Parse and validate a MongoDB URI.

    Returns a dict of the form::

        {
            'nodelist': <list of (host, port) tuples>,
            'username': <username> or None,
            'password': <password> or None,
            'database': <database name> or None,
            'collection': <collection name> or None,
            'options': <dict of MongoDB URI options>
        }

    :Parameters:
        - `uri`: The MongoDB URI to parse.
        - `default_port`: The port number to use when one wasn't specified
          for a host in the URI.
        - `validate`: If ``True`` (the default), validate and normalize all
          options.
        - `warn` (optional): When validating, if ``True`` then will warn
          the user then ignore any invalid options or values. If ``False``,
          validation will error when options are unsupported or values are
          invalid.

    .. versionchanged:: 3.5
        Return the original value of the ``readPreference`` MongoDB URI option
        instead of the validated read preference mode.

    .. versionchanged:: 3.1
        ``warn`` added so invalid options can be ignored.

def parse_uri(uri, default_port=DEFAULT_PORT, validate=True, warn=False):
    """Parse and validate a MongoDB URI.

    Returns a dict of the form::

        {
            'nodelist': <list of (host, port) tuples>,
            'username': <username> or None,
            'password': <password> or None,
            'database': <database name> or None,
            'collection': <collection name> or None,
            'options': <dict of MongoDB URI options>
        }

    :Parameters:
        - `uri`: The MongoDB URI to parse.
        - `default_port`: The port number to use when one wasn't specified
          for a host in the URI.
        - `validate`: If ``True`` (the default), validate and normalize all
          options.
        - `warn` (optional): When validating, if ``True`` then will warn
          the user then ignore any invalid options or values. If ``False``,
          validation will error when options are unsupported or values are
          invalid.

    .. versionchanged:: 3.5
        Return the original value of the ``readPreference`` MongoDB URI option
        instead of the validated read preference mode.

    .. versionchanged:: 3.1
        ``warn`` added so invalid options can be ignored.
    """
    if not uri.startswith(SCHEME):
        raise InvalidURI("Invalid URI scheme: URI "
                         "must begin with '%s'" % (SCHEME,))

    scheme_free = uri[SCHEME_LEN:]

    if not scheme_free:
        raise InvalidURI("Must provide at least one hostname or IP.")

    user = None
    passwd = None
    dbase = None
    collection = None
    options = {}

    host_part, _, path_part = _partition(scheme_free, '/')
    if not host_part:
        host_part = path_part
        path_part = ""

    if not path_part and '?' in host_part:
        raise InvalidURI("A '/' is required between "
                         "the host list and any options.")

    if '@' in host_part:
        userinfo, _, hosts = _rpartition(host_part, '@')
        user, passwd = parse_userinfo(userinfo)
    else:
        hosts = host_part

    if '/' in hosts:
        raise InvalidURI("Any '/' in a unix domain socket must be"
                         " percent-encoded: %s" % host_part)

    hosts = unquote_plus(hosts)
    nodes = split_hosts(hosts, default_port=default_port)

    if path_part:
        if path_part[0] == '?':
            opts = unquote_plus(path_part[1:])
        else:
            dbase, _, opts = map(unquote_plus, _partition(path_part, '?'))
            if '.' in dbase:
                dbase, collection = dbase.split('.', 1)

            if _BAD_DB_CHARS.search(dbase):
                raise InvalidURI('Bad database name "%s"' % dbase)

        if opts:
            options = split_options(opts, validate, warn)

    if dbase is not None:
        dbase = unquote_plus(dbase)
    if collection is not None:
        collection = unquote_plus(collection)

    return {
        'nodelist': nodes,
        'username': user,
        'password': passwd,
        'database': dbase,
        'collection': collection,
        'options': options
    }