[3.0.x] Fixed #31073 -- Prevented CheckboxInput.get_context() from mutating attrs.

Backport of 02eff7ef60 from master

django/forms/widgets.py

@@ -522,9 +522,7 @@ class CheckboxInput(Input):
 
     def get_context(self, name, value, attrs):
         if self.check_test(value):
-            if attrs is None:
-                attrs = {}
-            attrs['checked'] = True
+            attrs = {**(attrs or {}), 'checked': True}
         return super().get_context(name, value, attrs)
 
     def value_from_datadict(self, data, files, name):

tests/forms_tests/widget_tests/test_checkboxinput.py

@@ -89,3 +89,8 @@ class CheckboxInputTest(WidgetTest):
     def test_value_omitted_from_data(self):
         self.assertIs(self.widget.value_omitted_from_data({'field': 'value'}, {}, 'field'), False)
         self.assertIs(self.widget.value_omitted_from_data({}, {}, 'field'), False)
+
+    def test_get_context_does_not_mutate_attrs(self):
+        attrs = {'checked': False}
+        self.widget.get_context('name', True, attrs)
+        self.assertIs(attrs['checked'], False)

tests/postgres_tests/test_array.py

@@ -1003,6 +1003,17 @@ class TestSplitFormWidget(PostgreSQLWidgetTestCase):
             }
         )
 
+    def test_checkbox_get_context_attrs(self):
+        context = SplitArrayWidget(
+            forms.CheckboxInput(),
+            size=2,
+        ).get_context('name', [True, False])
+        self.assertEqual(context['widget']['value'], '[True, False]')
+        self.assertEqual(
+            [subwidget['attrs'] for subwidget in context['widget']['subwidgets']],
+            [{'checked': True}, {}]
+        )
+
     def test_render(self):
         self.check_html(
             SplitArrayWidget(forms.TextInput(), size=2), 'array', None,