Extract input data from the form to pass to a Requests session.

def _request(self, form, url=None, **kwargs):
        """Extract input data from the form to pass to a Requests session."""
        method = str(form.get("method", "get"))
        action = form.get("action")
        url = urllib.parse.urljoin(url, action)
        if url is None:  # This happens when both `action` and `url` are None.
            raise ValueError('no URL to submit to')

        # read https://www.w3.org/TR/html52/sec-forms.html
        data = kwargs.pop("data", dict())
        files = kwargs.pop("files", dict())

        # Use a list of 2-tuples to better reflect the behavior of browser QSL.
        # Requests also retains order when encoding form data in 2-tuple lists.
        data = [(k, v) for k, v in data.items()]

        # Process form tags in the order that they appear on the page,
        # skipping those tags that do not have a name-attribute.
        selector = ",".join("{}[name]".format(i) for i in
                            ("input", "button", "textarea", "select"))
        for tag in form.select(selector):
            name = tag.get("name")  # name-attribute of tag

            # Skip disabled elements, since they should not be submitted.
            if tag.has_attr('disabled'):
                continue

            if tag.name == "input":
                if tag.get("type", "").lower() in ("radio", "checkbox"):
                    if "checked" not in tag.attrs:
                        continue
                    value = tag.get("value", "on")
                else:
                    # browsers use empty string for inputs with missing values
                    value = tag.get("value", "")

                if tag.get("type", "").lower() == "file":
                    # read http://www.cs.tut.fi/~jkorpela/forms/file.html
                    # in browsers, file upload only happens if the form
                    # (or submit button) enctype attribute is set to
                    # "multipart/form-data". We don't care, simplify.
                    filename = value
                    if filename != "" and isinstance(filename, string_types):
                        content = open(filename, "rb")
                    else:
                        content = ""
                    # If value is the empty string, we still pass it for
                    # consistency with browsers (see #250).
                    files[name] = (filename, content)
                else:
                    data.append((name, value))

            elif tag.name == "button":
                if tag.get("type", "").lower() in ("button", "reset"):
                    continue
                else:
                    data.append((name, tag.get("value", "")))

            elif tag.name == "textarea":
                data.append((name, tag.text))

            elif tag.name == "select":
                # If the value attribute is not specified, the content will
                # be passed as a value instead.
                options = tag.select("option")
                selected_values = [i.get("value", i.text) for i in options
                                   if "selected" in i.attrs]
                if "multiple" in tag.attrs:
                    for value in selected_values:
                        data.append((name, value))
                elif selected_values:
                    # A standard select element only allows one option to be
                    # selected, but browsers pick last if somehow multiple.
                    data.append((name, selected_values[-1]))
                elif options:
                    # Selects the first option if none are selected
                    first_value = options[0].get("value", options[0].text)
                    data.append((name, first_value))

        if method.lower() == "get":
            kwargs["params"] = data
        else:
            kwargs["data"] = data

        return self.session.request(method, url, files=files, **kwargs)