Adding a property to an int value in python

Question

I have a value which is commonly used throughout my code in different units. E.g. representing a buffer size in number of BYTES, but in many places refering to the size as KB or as MB (this is just an example, not my real use case).

For elegance and ease of use, I want to avoid explicit conversions (e.g. size/1024, or b_to_mb(size)), because they are needed in so many different places.

I thought I can achieve that with properties, which makes the conversion easy (x.kb or x.mb), and also makes the caller unaware of the units of the actual value stored.

My code looks like this:

class BufferSize(int):
    @property
    def b(self):
        return int(self)
    @property
    def kb(self):
        return self.b / 1024
    @property
    def mb(self):
        return self.kb / 1024

This works, until I use the arithmetic operators:

s = BufferSize(500000)
s.kb
=> 488.28125   # ok
s.mb
=> 0.476837158203125  # ok
type(s + BufferSize(0))
=> int  # type lost...

Is there a way to ensure arithmetic operations preserve the type? That is, other than overriding each and every one of them?

Or maybe a different and better way to solve the original problem?

Answer 1

Well, it's not so much re-implement as wrap/cast the results of int's magic methods, at least those that return ints.. All, of them, unfortunately. Wonder if you could not use a decorator or metaclass to expedite that somewhat.

class BufferSize(int):
    @property
    def b(self):
        return int(self)
    @property
    def kb(self):
        return self.b / 1024
    @property
    def mb(self):
        return self.kb / 1024

    def __add__(self, *args, **kwds):
        return BufferSize(super(BufferSize, self).__add__(*args, **kwds))

v = BufferSize(1) + BufferSize(2)

print v, type(v)

output:

3 <class '__main__.BufferSize'>

I also thought about adding the property directly to int itself, but a quick test ruled that out.

>>> int.b = "b"

output:

 can't set attributes of built-in/extension type 'int'