def get_string(dev, index, langid = None):
    r"""Retrieve a string descriptor from the device.

    dev is the Device object which the string will be read from.

    index is the string descriptor index and langid is the Language
    ID of the descriptor. If langid is omitted, the string descriptor
    of the first Language ID will be returned.

    Zero is never the index of a real string. The USB spec allows a device to
    use zero in a string index field to indicate that no string is provided.
    So the caller does not have to treat that case specially, this function
    returns None if passed an index of zero, and generates no traffic
    to the device.

    The return value is the unicode string present in the descriptor, or None
    if the requested index was zero.

    It is a ValueError to request a real string (index not zero), if: the
    device's langid tuple is empty, or with an explicit langid the device does
    not support.
    """
    if 0 == index:
        return None

    from usb.control import get_descriptor
    langids = dev.langids

    if 0 == len(langids):
        raise ValueError("The device has no langid")
    if langid is None:
        langid = langids[0]
    elif langid not in langids:
        raise ValueError("The device does not support the specified langid")

    buf = get_descriptor(
                dev,
                255, # Maximum descriptor size
                DESC_TYPE_STRING,
                index,
                langid
            )
    if hexversion >= 0x03020000:
        return buf[2:buf[0]].tobytes().decode('utf-16-le')
    else:
        return buf[2:buf[0]].tostring().decode('utf-16-le')
r"""Retrieve a string descriptor from the device.

    dev is the Device object which the string will be read from.

    index is the string descriptor index and langid is the Language
    ID of the descriptor. If langid is omitted, the string descriptor
    of the first Language ID will be returned.

    Zero is never the index of a real string. The USB spec allows a device to
    use zero in a string index field to indicate that no string is provided.
    So the caller does not have to treat that case specially, this function
    returns None if passed an index of zero, and generates no traffic
    to the device.

    The return value is the unicode string present in the descriptor, or None
    if the requested index was zero.

    It is a ValueError to request a real string (index not zero), if: the
    device's langid tuple is empty, or with an explicit langid the device does
    not support.