isNumber? check

[Jeff Epler]

First, whatever test you use, you should probably encapsulate it in a
function, so that if you need to update the definition you can do it at
one site instead of many:

    def isnumeric(x):
        return isinstance(x, (int, long, float))

You could have a registry of numeric types:

    _numeric_types = ()
    def register_numeric_type(t):
        global _numeric_types
        if t in _numeric_types: return
        _numeric_types += (t,)
    
    for value in (0, 0., 0l, 0j):
        register_numeric_type(type(value))

    def isnumeric(x):
        return isinstance(x, _numeric_types)

Now, if someone wants to write a vector type, it merely needs to be
registered.

You could test that common numeric operations work:
    def isnumeric(x):
        try:
            if x*1 == x and x+0 == x:
                return 1
        except TypeError:
            pass
        return 0

You could just run your code and let the eventual TypeError speak for
itself..  instead of
    def f(x):
        if not isnumeric(x): raise TypeError, "can't f() a %s" % type(x)
        return x*x
just write
    def f2(x):
        return x*x
The difference in the quality of the error message is not large:
    >>> f("")
    Traceback (most recent call last):
      File "<stdin>", line 1, in ?
      File "<stdin>", line 2, in f
    TypeError: can't f() a <type 'str'>
    >>> f2("")
    Traceback (most recent call last):
      File "<stdin>", line 1, in ?
      File "<stdin>", line 2, in f2
    TypeError: unsupported operand type(s) for *: 'str' and 'str'

Jeff