How to read/copy ctype pointers into python class?

Question

This is a kind of follow-up from my last question if this can help you.

I'm defining a few ctype structures

class EthercatDatagram(Structure):
    _fields_ = [("header", EthercatDatagramHeader),
       ("packet_data_length", c_int),
       ("packet_data", POINTER(c_ubyte)),
       ("work_count", c_ushort)]

class EthercatPacket(Structure):
    _fields_ = [("ether_header", ETH_HEADER),
       ("Ethercat_header", EthercatHeader),
       ("data", POINTER(EthercatDatagram))]

note that this is parsed correctly by python, the missing classes are defined elsewhere. My problem is when I call the following code

packet = EthercatPacket()
ethercap.RecvPacket(byref(packet))
print packet.data.header

This is incorrect. As I understand the problem, data is some kind of pointer so it isn't (really) mapped to EthercatDatagram, hence, the parser doesn't know the underlying header field.

is there some way to read that field as well as any other field represented by POINTER()?

Answer 1

Ok I got it working

correct code was

print packet.data.header[0]

thanks to the 7 person who dared to look at the question

the google string for the answer was : python ctype dereference pointer 3rd hit

Answer 2

The square-bracket notation is indeed correct. For reference, here's a snippet from some ctypes code I recently made:

class Message(Structure):
    _fields_ = [ ("id", BYTE), ("data", POINTER(BYTE)), ("data_length", DWORD) ]
    def __repr__(self):
        d = ' '.join(["0x%02X" % self.data[i] for i in range(self.data_length)])
        return "<Message: id = 0x%02X, " % (self.id) + "data_length = " + str(self.data_length) + (self.data_length > 0 and (", data: " + d) or "") + ">"