[issue25537] Call `isinstance` instead of `issubclass` during exception handling

[Martin Panter]

Martin Panter added the comment:

[Avoiding UTF-8 error]

This is an interesting idea that never occurred to me. It would be nice to support it if the performance impact isn’t a problem. By the sound of Issue 12029, performance is barely affected until you actually try to catch a “virtual exception”. So the impact of changing from subclass to instance checking shouldn’t be much worse if we do it right.

I agree this would be a new feature for a new release, not for 2.7 or 3.5.

BTW here’s another approach using the same sys.exc_info() hack that also works in Python 3:

>>> def catch_not_implemented():
...     '''Helper to catch "API not implemented" exceptions'''
...     [_, exc, _] = sys.exc_info()
...     if isinstance(exc, NotImplementedError):
...         return BaseException  # Catch the exception
...     if isinstance(exc, EnvironmentError) and exc.errno == ENOSYS:
...         return BaseException  # Catch the exception
...     if isinstance(exc, HTTPError) and exc.code == HTTPStatus.NOT_IMPLEMENTED:
...         return BaseException  # Catch the exception
...     return ()  # Don't catch the exception
... 
>>> try: raise OSError(ENOSYS, "Dummy message")
... except catch_not_implemented() as err: print(repr(err))
... 
OSError(38, 'Dummy message')

----------
dependencies: +Catching virtual subclasses in except clauses
nosy: +martin.panter
versions:  -Python 2.7, Python 3.5

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<http://bugs.python.org/issue25537>
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