def secure_filename(filename):
    r"""Borrowed from :mod:`werkzeug.utils`, under the BSD 3-clause license.

    Pass it a filename and it will return a secure version of it.  This
    filename can then safely be stored on a regular file system and passed
    to :func:`os.path.join`.  The filename returned is an ASCII only string
    for maximum portability.

    On windows systems the function also makes sure that the file is not
    named after one of the special device files.

    >>> secure_filename("My cool movie.mov")
    'My_cool_movie.mov'
    >>> secure_filename("../../../etc/passwd")
    'etc_passwd'
    >>> secure_filename(u'i contain cool \xfcml\xe4uts.txt')
    'i_contain_cool_umlauts.txt'

    The function might return an empty filename.  It's your responsibility
    to ensure that the filename is unique and that you generate random
    filename if the function returned an empty one.

    :param filename: the filename to secure
    """
    _filename_ascii_strip_re = re.compile(r'[^A-Za-z0-9_.-]')
    _windows_device_files = ('CON', 'AUX', 'COM1', 'COM2', 'COM3', 'COM4',
                             'LPT1', 'LPT2', 'LPT3', 'PRN', 'NUL')
    if isinstance(filename, text_type):
        from unicodedata import normalize
        filename = normalize('NFKD', filename).encode('ascii', 'ignore')
        if not PY2:
            filename = filename.decode('ascii')
    for sep in os.path.sep, os.path.altsep:
        if sep:
            filename = filename.replace(sep, ' ')
    filename = str(_filename_ascii_strip_re.sub('', '_'.join(
                   filename.split()))).strip('._')

    # on nt a couple of special files are present in each folder.  We
    # have to ensure that the target file is not such a filename.  In
    # this case we prepend an underline
    if os.name == 'nt' and filename and \
       filename.split('.')[0].upper() in _windows_device_files:
        filename = '_' + filename

    return filename
def _subset_sounding_data(
        example_dict, netcdf_dataset_object, example_indices_to_keep,
        field_names_to_keep, heights_to_keep_m_agl):
    """Subsets sounding data by field and height.

    :param example_dict: See doc for `_subset_radar_data`.
    :param netcdf_dataset_object: Same.
    :param example_indices_to_keep: Same.
    :param field_names_to_keep: 1-D list of field names to keep.  If None, will
        keep all fields.
    :param heights_to_keep_m_agl: 1-D numpy array of heights to keep.  If None,
        will keep all heights.
    :return: example_dict: Same as input but with the following exceptions.

    [1] Keys "sounding_field_names" and "sounding_heights_m_agl" may have
        different values.
    [2] Key "sounding_matrix" has been added.
    """

    if field_names_to_keep is None:
        field_names_to_keep = copy.deepcopy(example_dict[SOUNDING_FIELDS_KEY])
    if heights_to_keep_m_agl is None:
        heights_to_keep_m_agl = example_dict[SOUNDING_HEIGHTS_KEY] + 0

    error_checking.assert_is_numpy_array(
        numpy.array(field_names_to_keep), num_dimensions=1
    )

    heights_to_keep_m_agl = numpy.round(heights_to_keep_m_agl).astype(int)
    error_checking.assert_is_numpy_array(
        heights_to_keep_m_agl, num_dimensions=1)

    sounding_matrix = numpy.array(
        netcdf_dataset_object.variables[SOUNDING_MATRIX_KEY][
            example_indices_to_keep, ...
        ],
        dtype=float
    )

    # TODO(thunderhoser): This is a HACK.
    spfh_index = example_dict[SOUNDING_FIELDS_KEY].index(
        soundings.SPECIFIC_HUMIDITY_NAME)
    temp_index = example_dict[SOUNDING_FIELDS_KEY].index(
        soundings.TEMPERATURE_NAME)
    pressure_index = example_dict[SOUNDING_FIELDS_KEY].index(
        soundings.PRESSURE_NAME)
    theta_v_index = example_dict[SOUNDING_FIELDS_KEY].index(
        soundings.VIRTUAL_POTENTIAL_TEMPERATURE_NAME)

    sounding_matrix[..., spfh_index][
        numpy.isnan(sounding_matrix[..., spfh_index])
    ] = 0.

    nan_example_indices, nan_height_indices = numpy.where(numpy.isnan(
        sounding_matrix[..., theta_v_index]
    ))

    if len(nan_example_indices) > 0:
        this_temp_matrix_kelvins = sounding_matrix[..., temp_index][
            nan_example_indices, nan_height_indices]

        this_pressure_matrix_pa = sounding_matrix[..., pressure_index][
            nan_example_indices, nan_height_indices]

        this_thetav_matrix_kelvins = (
            temp_conversion.temperatures_to_potential_temperatures(
                temperatures_kelvins=this_temp_matrix_kelvins,
                total_pressures_pascals=this_pressure_matrix_pa)
        )

        sounding_matrix[..., theta_v_index][
            nan_example_indices, nan_height_indices
        ] = this_thetav_matrix_kelvins

    these_indices = numpy.array([
        example_dict[SOUNDING_FIELDS_KEY].index(f)
        for f in field_names_to_keep
    ], dtype=int)

    sounding_matrix = sounding_matrix[..., these_indices]

    these_indices = numpy.array([
        numpy.where(example_dict[SOUNDING_HEIGHTS_KEY] == h)[0][0]
        for h in heights_to_keep_m_agl
    ], dtype=int)

    sounding_matrix = sounding_matrix[..., these_indices, :]

    example_dict[SOUNDING_FIELDS_KEY] = field_names_to_keep
    example_dict[SOUNDING_HEIGHTS_KEY] = heights_to_keep_m_agl
    example_dict[SOUNDING_MATRIX_KEY] = sounding_matrix

    return example_dict