Thanks for the patch. I agree with the interpretation of the format string. One thing is unclear though: Using this interpretation the multi-dimensional array notation in format strings only seems useful for pointers to arrays. The PEP isn't so clear on that, would you agree? I'm not done reviewing the patch, just a couple of nitpicks: - We need a function declaration of _ctypes_alloc_format_string_with_shape() in ctypes.h. - prefix_len = 32*(ndim+1) + 3: This is surely sufficient, but (ndim+1) is not obvious to me. I think we need (20 + 1) * ndim + 3. - I'd use "%zd" for Py_ssize_t (I know that in other parts of the code "%ld" is used, too).