mutable numeric type

[Andreas Beyer]

There has been quite some traffic about mutable and immutable data types 
on this list. I understand the issues related to mutable numeric data 
types. However, in my special case I don't see a better solution to the 
problem.
Here is what I am doing:

I am using a third party library that is performing basic numerical 
operations (adding, multiplying, etc.) with objects of unknown type. Of 
course, the objects must support the numerical operators. In my case the 
third party library is a graph algorithm library and the assigned 
objects are edge weights. I am using the library to compute node 
distances, etc.

I would like to be able to change the edge weights after creating the 
edges. Otherwise, I would have to remove the edges and re-create them 
with the new values, which is quite costly. Since I also didn't want to 
change the code of the graph library, I came up with a mutable numeric 
type, which implements all the numerical operators (instances are of 
course not hashable). This allows me to change the edge weights after 
creating the graph.

I can do the following:
 >>> x = MutableNumeric(10)
 >>> y = MutableNumeric(2)
 >>> x*y
20
 >>> x.value = 1.3
 >>> x*y
2.6000000000000001
 >>>

The effect of numerical operations is determined by the contained basic 
data types:
 >>> x.value = 3
 >>> x/2
1
 >>> x.value = 3.0
 >>> x/2
1.5
 >>>

Augmented operations change the instance itself:
 >>> x.value = 0
 >>> id(x)
-1213448500
 >>> x += 2
 >>> x
MutableNumeric(2)
 >>> id(x) # show that same instance
-1213448500
 >>>

Is there anything wrong with such design? I am a bit surprised that 
Python does not already come with such data type (which is really simple 
to implement). Is there something that I am missing here?

Thanks!
    Andreas