Add support for FormField widget

[ahosgood]

There is currently no widget for the FormField (https://wtforms.readthedocs.io/en/2.3.x/fields/#field-enclosures) enclosure to group form fields.

Adding one would allow you to define small, reusable FlaskForm objects with a bunch of fields (kind of like a pattern). This should reduce duplicated code.

The initial code I produced to test this was:

from markupsafe import Markup
from wtforms.widgets.core import html_params


class GovukFieldsetWidget(object):
    def __init__(self):
        pass

    def construct_subfield(self, subfield, field, options):
        subfield_id = subfield.id.replace(field.id + field.separator, "")
        subfield_options = options[subfield_id] if subfield_id in options else {}
        return subfield.widget(subfield, params=subfield_options)

    def __call__(self, field, **kwargs):
        kwargs.setdefault("id", field.id)
        options = kwargs["params"] if "params" in kwargs else {}
        fieldset_options = {}
        html = ["<div class='govuk-form-group'>"]
        html.append("<fieldset class='govuk-fieldset' %s>" % (html_params(**fieldset_options)))
        field_title = (
            options["fieldset"]["legend"]["text"]
            if "fieldset" in options and "legend" in options["fieldset"] and "text" in options["fieldset"]["legend"]
            else field.label.text
            if field.label
            else field.name
            if field.name
            else ""
        )
        if field_title:
            fieldset_classes = (
                options["fieldset"]["legend"]["classes"]
                if "fieldset" in options
                and "legend" in options["fieldset"]
                and "classes" in options["fieldset"]["legend"]
                else ""
            )
            html.append("<legend class='govuk-fieldset__legend %s'>%s</legend>" % (fieldset_classes, field_title))
        if hasattr(field, "task_order"):
            for subfield_name in field.task_order:
                subfield = field[subfield_name]
                html.append(self.construct_subfield(subfield, field, options))
        else:
            for subfield in field:
                html.append(self.construct_subfield(subfield, field, options))
        html.append("</fieldset>")
        html.append("</div>")
        return Markup("".join(html))

You could then use the GovukFieldsetWidget to create a group of fields as a single form where that "pattern" could be validated separately:

from flask_wtf import FlaskForm
from wtforms.fields import IntegerField, FormField, BooleanField
from GovukFieldsetWidget import GovukFieldsetWidget # see class above
from govuk_frontend_wtf.wtforms_widgets import GovCheckboxInput, GovTextInput


class ChargeAmount(FlaskForm):
    class Meta:
        csrf = False

    task_order = ["charge_amount", "undisclosed_amount"]
    charge_amount = IntegerField(
        widget=GovTextInput(),
        validators=[],
    )
    undisclosed_amount = BooleanField(widget=GovCheckboxInput())


pattern = FormField(
    ChargeAmount,
    widget=GovukFieldsetWidget(),
)