Add inherit_kwargs attribute to ClusterForm child formsets (wagtail#156)

This specifies a list of keyword argument names; when any of these are passed to the parent ClusterForm's constructor, they will also be passed on to the child forms of that formset (via the formset's form_kwargs keyword argument).

modelcluster/forms.py

@@ -74,6 +74,8 @@ def transientmodelformset_factory(model, formset=BaseTransientModelFormSet, **kw
 
 
 class BaseChildFormSet(BaseTransientModelFormSet):
+    inherit_kwargs = None
+
     def __init__(self, data=None, files=None, instance=None, queryset=None, **kwargs):
         if instance is None:
             self.instance = self.fk.remote_field.model()
@@ -171,7 +173,8 @@ def childformset_factory(
     parent_model, model, form=ModelForm,
     formset=BaseChildFormSet, fk_name=None, fields=None, exclude=None,
     extra=3, can_order=False, can_delete=True, max_num=None, validate_max=False,
-    formfield_callback=None, widgets=None, min_num=None, validate_min=False
+    formfield_callback=None, widgets=None, min_num=None, validate_min=False,
+    inherit_kwargs=None,
 ):
 
     fk = _get_foreign_key(parent_model, model, fk_name=fk_name)
@@ -203,6 +206,11 @@ def childformset_factory(
     }
     FormSet = transientmodelformset_factory(model, **kwargs)
     FormSet.fk = fk
+
+    # A list of keyword argument names that should be passed on from ClusterForm's constructor
+    # to child forms in this formset
+    FormSet.inherit_kwargs = inherit_kwargs
+
     return FormSet
 
 
@@ -296,7 +304,15 @@ def __init__(self, data=None, files=None, instance=None, prefix=None, **kwargs):
                 formset_prefix = "%s-%s" % (prefix, rel_name)
             else:
                 formset_prefix = rel_name
-            self.formsets[rel_name] = formset_class(data, files, instance=instance, prefix=formset_prefix)
+
+            child_form_kwargs = {}
+            if formset_class.inherit_kwargs:
+                for kwarg_name in formset_class.inherit_kwargs:
+                    child_form_kwargs[kwarg_name] = kwargs.get(kwarg_name)
+
+            self.formsets[rel_name] = formset_class(
+                data, files, instance=instance, prefix=formset_prefix, form_kwargs=child_form_kwargs
+            )
 
         if self.is_bound and not self._has_explicit_formsets:
             # check which formsets have actually been provided as part of the form submission -

tests/tests/test_cluster_form.py

@@ -158,6 +158,46 @@ class Meta:
         self.assertNotIn('release_date', form.formsets['albums'].forms[0].fields)
         self.assertEqual(Textarea, type(form.formsets['albums'].forms[0]['name'].field.widget))
 
+    def test_without_kwarg_inheritance(self):
+        # by default, kwargs passed to the ClusterForm do not propagate to child forms
+        class BandForm(ClusterForm):
+            class Meta:
+                model = Band
+                formsets = {
+                    'members': {'fields': ['name']}
+                }
+                fields = ['name']
+
+        form = BandForm(label_suffix="!!!:")
+        form_html = form.as_p()
+        # band name field should have label_suffix applied
+        self.assertInHTML('<label for="id_name">Name!!!:</label>', form_html)
+        # but this should not propagate to member form fields
+        self.assertInHTML('<label for="id_members-0-name">Name!!!:</label>', form_html, count=0)
+
+    def test_with_kwarg_inheritance(self):
+        # inherit_kwargs should allow kwargs passed to the ClusterForm to propagate to child forms
+        class BandForm(ClusterForm):
+            class Meta:
+                model = Band
+                formsets = {
+                    'members': {'fields': ['name'], 'inherit_kwargs': ['label_suffix']}
+                }
+                fields = ['name']
+
+        form = BandForm(label_suffix="!!!:")
+        form_html = form.as_p()
+        # band name field should have label_suffix applied
+        self.assertInHTML('<label for="id_name">Name!!!:</label>', form_html)
+        # and this should propagate to member form fields too
+        self.assertInHTML('<label for="id_members-0-name">Name!!!:</label>', form_html)
+
+        # the form should still work without a label_suffix kwarg
+        form = BandForm()
+        form_html = form.as_p()
+        self.assertInHTML('<label for="id_name">Name:</label>', form_html)
+        self.assertInHTML('<label for="id_members-0-name">Name:</label>', form_html)
+
     def test_custom_formset_form(self):
         class AlbumForm(ClusterForm):
             pass