Chris Colbert, 18.02.2011 03:23:
What is the rule of thumb when declaring functions from python's C-api when
comes to ref counting?
The general rule is to not declare them yourself. Instead, cimport them
from the cpython package. (See Cython/Includes/)
If I define a test case like so:
cdef extern from "Python.h":
object PyWeakref_NewRef(object, object)
object PyWeakref_GET_OBJECT(object)
class Foo(object):
pass
cdef class Test:
cdef object obj
cdef object wr
def __init__(self):
self.obj = Foo()
self.wr = PyWeakref_NewRef(self.obj, None)
def get_ref(self):
return PyWeakref_GET_OBJECT(self.wr)
I get these random python fatal errors:
In [8]: %timeit -n 1000 t.get_ref()
1000 loops, best of 3: 224 ns per loop
In [9]: %timeit -n 1000 t.get_ref()
Fatal Python error: deallocating None
Abort trap
However, if I redefine the macro signature and getter function to this:
from cpython cimport PyObject
cdef extern from "Python.h":
object PyWeakref_NewRef(object, object)
PyObject* PyWeakref_GET_OBJECT(object)
class Foo(object):
pass
cdef class Test:
cdef object obj
cdef object wr
def __init__(self):
self.obj = Foo()
self.wr = PyWeakref_NewRef(self.obj, None)
def clear_obj(self):
self.obj = None
def get_ref(self):
return<object>PyWeakref_GET_OBJECT(self.wr)
Then it runs without issue. I can other gather is has to due the
incref/decref going on in the generated C code. Should be doing something on
my end to manually manage ref counts when using the C-Api?
Check the CPython documentation. Whenever a function returns a borrowed
reference, you must declare it as PyObject* and cast it to <object>.
That being said, support for borrowed references has been long on the list
but no-one has shown interest in doing it (or getting it done) so far.
Stefan
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