Updated MultiValueDict.update() to mirror dict.update() behavior.

Changes in behavior include:

- Accepting iteration over empty sequences, updating nothing.
- Accepting iterable of 2-tuples providing key-value pairs.
- Failing with the same or comparable exceptions for invalid input.

Notably this replaces the previous attempt to catch TypeError which was
unreachable as the call to .items() resulted in AttributeError on
non-dict objects.

django/utils/datastructures.py

@@ -194,16 +194,15 @@ def update(self, *args, **kwargs):
         if len(args) > 1:
             raise TypeError("update expected at most 1 argument, got %d" % len(args))
         if args:
-            other_dict = args[0]
-            if isinstance(other_dict, MultiValueDict):
-                for key, value_list in other_dict.lists():
+            arg = args[0]
+            if isinstance(arg, MultiValueDict):
+                for key, value_list in arg.lists():
                     self.setlistdefault(key).extend(value_list)
             else:
-                try:
-                    for key, value in other_dict.items():
-                        self.setlistdefault(key).append(value)
-                except TypeError:
-                    raise ValueError("MultiValueDict.update() takes either a MultiValueDict or dictionary")
+                if isinstance(arg, Mapping):
+                    arg = arg.items()
+                for key, value in arg:
+                    self.setlistdefault(key).append(value)
         for key, value in kwargs.items():
             self.setlistdefault(key).append(value)
 

tests/utils_tests/test_datastructures.py

@@ -9,6 +9,7 @@
     CaseInsensitiveMapping, DictWrapper, ImmutableList, MultiValueDict,
     MultiValueDictKeyError, OrderedSet,
 )
+from django.utils.version import PY37
 
 
 class OrderedSetTests(SimpleTestCase):
@@ -204,6 +205,41 @@ def test_update_kwargs(self):
         x.update(a=4, b=5)
         self.assertEqual(list(x.lists()), [('a', [1, 4]), ('b', [2, 5]), ('c', [3])])
 
+    def test_update_with_empty_iterable(self):
+        for value in ['', b'', (), [], set(), {}]:
+            d = MultiValueDict()
+            d.update(value)
+            self.assertEqual(d, MultiValueDict())
+
+    def test_update_with_iterable_of_pairs(self):
+        for value in [(('a', 1),), [('a', 1)], {('a', 1)}]:
+            d = MultiValueDict()
+            d.update(value)
+            self.assertEqual(d, MultiValueDict({'a': [1]}))
+
+    def test_update_with_non_iterable_raises_typeerror(self):
+        for value in [None, True, False, 123, 123.45]:
+            type_ = type(value)
+            msg = "'%s' object is not iterable" % type_.__name__
+            with self.subTest(type_), self.assertRaisesMessage(TypeError, msg):
+                MultiValueDict().update(value)
+
+    def test_update_with_unspupported_non_empty_iterable_raises(self):
+        # TypeError: cannot convert dictionary update sequence element #0 to a sequence
+        for value in [b'123', b'abc', (1, 2, 3), [1, 2, 3], {1, 2, 3}]:
+            if PY37:
+                msg = 'cannot unpack non-iterable int object'
+            else:
+                msg = "'int' object is not iterable"
+            with self.subTest(value), self.assertRaisesMessage(TypeError, msg):
+                MultiValueDict().update(value)
+
+        # ValueError: dictionary update sequence element #0 has length 1; 2 is required
+        for value in ['123', 'abc', ('a', 'b', 'c'), ['a', 'b', 'c'], {'a', 'b', 'c'}]:
+            msg = 'not enough values to unpack (expected 2, got 1)'
+            with self.subTest(value), self.assertRaisesMessage(ValueError, msg):
+                MultiValueDict().update(value)
+
 
 class ImmutableListTests(SimpleTestCase):