[Numpy-discussion] numpy.ctypeslib.ndpointer and the restype attribute [patch]

Thomas Heller theller@ctypes....
Fri Mar 27 15:32:49 CDT 2009

> Sturla Molden schrieb:
>> On 3/26/2009 12:41 PM, Jens Rantil wrote:
>>> Wouldn't my code, or a tweak of it, be a nice feature in
>>> numpy.ctypeslib? Is this the wrong channel for proposing things like
>>> this?
>> If you look at
>>     http://svn.scipy.org/svn/numpy/trunk/numpy/ctypeslib.py
>> you will see that it does almost the same. I think it would be better to 
>> work out why ndpointer fails as restype and patch that.

Thomas Heller schrieb:

> ndpointer(...), which returns an _nptr instance, does not work as restype
> because neither it is a base class of one of the ctypes base types like
> ctypes.c_void_p, also it is not callable with one argument.
> There are two ways to fix this.  The first one is to make the _nptr callable
> The other way is to make _nptr a subclass of ctypes.c_void_p,
> the result that the foreign function call returns will then be
> an instance of this class.  Unfortunately, ctypes will not call
> __new__() to create this instance; so a custom __new__() implementation
> cannot return a numpy array and we are left with the _nptr instance.
> The only way to create and access the numpy array is to construct
> and return one from a method call on the _nptr instance, or a property
> on the _nptr instance.
> Ok, .errcheck could call that method and return the result.
Well, looking into the ctypes sources trying to invent a new protocol for
the restype attribute I found out that THERE IS ALREADY a mechanism for it,
but I had totally forgotten that it exists.

When the .restype attribute of a function is set to a SUBCLASS of a
ctypes type (c_void_p for example), an instance of this subclass is created.
After that, if this instance has a _check_retval_ method, this method is called
and the result of this call is returned.  So, it is indeed possible to create
a class that can be assigned to .restype, and which can convert the return value
of a function to whatever we like.

I will prepare a patch for numpy.ctypeslib.


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