Fixed #23883 -- Stopped flatatt modifying its argument

django/forms/utils.py

@@ -29,16 +29,17 @@ def flatatt(attrs):
 
     The result is passed through 'mark_safe'.
     """
+    key_value_attrs = []
     boolean_attrs = []
-    for attr, value in list(attrs.items()):
+    for attr, value in attrs.items():
-        if value is True:
+        if isinstance(value, bool):
-            boolean_attrs.append((attr,))
+            if value:
-            del attrs[attr]
+                boolean_attrs.append((attr,))
-        elif value is False:
+        else:
-            del attrs[attr]
+            key_value_attrs.append((attr, value))
 
     return (
-        format_html_join('', ' {0}="{1}"', sorted(attrs.items())) +
+        format_html_join('', ' {0}="{1}"', sorted(key_value_attrs)) +
         format_html_join('', ' {0}', sorted(boolean_attrs))
     )
 

tests/forms_tests/tests/test_util.py

@@ -27,6 +27,23 @@ class FormsUtilTestCase(TestCase):
         self.assertEqual(flatatt({'class': "news", 'title': "Read this", 'required': False}), ' class="news" title="Read this"')
         self.assertEqual(flatatt({}), '')
 
+    def test_flatatt_no_side_effects(self):
+        """
+        Fixes #23883 -- Check that flatatt does not modify the dict passed in
+        """
+        attrs = {'foo': 'bar', 'true': True, 'false': False}
+        attrs_copy = copy.copy(attrs)
+        self.assertEqual(attrs, attrs_copy)
+
+        first_run = flatatt(attrs)
+        self.assertEqual(attrs, attrs_copy)
+        self.assertEqual(first_run, ' foo="bar" true')
+
+        second_run = flatatt(attrs)
+        self.assertEqual(attrs, attrs_copy)
+
+        self.assertEqual(first_run, second_run)
+
     def test_validation_error(self):
         ###################
         # ValidationError #