Setting ctypes.Structure default values

Question

This doesn't work:

class ifinfomsg(ctypes.Structure):
    _fields_ = [
                 ('ifi_family',  ctypes.c_ubyte),
                 ('__ifi_pad',   ctypes.c_ubyte),
                 ('ifi_type',    ctypes.c_ushort),
                 ('ifi_index',   ctypes.c_int),
                 ('ifi_flags',   ctypes.c_uint),
                 ('ifi_change',  ctypes.c_uint(0xFFFFFFFF))
               ]

It errors with:

  File "rtnetlink.py", line 243, in <module>
    class ifinfomsg(ctypes.Structure):
TypeError: Error when calling the metaclass bases
    second item in _fields_ tuple (index 5) must be a C type

However I can set the values in __init__():

class ifinfomsg(ctypes.Structure):
    _fields_ = [
                 ('ifi_family',  ctypes.c_ubyte),
                 ('__ifi_pad',   ctypes.c_ubyte),
                 ('ifi_type',    ctypes.c_ushort),
                 ('ifi_index',   ctypes.c_int),
                 ('ifi_flags',   ctypes.c_uint),
                 ('ifi_change',  ctypes.c_uint)
               ]

    def __init__(self):
        self.__ifi_pad = 0
        self.ifi_change = 0xFFFFFFFF

Is this the 'correct' way to do this, via __init__?

At least two solutions in code can be found here: http://stackoverflow.com/questions/7946519/default-values-in-a-ctypes-structure — Chickenmarkus, Sep 17 '14 at 13:48

score 2 · Accepted Answer · answered Feb 20 '12 at 23:17

Your example errors because ctypes.c_uint(0xFFFFFFFF) is not a type. In the class definitions everything has to be a type (pointers would be POINTER not pointer).

I think that the __init__ method you use if just fine. I have wrapped some really large structures and typically use __init__ just like you have done to set default values.

In general if you have control of the C library that you are wrapping I think that you should not have default setting in both places (C and Python). In one large library I wrapped where I have access to the C I created C functions "set_defaults" that I call from the __init__. This way C only users have access to the defaults and there is only one set of code to maintain.

Setting ctypes.Structure default values

1 Answers1