subclassing float (or not)

[Steven Majewski]

On Tue, 26 Feb 2002, Thomas Heller wrote:

> It seems you always end up with implementing something like
>
> class F(float):
>   # float subclass
>   def __mul__(self, other):
>     return F(float(self) * other, ...)
>
> or
>
> class F:
>   # container class with a float instance variable
>   def __mul__(self, other):
>     return F(self.float * other, ...)
>
> The only *real* difference seems that you can pass float
> subclass instances to round(), log(), and these kind of functions.
> As you already noted, normally they return the wrong results ;-(
>

Well -- the other main advantage of subclassing float is that you
can do:  'isinstance( myFloatType( 1.0 ), float )'  and get true.


The other shortcut it gives you is that coercing to float or
int works without you having to actually write a coerce method:

  int( myFloatType( 1.0 ) )  --> 1


There may be other things lurking there that automatically work
correctly, that wouldn't if you used a container class. If you
subclass float instead, you don't have to wait around to see
what else breaks because you forgot to implement a method to
support it!

-- Steve