Default values in a ctypes Structure

Question

In a ctypes Structure, is it possible to specify default values?

For example, with a regular python function, you can do this:

def func(a, b=2):
    print a + b

That would allow for this behaviour:

func(1) # prints 3

func(1, 20) # prints 21

func(1, b=50) # prints 51

Is it possible to do this in a ctypes Structure?

for example:

class Struct(Structure):
    _fields_ = [("a", c_int), ("b", c_int)] # b default should be 2

    def print_values(self):
        print self.a, self.b

struct_instance = Struct(1)

struct_instance.print_values() # should somehow print 1, 2

score 9 · Accepted Answer · edited Apr 10 '17 at 21:20

9

Yes. Simply override the __init__ method.

class Struct(Structure):
    _fields_ = [("a", c_int), ("b", c_int)]

    def __init__(self, a, b=2):
        super(Struct, self).__init__(a, b)

    def print_values(self):
        print(self.a, self.b)

edited Apr 10 '17 at 21:20

Guillaume Jacquenot

11,217
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answered Oct 30 '11 at 17:41

Petr Viktorin

65,510
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81

Thanks! Not sure why I didn't think of that. :) – Lewis Ellis Oct 30 '11 at 17:51

Chickenmarkus · Answer 2 · 2022-03-08T08:35:23.077

There is a more comfortable way if you have a lot of structures with default values different to ctypes' ones. Expand the ctypes.Structure to another class variable _defaults_.

class BaseStructure(ctypes.Structure):

    def __init__(self, **kwargs):
        """
        Ctypes.Structure with integrated default values.

        :param kwargs: values different to defaults
        :type kwargs: dict
        """

        values = type(self)._defaults_.copy()
        values.update(kwargs)

        super().__init__(**values)            # Python 3 syntax



class YourStructure(BaseStructure):
    _fields_ = [ ("param1", ctypes.c_uint32),
                 ("param2", ctypes.c_uint32),
                 ("param3", ctypes.c_int32),
               ]
    _defaults_ = { "param1" : 42,
                   "param3" : -23,
                 }

You say "Python 3 syntax" – what would the equivalent in Python 2.7 be? — slhck, Apr 08 '15 at 12:53
In Python 2 you have to repeat yourself: ```super(YourStructure, self).__init__(**values)``` (http://stackoverflow.com/a/576183/2053808). — Chickenmarkus, Apr 14 '15 at 07:53

Default values in a ctypes Structure

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