subclassing float (or not)

[Steven Majewski]

On Fri, 22 Feb 2002, Sean 'Shaleh' Perry wrote:

>
> On 22-Feb-2002 Thomas Heller wrote:
> > I'm currently porting my own rewrite of Konrad Hinsen's PhysicalValue
> > module to Python 2.2.
> > Basically a PhysicalValue instance is a float conbined with a unit.
> > Should I subclass float or should I not? What would I gain by doing
> > this (except maybe some memory savings when I use __slots__)?
> >
>
> Generally when you subclass you are saying "I am a special type of foo" where
> foo is your parent.  I think in this instance it would make more sense to make
> a new class which contains a float and a unit.  Give it methods so it acts how
> you expect.
>

Except that how he expects it to act is exactly like a float except for
carrying and propagating a little extra bit of state at each operation.

Sounds like a good application for subclassing floats to me!

You can do it (and could have always done it) with a container class,
but there are always holes where you need the operation to be
coerced on the classes value and not the class itself. In some
cases that still may be what you really want (because you also want
to coerce the result to my_float rather than float), but at least
if you miss something, a subclassed float will do something closer
to the right thing.

An example of what I mean is 'sqrt(MyFloat)' -- if MyFloat is a
class that contains a float and has methods for __add__, __mul__,etc.
sqrt(MyFloat) will raise an exception if passed a container class --
you need to call 'sqrt(MyFloat.value)' , while:

>>> class F(float):
...     pass
...
>>> from math import *
>>> sqrt( F(1.0))
1.0

Will return a normal float where you may really want a custom F instance.
( Which is the better way to go wrong here may vary. )

Two other float-like classes that I've had use for that are easier to
do now with float subclasses:

Values that carry and propagate standard errors or standard deviations
with them.

Values that carry an "audit trail" ( for taking chained ratios of
experimental measures and trying to track down the inconsistent
data. )


-- Steve Majewski