python ctype recursive structures

Solution 1:

You almost certainly want to declare next_command as a pointer. Having a structure that contains itself isn't possible (in any language).

I think this is what you want:

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

Solution 2:

The reason why

EthercatDatagram._fields_.append(("next_command", EthercatDatagram))

does not work is that the machinery that creates the descriptor objects (see the source of the PyCStructType_setattro function) for accessing the next_command attribute is activated only upon assignment to the _fields_ attribute of the class. Merely appending the new field to the list goes completely unnoticed.

To avoid this pitfall, use always a tuple (and not a list) as the value of the _fields_ attribute: that will make it clear that you have to assign a new value to the attribute and not modify it in place.