How to add/subtract two ctypes.c_double in Python

Question

Simple question. I have two ctypes.c_double. I would like to add or subtract them together.

I want to be able to say something like this:

In [1]: c_double(2) + c_double(2)
Out[1]: c_double(4.0)

Currently, when I try this I get an error: TypeError: unsupported operand type(s) for +: 'c_double' and 'c_double'

My current workaround is this:

In [1]: result = c_double(2).value + c_double(2).value

In [2]: c_double(result)
Out[2]: c_double(4.0)

Is there a way to directly add/subtract ctypes.cdouble?

Alex_6 · Accepted Answer · 2019-07-24 15:59:13Z

3

You're using a class created by someone else, and it seems that he hasn't written a __add__ method, that handle the '+' operations.

I would suggest you write you're own method :

def add(self, other):
    return c_double(self.value + other.value)

Then you tell the c_double class that add is its __add__ method:

c_double.__add__ = add

I think it should work

answered Jul 24, 2019 at 15:59

Alex_6

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2 Comments

Jean-François Fabre Over a year ago

and it works. You just have to do c_double.__add__ = add in each module where you need addition.

Intrastellar Explorer Over a year ago

@Jean-FrançoisFabre would you do this in each module's global scope, after from ctypes import c_double?

Jean-François Fabre · Accepted Answer · 2019-07-24 16:02:52Z

1

you could inherit the c_double class to add a __add__ method

import ctypes

class my_c_double(ctypes.c_double):
    def __add__(self,other):
        return my_c_double(self.value + other.value)


a = my_c_double(10)
b = my_c_double(20)

print((a+b).value)

prints 30

Note that you may want to implement __radd__ and __iadd__ and check types to be able to directly left add with floats and integers.

answered Jul 24, 2019 at 16:02

Jean-François Fabre♦

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