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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def import_locations(self, marker_file):
 """Import trigpoint database files. ``import_locations()`` returns a dictionary with keys containing the trigpoint identifier, and values that are :class:`Trigpoint` objects. It expects trigpoint marker files in the format provided at alltrigs-wgs84.txt_, which is the following format:: H SOFTWARE NAME & VERSION I GPSU 4.04, S SymbolSet=0 W,500936,N52.066035,W000.281449, 37.0,Broom Farm W,501097,N52.010585,W000.173443, 97.0,Bygrave W,505392,N51.910886,W000.186462, 136.0,Sish Lane Any line not consisting of 6 comma separated fields will be ignored. The reader uses the :mod:`csv` module, so alternative whitespace formatting should have no effect. The above file processed by ``import_locations()`` will return the following ``dict`` object:: {500936: point.Point(52.066035, -0.281449, 37.0, "Broom Farm"), 501097: point.Point(52.010585, -0.173443, 97.0, "Bygrave"), 505392: point.Point(51.910886, -0.186462, 136.0, "Sish Lane")} Args: marker_file (iter):
 Trigpoint marker data to read Returns: dict: Named locations with :class:`Trigpoint` objects Raises: ValueError: Invalid value for ``marker_file`` .. _alltrigs-wgs84.txt: http://www.haroldstreet.org.uk/trigpoints/ """

        self._marker_file = marker_file
        field_names = ('tag', 'identity', 'latitude', 'longitude', 'altitude',
                       'name')
        pos_parse = lambda x, s: float(s[1:]) if s[0] == x else 0 - float(s[1:])
        latitude_parse = partial(pos_parse, 'N')
        longitude_parse = partial(pos_parse, 'E')
        # A value of 8888.0 denotes unavailable data
        altitude_parse = lambda s: None if s.strip() == '8888.0' else float(s)
        field_parsers = (str, int, latitude_parse, longitude_parse,
                         altitude_parse, str)

        data = utils.prepare_csv_read(marker_file, field_names)

        for row in (x for x in data if x['tag'] == 'W'):
            for name, parser in zip(field_names, field_parsers):
                row[name] = parser(row[name])
            del row['tag']
            try:
                self[row['identity']] = Trigpoint(**row)
            except TypeError:
                # Workaround formatting error in 506514 entry that contains
                # spurious comma
                del row[None]
                self[row['identity']] = Trigpoint(**row)