twisted.python.failure.Failure.trap(self, *errorTypes) method documentation

Trap this failure if its type is in a predetermined list.

This allows you to trap a Failure in an error callback. It will be automatically re-raised if it is not a type that you expect.

The reason for having this particular API is because it's very useful in Deferred errback chains:

    def _ebFoo(self, failure):
        r = failure.trap(Spam, Eggs)
        print('The Failure is due to either Spam or Eggs!')
        if r == Spam:
            print('Spam did it!')
        elif r == Eggs:
            print('Eggs did it!')

If the failure is not a Spam or an Eggs, then the Failure will be 'passed on' to the next errback. In Python 2 the Failure will be raised; in Python 3 the underlying exception will be re-raised.