import exceptions

class UserError(exceptions.Exception):
    def __init__(self, args=None):
        self.args = args

def bar():
    raise UserError, "test error message"

def foo():
    try:
        u = bar()
    except UserError, e:
        print e
        return

>>> foo()
('t', 'e', 's', 't', ' ', 'e', 'r', 'r', 'o', 'r', ' ', 'm', 'e', 's', 's', 'a', 'g', 'e')