[Python-ideas] Use `isinstance` check during exception handling

[Andrew Barnert]

On Nov 4, 2015, at 14:02, sjoerdjob at sjec.nl wrote:
> 
> Hi all,
> 
> TL;DR: Change exception checking logic from
>    err.__class__ in exc.__mro__
> to
>    isinstance(err, exc)
> Because it is more powerful, more consistent and allows for cleaner
> code.

Correct me if I'm wrong, but doesn't Python allow you to raise a class and handle it without ever creating an instance? If an instance is needed (e.g., because the exception is caught by a handler with an as clause), one is created by calling exc(), but if it's not needed, that doesn't happen.

Assuming I'm right, your proposal would mean the instance is _always_ needed, so if you raise a type, exc() is always called. So, for example:

    class MyException(Exception):
        def __init__(self, thingy, *a, **kw):
            super().__init__(*a, **kw)
            self.thingy = thingy

    try:
        raise MyException
    except MyException:
        print('whoops')

Instead of printing "whoops", it would now (I think) abort with a TypeError about trying to call MyException with the wrong number of arguments.

Plus, there's almost certainly code where someone skips creating the instance as an optimization—probably it's usually a useless premature one, but maybe someone has tested and it makes a difference in a real program.

One obvious possibility is: if exc is itself a subclass of BaseException, you use the existing subclass check; if not, instead of using the subclass check on its type, you use the new instance check. But you'd have to think that through to make sure it makes sense.
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