[Python-Dev] PyInstance_Check() and new-style classes

[Eric Wilhelm]

# The following was supposedly scribed by
# Phillip J. Eby
# on Monday 12 July 2004 11:12 am:

>>I'm not sure if I understand what this flag is telling me.  According to
>> the api/type-structs.html, Py_TPFLAGS_HEAPTYPE is set when the object is
>> allocated.  To me, that reads that it would be set for builtin tuples, etc
>> even if they were not subclassed.
>
>A "heap type" is a type whose type struct is in heap memory.  A "non-heap"
>type is one whose type struct is declared statically, i.e. in C code.
>
>So, a non-heap type is either a built-in type, or an extension type
>provided by a C extension module.  A heap type is either one coded in pure
>Python, or potentially a C extension coded in an unusual fashion (e.g. via
>Pyrex).

Okay, looks like this will work just beautifully.  Heaptype is true for an 
instance of a class which inherits from a builtin type, but not for a simple 
instance of a builtin type.

True, if I were trying to wrap a C extension module via Inline::Python, 
heaptype would not be true, but a really easy work-around should exist by 
simply defining a python class which does nothing but inherit the C 
extension.  That would give the Inline::Python code something to grab onto.

Anyway, this is much cleaner:

http://ericwilhelm.homeip.net/svn/Inline-Python/trunk/xs-based/py2pl.c

-       int type_sum =
-                       PyString_Check(obj)+
-                       PyNumber_Check(obj)+
<snip lots of PyFoo_Check(obj) + lines>

        /* wrap an instance of a Python class */
        /* elw: here we need to make these look like instances: */
-       if ((! type_sum && PyType_Check(PyObject_Type(obj))) || 
PyInstance_Check(obj)) {
+       if ((obj->ob_type->tp_flags & Py_TPFLAGS_HEAPTYPE) || 
PyInstance_Check(obj)) {

So,  I think, with everybody's help, I may have found "the right way" (TM.)

Should this be better documented, or is this an obscure usage?  Will it break 
in future versions of Python?

Thanks,
Eric
-- 
I arise in the morning torn between a desire to improve the world and a
desire to enjoy the world. This makes it hard to plan the day. 
                                        --E.B. White