def open(server=None, url=None, ip=None, port=None, name=None, https=None, auth=None, verify_ssl_certificates=True,
             proxy=None, cookies=None, verbose=True, _msgs=None):
        r"""
        Establish connection to an existing H2O server.

        The connection is not kept alive, so what this method actually does is it attempts to connect to the
        specified server, and checks that the server is healthy and responds to REST API requests. If the H2O server
        cannot be reached, an :class:`H2OConnectionError` will be raised. On success this method returns a new
        :class:`H2OConnection` object, and it is the only "official" way to create instances of this class.

        There are 3 ways to specify the target to connect to (these settings are mutually exclusive):

            * pass a ``server`` option,
            * pass the full ``url`` for the connection,
            * provide a triple of parameters ``ip``, ``port``, ``https``.

        :param H2OLocalServer server: connect to the specified local server instance. There is a slight difference
            between connecting to a local server by specifying its ip and address, and connecting through
            an H2OLocalServer instance: if the server becomes unresponsive, then having access to its process handle
            will allow us to query the server status through OS, and potentially provide snapshot of the server's
            error log in the exception information.
        :param url: full url of the server to connect to.
        :param ip: target server's IP address or hostname (default "localhost").
        :param port: H2O server's port (default 54321).
        :param name: H2O cluster name.
        :param https: if True then connect using https instead of http (default False).
        :param verify_ssl_certificates: if False then SSL certificate checking will be disabled (default True). This
            setting should rarely be disabled, as it makes your connection vulnerable to man-in-the-middle attacks. When
            used, it will generate a warning from the requests library. Has no effect when ``https`` is False.
        :param auth: authentication token for connecting to the remote server. This can be either a
            (username, password) tuple, or an authenticator (AuthBase) object. Please refer to the documentation in
            the ``requests.auth`` module.
        :param proxy: url address of a proxy server. If you do not specify the proxy, then the requests module
            will attempt to use a proxy specified in the environment (in HTTP_PROXY / HTTPS_PROXY variables). We
            check for the presence of these variables and issue a warning if they are found. In order to suppress
            that warning and use proxy from the environment, pass ``proxy="(default)"``.
        :param cookies: Cookie (or list of) to add to requests
        :param verbose: if True, then connection progress info will be printed to the stdout.
        :param _msgs: custom messages to display during connection. This is a tuple (initial message, success message,
            failure message).

        :returns: A new :class:`H2OConnection` instance.
        :raises H2OConnectionError: if the server cannot be reached.
        :raises H2OServerError: if the server is in an unhealthy state (although this might be a recoverable error, the
            client itself should decide whether it wants to retry or not).
        """
        if server is not None:
            assert_is_type(server, H2OLocalServer)
            assert_is_type(ip, None, "`ip` should be None when `server` parameter is supplied")
            assert_is_type(url, None, "`url` should be None when `server` parameter is supplied")
            assert_is_type(name, None, "`name` should be None when `server` parameter is supplied")
            if not server.is_running():
                raise H2OConnectionError("Unable to connect to server because it is not running")
            ip = server.ip
            port = server.port
            scheme = server.scheme
            context_path = ''
        elif url is not None:
            assert_is_type(url, str)
            assert_is_type(ip, None, "`ip` should be None when `url` parameter is supplied")
            assert_is_type(name, str, None)
            # We don't allow any Unicode characters in the URL. Maybe some day we will...
            match = assert_matches(url, H2OConnection.url_pattern)
            scheme = match.group(1)
            ip = match.group(2)
            port = int(match.group(3))
            context_path = '' if match.group(4) is None else "%s" % (match.group(4))
        else:
            if ip is None: ip = str("localhost")
            if port is None: port = 54321
            if https is None: https = False
            if is_type(port, str) and port.isdigit(): port = int(port)
            assert_is_type(ip, str)
            assert_is_type(port, int)
            assert_is_type(name, str, None)
            assert_is_type(https, bool)
            assert_matches(ip, r"(?:[\w-]+\.)*[\w-]+")
            assert_satisfies(port, 1 <= port <= 65535)
            scheme = "https" if https else "http"
            context_path = ''

        if verify_ssl_certificates is None: verify_ssl_certificates = True
        assert_is_type(verify_ssl_certificates, bool)
        assert_is_type(proxy, str, None)
        assert_is_type(auth, AuthBase, (str, str), None)
        assert_is_type(cookies, str, [str], None)
        assert_is_type(_msgs, None, (str, str, str))

        conn = H2OConnection()
        conn._verbose = bool(verbose)
        conn._local_server = server
        conn._base_url = "%s://%s:%d%s" % (scheme, ip, port, context_path)
        conn._name = server.name if server else name
        conn._verify_ssl_cert = bool(verify_ssl_certificates)
        conn._auth = auth
        conn._cookies = cookies
        conn._proxies = None
        if proxy and proxy != "(default)":
            conn._proxies = {scheme: proxy}
        elif not proxy:
            # Give user a warning if there are any "*_proxy" variables in the environment. [PUBDEV-2504]
            # To suppress the warning pass proxy = "(default)".
            for name in os.environ:
                if name.lower() == scheme + "_proxy":
                    warn("Proxy is defined in the environment: %s. "
                         "This may interfere with your H2O Connection." % name)

        try:
            retries = 20 if server else 5
            conn._stage = 1
            conn._timeout = 3.0
            conn._cluster = conn._test_connection(retries, messages=_msgs)
            # If a server is unable to respond within 1s, it should be considered a bug. However we disable this
            # setting for now, for no good reason other than to ignore all those bugs :(
            conn._timeout = None
            # This is a good one! On the surface it registers a callback to be invoked when the script is about
            # to finish, but it also has a side effect in that the reference to current connection will be held
            # by the ``atexit`` service till the end -- which means it will never be garbage-collected.
            atexit.register(lambda: conn.close())
        except Exception:
            # Reset _session_id so that we know the connection was not initialized properly.
            conn._stage = 0
            raise
        return conn
r"""
        Establish connection to an existing H2O server.

        The connection is not kept alive, so what this method actually does is it attempts to connect to the
        specified server, and checks that the server is healthy and responds to REST API requests. If the H2O server
        cannot be reached, an :class:`H2OConnectionError` will be raised. On success this method returns a new
        :class:`H2OConnection` object, and it is the only "official" way to create instances of this class.

        There are 3 ways to specify the target to connect to (these settings are mutually exclusive):

            * pass a ``server`` option,
            * pass the full ``url`` for the connection,
            * provide a triple of parameters ``ip``, ``port``, ``https``.

        :param H2OLocalServer server: connect to the specified local server instance. There is a slight difference
            between connecting to a local server by specifying its ip and address, and connecting through
            an H2OLocalServer instance: if the server becomes unresponsive, then having access to its process handle
            will allow us to query the server status through OS, and potentially provide snapshot of the server's
            error log in the exception information.
        :param url: full url of the server to connect to.
        :param ip: target server's IP address or hostname (default "localhost").
        :param port: H2O server's port (default 54321).
        :param name: H2O cluster name.
        :param https: if True then connect using https instead of http (default False).
        :param verify_ssl_certificates: if False then SSL certificate checking will be disabled (default True). This
            setting should rarely be disabled, as it makes your connection vulnerable to man-in-the-middle attacks. When
            used, it will generate a warning from the requests library. Has no effect when ``https`` is False.
        :param auth: authentication token for connecting to the remote server. This can be either a
            (username, password) tuple, or an authenticator (AuthBase) object. Please refer to the documentation in
            the ``requests.auth`` module.
        :param proxy: url address of a proxy server. If you do not specify the proxy, then the requests module
            will attempt to use a proxy specified in the environment (in HTTP_PROXY / HTTPS_PROXY variables). We
            check for the presence of these variables and issue a warning if they are found. In order to suppress
            that warning and use proxy from the environment, pass ``proxy="(default)"``.
        :param cookies: Cookie (or list of) to add to requests
        :param verbose: if True, then connection progress info will be printed to the stdout.
        :param _msgs: custom messages to display during connection. This is a tuple (initial message, success message,
            failure message).

        :returns: A new :class:`H2OConnection` instance.
        :raises H2OConnectionError: if the server cannot be reached.
        :raises H2OServerError: if the server is in an unhealthy state (although this might be a recoverable error, the
            client itself should decide whether it wants to retry or not).