Re: [ctypes-users] Testing for an array type of unknown length

[Terry G. <te...@ax...>]

Thomas Heller <th...@py...> writes:
> "Terry Griffin" <te...@ax...> writes:
>
>> Greetings,
>>
>> If I have a structure:
>>
>>     class MyStruct(Structure):
>>         _fields_ = [(whatever...
>>
>> Then I can do this:
>>
>>     mystructdata = MyStruct()
>>
>>     if type(mystructdata) == MyStruct:
>>         operate on mystructdata
>>     else:
>>         raise TypeError,...
>>
>> Fine so far. Now say I want my 'if' construct is to support both
>> singletons and arrays using logic like this:
>>
>>     MyStructArray = MyStruct * 10 # or some other size
>>     mystructdata = MyStructArray()
>>
>>     if type(mystructdata) == MyStruct:
>>         operate on mystructdata, the singleton
>>     elif type(mystructdata) is an array of MyStruct of unknown length:
>>         alen = len(mystructdata)
>>         operate on mystructdata, an array of MyStruct of length 'alen'
>>     else:
>>         raise TypeError,...
>>
>> How do I do 'elif' condition?
>
> First possibility is this one:
>
> elif type(mystructdata[0]) == MyStruct:
>
> But it doesn't work for simple types like c_int for example, because
> indexing an array of c_int items gets you normal Python integers, not
> c_int instances.
>
> And the second one is to use the undocumented _type_ attribute:
>
> elif type(mystructdata)._type_ == MyStruct:
>
> Thomas
>

The first option crossed my mind, but it could be fooled if mystructdata
was a list, tuple, or other container with an iterator. Some preliminary
checks for ListType etc could catch those.

The second method would raise an exception if mystructdata was not a
ctypes array at all, which in my case is probably ok since I was going to
raise an exception on a type mismatch anyway.

Anyone know if there any plans for a general (and documented) solution for
this in future versions of ctypes?

Thanks,
Terry

-- 
Terry Griffin
Axian Inc.
http://www.axian.com/