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

[Michael Selik]

Hi Sjoerd,

I'm not clear on what problem this suggestion is solving. PEP 3151 was
solving the problem of existing code failing to be specific with IOError
and OSError error numbers. Can you provide some less abstract snippets as
examples, including surrounding context or a link to a project that has the
problem?

If I saw `except Exception as e` followed by `isinstance(e, SomeError)` I
would be very confused as to why `except SomeError` was not better. Is this
simply for backporting FileNotFoundError?

Michael

On Wed, Nov 4, 2015 at 8:39 PM Terry Reedy <tjreedy at udel.edu> wrote:

>
>
> On 11/4/2015 5:02 PM, sjoerdjob at sjec.nl wrote:
> > TL;DR: Change exception checking logic  from
> >      err.__class__ in exc.__mro__
>
> This is backwards. err.__class__ must be a (true) subclass, not a
> superclass, of exc, where err = exception instance, exc = exception class.
>
> There are two checks in the exception process.  First, for 'raise err',
> is that err is a instance of StopIteration, which is to say,
> err.__class__ is a subclass of StopIteration.  To avoid the fakery of
> __instancecheck__ and __subclasscheck__, I presume this is implemented
> as something like one of
>    StopIteration in err.__class__.__mro__
>    StopIteration in type(err).__mro__
> (There was a recent pydev discussion about when and why these may diverge.)
>
> The second, for 'except exc', is that exc is a superclass (other than
> object) of type(err).  I presume this is implemented as something like
>    exc in type(err)[:-1]  # exclude object class
> This assume that err has already passed the first check above.
>
> The `try` doc says "An object is compatible with an exception if it is
> the class or a base class of the exception object".  This text fails to
> exclude the object class as a compatible object.  CPython code rejects
> 'except object:' with "TypeError: catching classes that do not inherit
> from BaseException is not allowed"
>
> > to
> >      isinstance(err, exc)
> > Because it is more powerful,
>
> ?? This predicate is True more often than the current one because it a)
> does not exclude exc = object
>
>  >>> isinstance(Exception(), object)
> True
>
> and b) accesses exc.__instancecheck__ (which is part of your point).
> This makes it a weaker predicate.
>
> I don't have strong opinions at the moment about the proposal itself,
> but semantic changes in core python syntax need a PEP (once you think
> there is *some* support).
>
> --
> Terry Jan Reedy
>
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