LBYL vs EAFP

[Steven D'Aprano]

The eternal conflict between "Look Before You Leap" and "Easier to Ask for
Forgiveness than Permission" (LBYL vs EAFP) continues... 

I want to check that a value is a number. Let's say I don't care what sort
of number -- float, int, complex, Fraction, Decimal, something else -- just
that it is a number. Should I:

Look Before I Leap:

    from numbers import Number
    if isinstance(x, Number):
        ...
    else:
        raise TypeError


or Ask Forgiveness:

    x + 0
    ...
        

where in both cases the ellipsis ... is the code I actually care about.

The second version is more compact and easier to write, but is it easier to
read?

Does your answer change if I then go on to check the range of the number,
e.g. to test that x is positive?



A third option is not to check x at all, and hope that it will blow up at
some arbitrary place in the middle of my code rather than silently do the
wrong thing. I don't like this idea because, even if it fails, it is better
to fail earlier than later.

Comments, thoughts and opinions please.



-- 
Steven