@Appender((_shared_docs['valid_index'] % {'position': 'first', 'klass': 'NDFrame'}))
def first_valid_index(self):
    return self._find_valid_index('first')
@Appender((_shared_docs['valid_index'] % {'position': 'first', 'klass': 'NDFrame'}))
def first_valid_index(self):
    return self._find_valid_index('first')