yourlabs#1333 clone forwards

When Django creates a Form instance from a form class, the fields and widgets are _copied_ instead of instantiated. This means that every field/widget must implement __deepcopy__ properly to create clones of any references it holds.

Implement `__deepcopy__` in the `WidgetMixin` and copy the `forward` list to prevent forwards from leaking into unrelated forms when using a common base class.

src/dal/widgets.py

@@ -47,10 +47,18 @@ class WidgetMixin(object):
     def __init__(self, url=None, forward=None, *args, **kwargs):
         """Instanciate a widget with a URL and a list of fields to forward."""
         self.url = url
-        self.forward = forward or []
+        if forward:
+            self.forward = list(forward).copy()
+        else:
+            self.forward = []
         self.placeholder = kwargs.get("attrs", {}).get("data-placeholder")
         super(WidgetMixin, self).__init__(*args, **kwargs)
 
+    def __deepcopy__(self, memo):
+        clone = super().__deepcopy__(memo)
+        clone.forward = self.forward.copy()
+        return clone
+
     def build_attrs(self, *args, **kwargs):
         """Build HTML attributes for the widget."""
         attrs = super(WidgetMixin, self).build_attrs(*args, **kwargs)

test_project/tests/test_widgets.py

@@ -1,5 +1,7 @@
 """Test the base widget."""
+from django.contrib.auth import get_user_model
 
+from dal import forward
 from dal.autocomplete import Select2
 from dal.widgets import Select, WidgetMixin
 
@@ -10,6 +12,9 @@
 from django import http
 from django import test
 from django.urls import re_path as url
+
+from dal_select2.widgets import ModelSelect2
+
 try:
     from django.urls import reverse
 except ImportError:
@@ -158,3 +163,28 @@ def render_forward_conf(self, id):
         # attrs with id
         observed = widget.render('myname', '', attrs={'id': 'myid'})
         self.assertEqual(observed, 'myid')
+
+    def test_forwards_are_cloned(self):
+        User = get_user_model()
+
+        class BaseForm(forms.Form):
+            choice_field = forms.ModelChoiceField(User.objects.all(), widget=ModelSelect2(url='/somewhere'))
+
+        class Form1(BaseForm):
+            def __init__(self, *args, **kwargs):
+                super().__init__(*args, **kwargs)
+                self.fields['choice_field'].widget.forward.append(forward.Const(True, 'filter_a'))
+
+        class Form2(BaseForm):
+            def __init__(self, *args, **kwargs):
+                super().__init__(*args, **kwargs)
+                self.fields['choice_field'].widget.forward.append(forward.Const(True, 'filter_b'))
+
+        form1 = Form1()
+        form2 = Form2()
+
+        self.assertEqual(1, len(form1.fields['choice_field'].widget.forward))
+        self.assertEqual('filter_a', form1.fields['choice_field'].widget.forward[0].dst)
+
+        self.assertEqual(1, len(form2.fields['choice_field'].widget.forward))
+        self.assertEqual('filter_b', form2.fields['choice_field'].widget.forward[0].dst)