Nick Coghlan added the comment:
Thanks for being understanding of the decision. Regarding my comment above
about __subclasscheck__ potentially letting you implement this without changing
the signature of suppress, here's an example of how you might be able to do
that.
First, define a metaclass that delegates type checks to the type itself
(similar to abc.ABCMeta):
class FilteredExceptionMeta(type):
def __subclasscheck__(cls, other):
return cls.__subclasshook__(other)
def __instancecheck__(cls, other):
return cls.__subclasshook__(type(other))
Then, define a factory function to create custom classes based on that
metaclass:
def filtered_exc(exc_type, *, unless=()):
class _FilteredException(metaclass=FilteredExceptionMeta):
@classmethod
def __subclasshook__(cls, other):
return (issubclass(other, exc_type)
and not issubclass(other, unless))
return _FilteredException
>>> from contextlib import suppress
>>> selective_filter = suppress(filtered_exc(OSError, unless=FileNotFoundError))
>>> with selective_filter:
... raise OSError("Suppressed")
...
>>> with selective_filter:
... raise FileNotFoundError("Not suppressed")
...
Traceback (most recent call last):
File "<stdin>", line 2, in <module>
FileNotFoundError: Not suppressed
This works because suppress() calls issubclass() explicitly, unlike the current
implementation of except clause processing.
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<http://bugs.python.org/issue27814>
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