Fixed mapping checks against Django's MultiValueDict

Fixes #419.

Author: agronholm

docs/versionhistory.rst

@@ -16,6 +16,8 @@ This library adheres to
   (`#432 <https://github.com/agronholm/typeguard/issues/432>`_, PR by Yongxin Wang)
 - Fixed detection of optional fields (``NotRequired[...]``) in ``TypedDict`` when using
   forward references (`#424 <https://github.com/agronholm/typeguard/issues/424>`_)
+- Fixed mapping checks against Django's ``MultiValueDict``
+  (`#419 <https://github.com/agronholm/typeguard/issues/419>`_)
 
 **4.1.5** (2023-09-11)
 

src/typeguard/_config.py

@@ -1,6 +1,6 @@
 from __future__ import annotations
 
-from collections.abc import Collection
+from collections.abc import Iterable
 from dataclasses import dataclass
 from enum import Enum, auto
 from typing import TYPE_CHECKING, TypeVar
@@ -49,11 +49,11 @@ class CollectionCheckStrategy(Enum):
     FIRST_ITEM = auto()
     ALL_ITEMS = auto()
 
-    def iterate_samples(self, collection: Collection[T]) -> Collection[T]:
+    def iterate_samples(self, collection: Iterable[T]) -> Iterable[T]:
         if self is CollectionCheckStrategy.FIRST_ITEM:
-            if len(collection):
+            try:
                 return [next(iter(collection))]
-            else:
+            except StopIteration:
                 return ()
         else:
             return collection

tests/test_checkers.py

@@ -433,6 +433,14 @@ def test_bad_value_type_full_check(self):
             collection_check_strategy=CollectionCheckStrategy.ALL_ITEMS,
         ).match("value of key 'y' of dict is not an instance of int")
 
+    def test_custom_dict_generator_items(self):
+        class CustomDict(dict):
+            def items(self):
+                for key in self:
+                    yield key, self[key]
+
+        check_type(CustomDict(a=1), Dict[str, int])
+
 
 class TestTypedDict:
     @pytest.mark.parametrize(