check if object is number

[Steven Bethard]

John Lenton wrote:
> On Fri, Feb 11, 2005 at 01:17:55PM -0700, Steven Bethard wrote:
> 
>>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?
> 
> 
> Why don't you express just this need as an assertion?
> 
>     assert 0 <= max <= max + 1, 'Argument must not be zany'

Well a TypeError might be raised in executing the expression:

py> max = complex(0, 1)
py> assert 0 <= max <= max + 1, 'Argument must not be zany'
Traceback (most recent call last):
   File "<interactive input>", line 1, in ?
TypeError: cannot compare complex numbers using <, <=, >, >=

but I do like the max <= max + 1 idea.  Maybe I should do something like:

py> def assertnumber(x):
...     try:
...         1 < x
...     except TypeError:
...         raise TypeError('%s is not comparable to int' %
...                         type(x).__name__)
...     try:
...         if not x <= x + 1:
...             raise TypeError
...     except TypeError:
...         raise TypeError('%s is not monotonic' %
...                         type(x).__name__)
...
py> assertnumber(1)
py> assertnumber(1.0)
py> assertnumber(complex(0, 1))
Traceback (most recent call last):
   File "<interactive input>", line 1, in ?
   File "<interactive input>", line 5, in assertnumber
TypeError: complex is not comparable to int
py> assertnumber('a')
Traceback (most recent call last):
   File "<interactive input>", line 1, in ?
   File "<interactive input>", line 11, in assertnumber
TypeError: str is not monotonic
py> class C(object):
...     def __add__(self, other):
...         return 0
...     def __lt__(self, other):
...         return True
...
py> assertnumber(C())
Traceback (most recent call last):
   File "<interactive input>", line 1, in ?
   File "<interactive input>", line 11, in assertnumber
TypeError: C is not monotonic

Steve