Checking for an exception

[Steve D'Aprano]

On Sun, 25 Jun 2017 10:49 am, Ben Finney wrote:

> Steve D'Aprano <steve+python at pearwood.info> writes:
> 
>> What's the right/best way to test whether an object is an exception
>> ahead of time? (That is, without trying to raise from it.)
> 
> This being Python, it is Easier to Ask for Forgiveness than for
> Permission.

Sometimes...


> The corollary of that is, if you try to ask permission first (to Look
> Before You Leap), it will likely not be as easy as simply using the
> object as you intend to use it.
> 
> So, EAFP would suggest just raising the object:
> 
>     raise obj

Unfortunately it's not that simple, as the result of passing a non-exception to
raise is to raise an exception, so I cannot trivially distinguish
between "caller passes an exception" and "caller passes a non-exception"
(result is still an exception).

I could write something like:

def is_exception(obj):
    try:
        raise obj  # This unconditionally raises.
    except TypeError:
        # Either obj is an instance or subclass of TypeError,
        # or it's some arbitrary non-exception object.
        if isinstance(obj, TypeError):
            return True
        try:
            return issubclass(obj, TypeError)
        except TypeError:
            return False
    except:
        # Any exception other than TypeError means obj is that exception.
        return True
    else:
        # In Python 2.4 and 2.5 you can (but shouldn't) raise strings.
        assert isinstance(obj, str)
        return False


but I don't think that's much of an improvement :-)




-- 
Steve
“Cheer up,” they said, “things could be worse.” So I cheered up, and sure
enough, things got worse.