global retcodes
retcodes = {0: "OK", 1: "Error_X", 2: "Error_Y"}

#############################
# [A] one custom exception for all purposes, but with a descriptive message
#############################

class single_custom_exception(Exception):
    def __init__(self, retcode, message):
        self.retcode = retcode
        message += " [%s]" % retcodes.get(retcode)
        Exception.__init__(self, message)

#############################
# [B] generating custom exceptions with a class factory
#############################

class BaseError(Exception): pass

def throw(retcode, message):
    """class factory that generates an error class for
    each return code label"""
    class custom_exception(BaseError):
        def __init__(self):
            BaseError.__init__(self, message)
    exception = retcodes.get(retcode)
    custom_exception.__name__ = exception
    return custom_exception

#######
# demonstrate use
#######

def someFunc(scenario):
    """a nonsensical function"""
    retcode = 1
    if retcode > 0:
        message = "error calling %r" % someFunc.__name__
        if scenario == "[A]":
            raise single_custom_exception(retcode, message)
        elif scenario == "[B]":
            raise throw(retcode, message)

someFunc("[A]")
someFunc("[B]")

# this may be useful
print issubclass(throw(1, "some message"), BaseError)  # returns True