Expose PyErr_GivenExceptionMatches in inspect

Ideas
#1

the try/except exception matching is now documented such that “except does not match virtual subclasses of the specified exception type”

see [doc] clarify that except does not match virtual subclasses of the specified exception type · Issue #56238 · python/cpython · GitHub

I’d like to be able to emulate this behaviour in a few places, such as implementing context managers with def __exit__( eg:

class ExampleContextManager:
    def __enter__(self):
        return self

    def __exit__(self, typ, value, traceback):
        if typ is None:
            return False

        if value is None:
            value = typ()

        if inspect.exceptionmatches(exc, SomeType):
            # handle SomeType case

So I’m proposing a python implementation of PyErr_GivenExceptionMatches be made available as inspect.exceptionmatches

#2

Since PyErr_GivenExceptionMatches is part of the public c api, I don’t see any reason not to expose it in python.

2 Likes
#3

Perhaps it should go in operator, since that’s where function versions of other fundamental operations are exposed. Though it’s obviously very inefficient, here’s a method for a pure-Python implementation:

def exception_matches(exc, expected):
    traceback = exc.__traceback__
    context = exc.__context__
    cause = exc.__cause__
    suppress = exc.__suppress_context__
    try:
        try:
            raise exc
        except expected:
            return True
        except:
            return False
    finally:
        exc.__traceback__ = traceback
        exc.__context__ = context
        exc.__cause__ = cause
        exc.__suppress_context__ = suppress
#4

Wouldn’t a simpler Python implementation just duplicate the high-level exception matching logic?

def exception_matches(exc, expected):
    """Return whether or not "raise exc" will be caught by "except expected"."""
    if isinstance(exc, type):
        return isubclass(exc, expected)
    return isinstance(exc, excepted)

Have I missed any cases, or some other issue?

It would be nice to have an official way to do this. The operator module makes sense to me.

#5

Unlike regular instance checks, exception handling skips all of the various hooks - the abc modules, __class__, __instancecheck__() and __subclasscheck__(), etc. Examining the implementation of PyErr_GivenExceptionMatches() though suggests perhaps a better function to expose PyType_IsSubtype(), since the PyErr_GivenExceptionMatches() just recursively handles tuples and verifies they are exceptions.

1 Like