Re: [ctypes-users] fun with ctypes

[Thomas H. <th...@ct...>]

Simon Burton schrieb:
> On Wed, 28 Feb 2007 21:05:33 +0100
> Thomas Heller <th...@ct...> wrote:
> 
>>  Sure, you can access the 'PyObject' fields with it, but to
>> call functions from the Python api the ctypes.py_object type should
>> be used instead.
> 
>>>> import ctypes
>>>> class A(object):pass
> ...
>>>> a=A()
>>>> a.b=1
>>>> aa=ctypes.py_object(a)
>>>> capi=ctypes.pythonapi
>>>> b=capi.PyObject_GetAttrString(aa,'b')
>>>> b
> 134537008
>>>> type(b)
> <type 'int'>
>>>>
> 
> well... i assume this b is the address of the int object.. is there
> a way to get a py_object from an int address ?

Yes: py_object.from_address(an_integer_address).  Works for any ctypes type.

But easier, and safer, is to set the restype attribute of the PyObject_FromString function:

capi.PyObject_GetAttrString.restype = py_object
b = capi.PyObject_GetAttrString(aa,'b')

In this way it will return the integer 1, and refcounts will be managed correctly.
Also, you will get an exception when the PyObject_GetAttrString returns NULL.

If you even set argtypes:

capi.PyObject_GetAttrString.argtypes = (py_object, c_char_p)

you can save the wrapping of the A() object into a py_object call,
the function call will do this automatically, and will check if you pass
a string as second parameter.

Also, to answer the question in your other post:

Yes, ctypes releases the GIL during function calls, but only for function calls
done via cdll or windll, not via pydll or the pythonapi object.  Also, pydll calls
check the PyErr_Occurred() function after the call.  This is outlined in the docs.

BTW: I never had the use case myself to call Python API functions via ctypes,
but I know that some people have sucessfully used it.  If you need to do this
I strongly recommend to use the module that Lenard Lindstrom contributed; it is
in the ctypeslib package that you can download with 'easy_install ctypeslib'
(or was it 'easy_install ctypeslib==dev' ? not sure).

Thomas