<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _prepare_source(selector, source):
 """Normalize source rows and selectors."""

    tablename, fields = get_data_specifier(selector)
    if len(fields) != 1:
        raise ItsdbError(
            'Selector must specify exactly one data column: {}'
            .format(selector)
        )
    if isinstance(source, TestSuite):
        if not tablename:
            tablename = source.relations.find(fields[0])[0]
        source = source[tablename]
    cols = list(source.fields.keys()) + fields
    return source, cols