Python number base class OR how to determine a value is a number

Question

Python seems to lacks a base class for "all numbers", e. g. int, float, complex, long (in Python2). This is unfortunate and a bit inconvenient to me.

I'm writing a function for matching data types onto each other (a tree matching / searching algorithm). For this I'd like to test the input values for being lists, dicts, strings, or "numbers". Each of the four cases is handled separately, but I do not want to distinguish between int, long, float, or complex (though the latter will probably not appear). This would be rather easy to achieve if all number types would be derived from a base type number, but unfortunately, they are not, AFAICS.

This enforcement of explicitness makes it obvious that the unusual complex is included. Which typically raises questions which I rather not want to think about. My design rather says "all number types" than that explicit list. I also do not want to explicitly list all possible number types coming from other libraries like numpy or similar.

First question, rather a theoretical one: Why didn't the designers make all number types inherit a common number base class? Is there a good reason for this, maybe a theory about it which lets it seem not recommended? Or would it make sense to propose this idea for later versions of Python?

Second question, the more practical one: Is there a recommended way of checking a value for being a number, maybe a library call I'm not aware of? The straight-forward version of using isinstance(x, (int, float, complex, long)) does look like a clutch to me, isn't compatible to Python3 which doesn't know a long type anymore, and doesn't include library-based number types like numpy.int32.

Answer 1

There is actually a base class for those types you listed.

If you're not looking at numpy types, a good starting point would be numbers.Complex:

>>> import numbers
>>> isinstance(1+9j, numbers.Complex)
True
>>> isinstance(1L, numbers.Complex)
True
>>> isinstance(1., numbers.Complex)
True
>>> isinstance(1, numbers.Complex)
True

It gets a bit messier when you start to include those from numpy, however, the numbers.Complex abstract base class already handles a good number of the mentioned cases.

Answer 2

Sorry, a bit late to the party. But this might be helpful for future readers of this page. I suggest you use Python's "duck typing" character and its EAFP ("Easier to Ask Forgiveness than Permission") philosophy: in other words, just try using the object in question as a number. Write something like this:

def isnumber(thing):
    try:
        thing + 0
        return True
    except TypeError:
        return False

It should work for any type of number, including user-defined classes.