Amaury Forgeot d'Arc added the comment:
Most concrete variable-sized types don't use tp_basicsize to know where the
data lives. For example, PyBytesObject has a "char ob_sval[1];" member, and
PyBytes_AsString() looks at this fixed offset.
For this reason, additional data cannot be stored before the variable-length
section, only after.
It is possible to store data after the variable section, and CPython does it
already for the __dict__ slot of user-defined classes::
>>> bytes.__basicsize__
33
>>> class B(bytes): pass
...
>>> B.__basicsize__
41
>>> B.__dictoffset__
-8
Note that tp_basicsize was increased by the size of the additional data (here a
PyObject*).
To access your data, the logic is something like::
tp->tp_basicsize + (obj->ob_size * tp->tp_itemsize) - sizeof(MyData)
The function _PyObject_GetDictPtr() has similar code, and already aligns to the
pointer size.
Of course, at the end of one object you cannot have *both* a __dict__ slot and
custom data.
The custom type needs to either disallow subclassing, or provide a (negative)
tp_dictoffset.
Also, some care may be required if the base class uses a custom allocator.
Overall, I think it's a bit involved, but doable.
I close the issue as "works for me", someone can reopen if there is something
wrong in the base types.
----------
nosy: +amaury.forgeotdarc
resolution: -> works for me
status: open -> closed
_______________________________________
Python tracker <[email protected]>
<http://bugs.python.org/issue515074>
_______________________________________
_______________________________________________
Python-bugs-list mailing list
Unsubscribe:
http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
