struct will look like trash regardless of what flavor of CO was passed.

So BDW has to get at Tcl's memory too, or _tkinter.c has to maintain a list
of allocated PythonCmd_ClientData thingies (until PythonCmdDelete is called
back from Tcl), or ...

It's convoluted for sure, but I do expect you'll find "the answer" at the
bottom of it.

rc-is-so-simple<wink>-ly y'rs  - tim
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