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
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