Open an existing database

def open(self):
        """Open an existing database"""
        if self._table_exists():
            self.mode = "open"
            # get table info
            self._get_table_info()
            self.types = dict([ (f[0],self.conv_func[f[1].upper()]) 
                for f in self.fields if f[1].upper() in self.conv_func ])
            return self
        else:
            # table not found
            raise IOError,"Table %s doesn't exist" %self.name