How to use the webtest.sel.Element function in WebTest

def get(self, tag, **kwargs):
        """Return an element matching ``tag``, an ``attribute`` and an
        ``index``. For example:

            resp.doc.get('input', name='go') => xpath=//input[@name="go"]
            resp.doc.get('li', description='Item') => xpath=//li[.="Item"]
        """
        locator = _eval_xpath(tag, **kwargs)
        return Element(self.resp, locator)

def css(self, selector):
        """Get an :class:`~webtest.browser.Element` using a css selector"""
        return Element(self.resp, 'css=%s' % selector)

class Field(forms.Field, Element):

    classes = {}

    def __init__(self, *args, **kwargs):
        super(Field, self).__init__(*args, **kwargs)
        self.browser = self.form.browser
        self.options = []
        self.selectedIndices = []
        self._forced_values = []
        self.locator = _eval_xpath(self.tag,
                                   locator=self.form.locator,
                                    name=self.name)

    value = property(Element.value__get, Element.value__set)


class Select(Field):
    """Field representing ``<select>``"""

    def force_value(self, value):
        self.select('value=%s' % value)

    def value__set(self, value):
        self.select('value=%s' % value)

    def value__get(self):
        return self.getSelectedValue()

    value = property(value__get, value__set)
</select>

def __contains__(self, s):
        if isinstance(s, Element):
            return s.exist()
        return self.browser.isTextPresent(json.dumps(s))

    def __call__(self, locator):
        return Element(locator)


###########
# Forms
###########


class Field(forms.Field, Element):

    classes = {}

    def __init__(self, *args, **kwargs):
        super(Field, self).__init__(*args, **kwargs)
        self.browser = self.form.browser
        self.options = []
        self.selectedIndices = []
        self._forced_values = []
        self.locator = _eval_xpath(self.tag,
                                   locator=self.form.locator,
                                    name=self.name)

    value = property(Element.value__get, Element.value__set)

def __call__(self, locator):
        return Element(locator)

def xpath(self, path):
        """Get an :class:`~webtest.browser.Element` using xpath"""
        return Element(self.resp, 'xpath=%s' % path)

def __contains__(self, s):
        if isinstance(s, Element):
            return s.isElementPresent()
        return s in self.text

Field.classes['button'] = Submit

Field.classes['image'] = Submit


class Forms(object):

    def __init__(self, resp):
        self.resp = resp

    def __getitem__(self, key):
        return Form(self.resp, key)


class Form(forms.Form, Element):
    """See :class:`~webtest.Form`"""

    FieldClass = Field

    def __init__(self, resp, id):
        self.resp = resp
        self.test_app = resp.test_app
        self.browser = resp.browser
        if isinstance(id, int):
            self.locator = _eval_xpath('form', index=id)
        else:
            self.locator = _eval_xpath('form', id=id)
        if not self:
            raise LookupError('No form found at %s' % self.locator)
        form = self.eval('e.innerHTML')
        super(Form, self).__init__(resp, '<form>%s</form>' % form)