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

[Travis E. Oliphant]

Thomas Heller wrote:
> Thomas Heller schrieb:
>   
>>> 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.
>>
>>     
>
> It seems there isn't much interest in a patch - true?
>   
No, I'm very interested in the patch.   Thanks for it.

-Travis