为何多父类派生的异常无法捕获其父类异常实例？

Great question! Let's break down exactly why this happens in Python:

Core Rule of Python Exception Handling #

Python's exception catching works by checking if the raised exception instance is an instance of (or subclass of) the exception class listed in the except clause. In other words:

• If you raise ChildError(), it can be caught by except ParentError: (since ChildError is a subclass of ParentError)
• But if you raise ParentError(), it cannot be caught by except ChildError: (because ParentError is not a subclass of ChildError)

Applying This to Your Code #

Let's map this to your example:

• MixedError inherits from both Error1 and Error2, so issubclass(MixedError, Error2) returns True
• However, the instance you're raising is Error2() — when we check isinstance(Error2(), MixedError), this returns False

That's the key! The except MixedError: clause only catches instances that are either:

• Directly MixedError instances, or
• Instances of subclasses of MixedError

Since Error2 is the parent class of MixedError, not a child, its instances don't match the MixedError type check.

Test This Yourself #

If you add a quick check to your code, you'll see this clearly:

print(isinstance(Error2(), MixedError))  # Output: False
print(isinstance(MixedError(), Error2))  # Output: True

Fixing the Behavior #

If you want the except MixedError: clause to trigger, you need to raise a MixedError instance instead:

try:
    print('before 1')
    raise MixedError()  # Changed from Error2()
    print('after 1')
except MixedError:
    print('Caught it with a mixin 1!')
except Exception:
    print('Big catcher here 1!')

This will output: before 1 Caught it with a mixin 1!

Alternatively, if you want to catch Error2 instances, use except Error2: directly instead of relying on the subclass.

内容的提问来源于stack exchange，提问作者Jon