This is rather a feature request instead of a bug report. 
Below is the mail I posted to the ctypes-users mailing list. In short: 
When ctypes checks input argument types using the "argtypes" attribute, 
it would be useful if it would try to convert the input value 
automatically if it isn't already an appropriate ctypes type (but a 
"compatible" Python type).

Here is the full mail with some examples:

I'm wrapping a couple of C functions from a DLL and I'm using the 
argtypes attribute to declare the types of the input arguments. I can 
call the functions just fine, but I was wondering why I have to provide 
the exact ctypes type as input when using more complex types such as 
arrays or callbacks (whereas Python floats are automatically converted).

Here is an example code snippet (I was using Python 2.5 on WinXP). The 
library ri.dll contains the functions RiColor() which takes an array of 
3 floats as input and a function RiErrorHandler() which takes a pointer 
to a function as input:

   # Create the required types...
   RtColor = 3*c_float
   RtErrorHandler = CFUNCTYPE(None, c_int, c_int, c_char_p)

   # Load the library and declare the input arguments...
   ri = cdll.LoadLibrary("ri.dll")
   ri.RiColor.argtypes = [RtColor]
   ri.RiErrorHandler.argtypes = [RtErrorHandler]

Now I can call the color function like this:

   ri.RiColor(RtColor(1,0,0))

But sometimes it would be more convenient to work with other types like 
tuples, lists or, in this case, a special vector type (that may come 
from another module but that behaves like a list of 3 floats).
But when I try to pass in just a Python tuple or list I get the 
following errors:

   ri.RiColor((1,0,0))

   --> ctypes.ArgumentError: argument 1: <type 'exceptions.TypeError'>: 
Don't know how to convert parameter 1

   ri.RiColor([1,0,0])

   --> ctypes.ArgumentError: argument 1: <type 'exceptions.TypeError'>: 
expected c_float_Array_3 instance instead of list

Similarly with the error handler function. I have to wrap a Python 
function with the RtErrorHandler type, otherwise ctypes won't accept it:

   def errHandler(code, severity, message):
       pass

   ri.RiErrorHandler(RtErrorHandler(errHandler)) # works
   ri.RiErrorHandler(errHandler)                 # produces a TypeError


So whenever an input type doesn't match what was specified in argtypes, 
couldn't ctypes try to convert the value before issuing an error? (it 
works with floats, so why not with other types as well?)
The conversion could just be done by passing the value to the 
constructor of the required type. In the color example this also means 
that array types should also accept sequences as input (i.e. anything 
that supports iteration and has the right number of elements).

I think this modification to ctypes would make the wrapped functions 
more flexible without having to write additional wrapper functions in 
Python.