On Sun, Aug 16, 2009 at 11:53 PM, "Martin v. Löwis"<mar...@v.loewis.de> wrote: >> Thanks for the pointer. I noticed that subtype_dealloc is only called for >> types >> that are allocated using type_new(). Does this mean that it is not >> safe to create >> types in C using just PyType_Ready() and set Py_TPFLAGS_HEAPTYPE on >> them? The documentation is not clear on this point. > > As Benjamin says, this is getting off-topic - python-dev is not a place > to ask for help in your project.
Please let me know where is a more suitable place to discuss the implementation of the cPython as it pertains to C extensions. I wrote to python-dev only because the other lists appeared to be more focused on Python-the-language. > I believe setting flags on a type is inherently unsafe. Clearly this is not true in general. Take Py_TPFLAGS_BASETYPE, which C types are expected to set if they can be subclassed. Or Py_TPFLAGS_HAVE_GC, which C types set if they participate in cyclic reference collection. The docs do not distinguish (AFAICS) between flags that C types may set directly and those that they may not. My reading of the docs left me with the impression that a type could set Py_TPFLAGS_HEAPTYPE if it had allocated that type on the heap and wanted it INCREF'd and DECREF'd by instances. I now know that there is much more to this flag than I anticipated (see http://thread.gmane.org/gmane.comp.python.devel/105648), I am just giving you feedback about why the docs led me to this incorrect conclusion. In any case, I think I will experiment with a different approach, where instead of creating types in C dynamically at runtime, I will create a type whose instances "pretend" to be types (they will create instances when called). Still, I would appreciate knowing where I should direct further questions of this type, which are not questions about how to use Python but rather questions about how to properly implement extensions. >> Here is what I would like to do when I create my types dynamically: >> >> - implement tp_alloc and tp_dealloc() to INCREF and DECREF the type. >> - not set Py_TPFLAGS_HEAPTYPE. >> - set Py_TPFLAGS_HAVE_GC (because instances of my obj can create cycles) >> >> Does this seem safe? I notice that subtype_dealloc() does some funky >> GC/trashcan stuff. Is it safe for me not to call subtype_dealloc? Can I >> safely implement my tp_dealloc function like this? > > If you bypass documented API, you really need to study the code, > understand its motivation, judge whether certain usage is "safe" wrt. > to the current implementation, and judge the likelihood of this code > not getting changed in future versions. It was not my intention to bypass the documented API. Py_TPFLAGS_HEAPTYPE is documented here, with no note that the flag should not be set explicitly by C types: http://docs.python.org/c-api/typeobj.html#Py_TPFLAGS_HEAPTYPE Also, INCREF'ing and DECREF'ing my type from the tp_new and tp_dealloc functions doesn't seem outside of the documented API. Thanks, Josh _______________________________________________ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
