How to use the mechanicalsoup.stateful_browser._BrowserState function in MechanicalSoup

a download of a file. All other arguments are forwarded to
        :func:`Browser.submit`.
        """
        self.form.choose_submit(btnName)

        referer = self.url
        if referer is not None:
            if 'headers' in kwargs:
                kwargs['headers']['Referer'] = referer
            else:
                kwargs['headers'] = {'Referer': referer}

        resp = self.submit(self.__state.form, url=self.__state.url,
                           *args, **kwargs)
        if update_state:
            self.__state = _BrowserState(page=resp.soup, url=resp.url,
                                         request=resp.request)
        return resp

Any change (`select_form`, or any value filled-in in the form) made to
        the current page before refresh is discarded.

        :raise ValueError: Raised if no refreshable page is loaded, e.g., when
            using the shallow ``Browser`` wrapper functions.

        :return: Response of the request."""
        old_request = self.__state.request
        if old_request is None:
            raise ValueError('The current page is not refreshable. Either no '
                             'page is opened or low-level browser methods '
                             'were used to do so')

        resp = self.session.send(old_request)
        Browser.add_soup(resp, self.soup_config)
        self.__state = _BrowserState(page=resp.soup, url=resp.url,
                                     request=resp.request)
        return resp

def open(self, url, *args, **kwargs):
        """Open the URL and store the Browser's state in this object.
        All arguments are forwarded to :func:`Browser.get`.

        :return: Forwarded from :func:`Browser.get`.
        """
        if self.__verbose == 1:
            sys.stdout.write('.')
            sys.stdout.flush()
        elif self.__verbose >= 2:
            print(url)

        resp = self.get(url, *args, **kwargs)
        self.__state = _BrowserState(page=resp.soup, url=resp.url,
                                     request=resp.request)
        return resp

def _update_state(self, response):
        """Dry up the setters from http post and get methods."""

        self._set_raw_content(response.content)
        self._set_status(response.status_code)
        self._add_soup(response, self.soup_config)
        self.__state = _BrowserState(page=response.soup,
                                     url=response.url,
                                     request=response.request)

def open_fake_page(self, page_text, url=None, soup_config=None):
        """Mock version of :func:`open`.

        Behave as if opening a page whose text is ``page_text``, but do not
        perform any network access. If ``url`` is set, pretend it is the page's
        URL. Useful mainly for testing.
        """
        soup_config = soup_config or self.soup_config
        self.__state = _BrowserState(
            page=bs4.BeautifulSoup(page_text, **soup_config),
            url=url)

def __init__(self, *args, **kwargs):
        self._set_raw_content(content=b'')
        self._set_status(status='')
        self.__state = _BrowserState()
        self._test_true = False
        self.timeout_exception = False
        self.flags = kwargs.pop('flags', None)
        self.priority = kwargs.pop('priority', 0)
        if kwargs.pop('meta', None):
            self._meta = dict(kwargs.pop('meta'))
        else:
            self._meta = None
        self.callback = None
        self.errback = None
        super().__init__(*args, **kwargs)

def open_fake_page(self, page_text, status_code=None, url=None, soup_config=None):
        """Mock version of :func:`open`.

        Behave as if opening a page whose text is ``page_text``, but do not
        perform any network access. If ``url`` is set, pretend it is the page's
        URL. Useful mainly for testing.
        """
        soup_config = soup_config or self.soup_config
        self._test_true = True
        self._set_raw_content(page_text.encode())
        self._set_status(status_code)

        self.__state = _BrowserState(
            page=bs4.BeautifulSoup(page_text, **soup_config),
            url=url)