check if object is number

[George Sakkis]

> George Sakkis wrote:
> > "Steven Bethard" <steven.bethard at gmail.com> wrote in message
> > news:gcidnb9g_ojxnpDfRVn-tA at comcast.com...
> >
> >>Is there a good way to determine if an object is a numeric type?
> >
> > In your example, what does your application consider to be numeric?
>
> Well, here's the basic code:
>
> def f(max=None):
>      ...
>      while max is None or n <= max:
>          ...
>          # complicated incrementing of n
>
> So for 'max', technically all I need is <= support.  However, the code
> also depends on the fact that after incrementing 'n' enough, it will
> eventually exceed 'max'.  Currently, ints, longs, floats, and Decimals
> will all meet this behavior.  But I'd rather not specify only those 4
> (e.g. with a typecheck), since someone could relatively easily create
> their own new numeric type with the same behavior.  Do you know a better
> way to test for this kind of behavior?
>
> Steve

The problem is more conceptional rather than technical. So what you're saying is that 3 <= "3.0"
should not be allowed, but 3 <= SomeUserDefinedNumericClass(3) is ok, although your program knows
nothing a priori about SomeUserDefinedNumericClass. The idea suggested before, try if "x+1" fails or
not does not get you far; any class that overrides __add__ may fool this test:

class Dummy:
    def __add__(self,other):
        return 0

Is Dummy 'numeric' ? Probably not. Whether a given instance is "numeric" (or "sequence",
"file-like", etc. for that matter) is a semantic issue. It cannot be resolved by relying on which
methods are supported or not, but what these methods *do*; this is something that no programming
language can tell you now and in the (foreseeable) future.

George