22.9 Mapping for Exceptions

The mapping for exceptions is based on the inheritance hierarchy shown in Figure 22.1

Figure 22.1. Inheritance structure for Ice exceptions.

The ancestor of all exceptions is exceptions.Exception, from which Ice.Exception is derived. Ice.LocalException and Ice.UserException are derived from Ice.Exception and form the base for all run-time and user exceptions.

Here is a fragment of the Slice definition for our world time server from Section 4.10.5 on page 125 once more:

exception GenericError {
    string reason;
};
exception BadTimeVal extends GenericError {};
exception BadZoneName extends GenericError {};

These exception definitions map as follows:

class GenericError(Ice.UserException):
    def __init__(self, reason=''):
        self.reason = reason

    def ice_name(self):
        # ...

    def __str__(self):
        # ...

class BadTimeVal(GenericError):
    def __init__(self, reason=''):
        GenericError.__init__(self, reason)

    def ice_name(self):
        # ...

    def __str__(self):
        # ...

class BadZoneName(GenericError):
    def __init__(self, reason=''):
        GenericError.__init__(self, reason)

    def ice_name(self):
        # ...

    def __str__(self):
        # ...

Each Slice exception is mapped to a Python class with the same name. The inheritance structure of the Slice exceptions is preserved for the generated classes, so BadTimeVal and BadZoneName inherit from GenericError.

Each exception member corresponds to an attribute of the instance, which the constructor initializes to a default value appropriate for its type. Although BadTimeVal and BadZoneName do not declare data members, their constructors still accept a value for the inherited data member reason in order to pass it to the constructor of the base exception GenericError.

Each exception also defines the ice_name method to return the name of the exception, and the special method __str__ to return a stringified representation of the exception and its members.

All user exceptions are derived from the base class Ice.UserException. This allows you to catch all user exceptions generically by installing a handler for Ice.UserException. Similarly, you can catch all Ice run-time exceptions with a handler for Ice.LocalException, and you can catch all Ice exceptions with a handler for Ice.Exception.