widget: allow setting empty value in MultiSelectWidget (#77469)

gadjo/forms/widgets.py

@@ -36,3 +36,6 @@ class MultiSelectWidget(forms.MultiWidget):
 
     def id_for_label(self, id_):
         return id_
+
+    def value_omitted_from_data(self, *args, **kwargs):
+        return super(forms.MultiWidget, self).value_omitted_from_data(*args, **kwargs)

tests/test_widgets.py

@@ -26,3 +26,26 @@ def test_multiselect_widget():
     assert len(PyQuery(rendered).find('select')) == 2
     assert PyQuery(rendered).find('option[selected]').text() == 'Aa Bb'
     assert ctx['form'].cleaned_data == {'choices': ['a', 'b']}
+
+
+def test_multiselect_widget_omitted_from_data():
+    class ExampleForm(forms.Form):
+        choices = forms.MultipleChoiceField(
+            label='choices', choices=[('a', 'Aa'), ('b', 'Bb'), ('c', 'Cc')], widget=MultiSelectWidget
+        )
+
+    form = ExampleForm(data={})
+    assert (
+        form.fields['choices'].widget.value_omitted_from_data(
+            form.data, form.files, form.add_prefix('choices')
+        )
+        is True
+    )
+
+    form = ExampleForm(data={'choices': 'a'})
+    assert (
+        form.fields['choices'].widget.value_omitted_from_data(
+            form.data, form.files, form.add_prefix('choices')
+        )
+        is False
+    )