Refactor GovukCheckboxField and all variants

Aims to make the structure of their code more like
GovukTextInputField so this convention can be
extended for radios.

It does that by:
- moving the code in their widget methods out into
  the govuk_checkbox_field_widget helper function
- moving the code that wraps the checkboxes
  in HTML to make them collapsible into the helper
  function
- remove GovukCheckboxesMixin in favour of having
  the extend_params method as a stand-alone
  function*

*The code is generic enough that it can be used
with other fields that share the same type of
data.

These changes also:
- don't alter the interface of any classes
  involved
- don't break any tests

app/main/forms.py

@@ -688,9 +688,7 @@ class RegisterUserFromOrgInviteForm(StripWhitespaceForm):
     auth_type = HiddenField('auth_type', validators=[DataRequired()])
 
 
-class GovukCheckboxesMixin:
+def extend_params(params, extensions):
-
-    def extend_params(self, params, extensions):
     items = None
     param_items = len(params['items']) if 'items' in params else 0
 
@@ -714,17 +712,7 @@ class GovukCheckboxesMixin:
                     params['items'][idx].update(items[idx])
 
 
-class GovukCheckboxField(GovukCheckboxesMixin, BooleanField):
+def govuk_checkbox_field_widget(self, field, param_extensions=None, **kwargs):
-
-    def __init__(self, label='', validators=None, param_extensions=None, **kwargs):
-        super(GovukCheckboxField, self).__init__(label, validators, false_values=None, **kwargs)
-        self.param_extensions = param_extensions
-
-    # self.__call__ renders the HTML for the field by:
-    # 1. delegating to self.meta.render_field which
-    # 2. calls field.widget
-    # this bypasses that by making self.widget a method with the same interface as widget.__call__
-    def widget(self, field, param_extensions=None, **kwargs):
 
     # error messages
     error_message = None
@@ -755,42 +743,29 @@ class GovukCheckboxField(GovukCheckboxesMixin, BooleanField):
 
     # extend default params with any sent in during instantiation
     if self.param_extensions:
-            self.extend_params(params, self.param_extensions)
+        extend_params(params, self.param_extensions)
 
     # add any sent in though use in templates
     if param_extensions:
-            self.extend_params(params, param_extensions)
+        extend_params(params, param_extensions)
 
     return Markup(
         render_template('forms/fields/checkboxes/macro.njk', params=params))
 
 
-# based on work done by @richardjpope: https://github.com/richardjpope/recourse/blob/master/recourse/forms.py#L6
+def govuk_checkboxes_field_widget(self, field, wrap_in_collapsible=False, param_extensions=None, **kwargs):
-class GovukCheckboxesField(GovukCheckboxesMixin, SelectMultipleField):
 
-    render_as_list = False
+    def _wrap_in_collapsible(field_label, checkboxes_string):
+        # wrap the checkboxes HTML in the HTML needed by the collapisble JS
+        result = Markup(
+            f'<div class="selection-wrapper"'
+            f'     data-module="collapsible-checkboxes"'
+            f'     data-field-label="{field_label}">'
+            f'  {checkboxes_string}'
+            f'</div>'
+        )
 
-    def __init__(self, label='', validators=None, param_extensions=None, **kwargs):
+        return result
-        super(GovukCheckboxesField, self).__init__(label, validators, **kwargs)
-        self.param_extensions = param_extensions
-
-    def get_item_from_option(self, option):
-        return {
-            "name": option.name,
-            "id": option.id,
-            "text": option.label.text,
-            "value": str(option.data),  # to protect against non-string types like uuids
-            "checked": option.checked
-        }
-
-    def get_items_from_options(self, field):
-        return [self.get_item_from_option(option) for option in field]
-
-    # self.__call__ renders the HTML for the field by:
-    # 1. delegating to self.meta.render_field which
-    # 2. calls field.widget
-    # this bypasses that by making self.widget a method with the same interface as widget.__call__
-    def widget(self, field, param_extensions=None, **kwargs):
 
     # error messages
     error_message = None
@@ -824,51 +799,85 @@ class GovukCheckboxesField(GovukCheckboxesMixin, SelectMultipleField):
 
     # extend default params with any sent in during instantiation
     if self.param_extensions:
-            self.extend_params(params, self.param_extensions)
+        extend_params(params, self.param_extensions)
 
     # add any sent in though use in templates
     if param_extensions:
-            self.extend_params(params, param_extensions)
+        extend_params(params, param_extensions)
 
+    if wrap_in_collapsible:
+
+        # add a blank hint to act as an ARIA live-region
+        params.update(
+            {"hint": {"html": "<div class=\"selection-summary\" role=\"region\" aria-live=\"polite\"></div>"}})
+
+        return _wrap_in_collapsible(
+            self.field_label,
+            Markup(render_template('forms/fields/checkboxes/macro.njk', params=params))
+            )
+    else:
         return Markup(
             render_template('forms/fields/checkboxes/macro.njk', params=params))
 
 
-# Extends fields using the GovukCheckboxesField interface to wrap their render in HTML needed by the collapsible JS
+class GovukCheckboxField(BooleanField):
-class GovukCollapsibleCheckboxesMixin:
+
+    def __init__(self, label='', validators=None, param_extensions=None, **kwargs):
+        super(GovukCheckboxField, self).__init__(label, validators, false_values=None, **kwargs)
+        self.param_extensions = param_extensions
+
+    # self.__call__ renders the HTML for the field by:
+    # 1. delegating to self.meta.render_field which
+    # 2. calls field.widget
+    # this bypasses that by making self.widget a method with the same interface as widget.__call__
+    def widget(self, field, param_extensions=None, **kwargs):
+        return govuk_checkbox_field_widget(self, field, param_extensions=param_extensions, **kwargs)
+
+
+# based on work done by @richardjpope: https://github.com/richardjpope/recourse/blob/master/recourse/forms.py#L6
+class GovukCheckboxesField(SelectMultipleField):
+
+    render_as_list = False
+
+    def __init__(self, label='', validators=None, param_extensions=None, **kwargs):
+        super(GovukCheckboxesField, self).__init__(label, validators, **kwargs)
+        self.param_extensions = param_extensions
+
+    def get_item_from_option(self, option):
+        return {
+            "name": option.name,
+            "id": option.id,
+            "text": option.label.text,
+            "value": str(option.data),  # to protect against non-string types like uuids
+            "checked": option.checked
+        }
+
+    def get_items_from_options(self, field):
+        return [self.get_item_from_option(option) for option in field]
+
+    # self.__call__ renders the HTML for the field by:
+    # 1. delegating to self.meta.render_field which
+    # 2. calls field.widget
+    # this bypasses that by making self.widget a method with the same interface as widget.__call__
+    def widget(self, field, param_extensions=None, **kwargs):
+        return govuk_checkboxes_field_widget(self, field, param_extensions=param_extensions, **kwargs)
+
+
+# Wraps checkboxes rendering in HTML needed by the collapsible JS
+class GovukCollapsibleCheckboxesField(GovukCheckboxesField):
     def __init__(self, label='', validators=None, field_label='', param_extensions=None, **kwargs):
 
-        super(GovukCollapsibleCheckboxesMixin, self).__init__(label, validators, param_extensions, **kwargs)
+        super(GovukCollapsibleCheckboxesField, self).__init__(label, validators, param_extensions, **kwargs)
         self.field_label = field_label
 
     def widget(self, field, **kwargs):
-
+        return govuk_checkboxes_field_widget(self, field, wrap_in_collapsible=True, param_extensions=None, **kwargs)
-        # add a blank hint to act as an ARIA live-region
-        if self.param_extensions is not None:
-            self.param_extensions.update(
-                {"hint": {"html": "<div class=\"selection-summary\" role=\"region\" aria-live=\"polite\"></div>"}})
-        else:
-            self.param_extensions = \
-                {"hint": {"html": "<div class=\"selection-summary\" role=\"region\" aria-live=\"polite\"></div>"}}
-
-        # wrap the checkboxes HTML in the HTML needed by the collapisble JS
-        return Markup(
-            f'<div class="selection-wrapper"'
-            f'     data-module="collapsible-checkboxes"'
-            f'     data-field-label="{self.field_label}">'
-            f'  {super(GovukCollapsibleCheckboxesMixin, self).widget(field, **kwargs)}'
-            f'</div>'
-        )
 
 
-class GovukCollapsibleCheckboxesField(GovukCollapsibleCheckboxesMixin, GovukCheckboxesField):
+# GovukCollapsibleCheckboxesField adds an ARIA live-region to the hint and wraps the render in HTML needed by the
-    pass
-
-
-# GovukCollapsibleCheckboxesMixin adds an ARIA live-region to the hint and wraps the render in HTML needed by the
 # collapsible JS
 # NestedFieldMixin puts the items into a tree hierarchy, pre-rendering the sub-trees of the top-level items
-class GovukCollapsibleNestedCheckboxesField(GovukCollapsibleCheckboxesMixin, NestedFieldMixin, GovukCheckboxesField):
+class GovukCollapsibleNestedCheckboxesField(NestedFieldMixin, GovukCollapsibleCheckboxesField):
     NONE_OPTION_VALUE = None
     render_as_list = True