# 异常

# '''
try:
    x = int(input("Please enter a number: "))
except ValueError:
    print("Oops!  That was no valid number.  Try again   ")
finally:
    print("xxxxxxxx")
# '''


class Error(Exception):
    """Base class for exceptions in this module."""
    pass


class InputError(Error):
    """Exception raised for errors in the input.

    Attributes:
        expression -- input expression in which the error occurred
        message -- explanation of the error
    """

    def __init__(self, expression, message):
        self.expression = expression
        self.message = message


class TransitionError(Error):
    """Raised when an operation attempts a state transition that's not
    allowed.

    Attributes:
        previous -- state at beginning of transition
        next -- attempted new state
        message -- explanation of why the specific transition is not allowed
    """

    def __init__(self, previous, next, message):
        self.previous = previous
        self.next = next
        self.message = message