Fix for #156 - if has_entry() has only a single matching key, report …

…the mismatching value.

src/hamcrest/library/collection/isdict_containing.py

@@ -1,4 +1,4 @@
-from typing import Hashable, Mapping, TypeVar, Union
+from typing import Hashable, Mapping, MutableMapping, TypeVar, Union
 
 from hamcrest.core.base_matcher import BaseMatcher
 from hamcrest.core.description import Description
@@ -32,6 +32,24 @@ def describe_to(self, description: Description) -> None:
             self.key_matcher
         ).append_text(": ").append_description_of(self.value_matcher).append_text("]")
 
+    def describe_mismatch(self, item: Mapping[K, V], mismatch_description: Description) -> None:
+        key_matches: MutableMapping[K, V] = {}
+        if hasmethod(item, "items"):
+            for key, value in item.items():
+                if self.key_matcher.matches(key):
+                    key_matches[key] = value
+        if len(key_matches) == 1:
+            key, value = key_matches.popitem()
+            mismatch_description.append_text("value for ").append_description_of(key).append_text(
+                " "
+            )
+            self.value_matcher.describe_mismatch(value, mismatch_description)
+        else:
+            super().describe_mismatch(item, mismatch_description)
+
+    def describe_match(self, item: Mapping[K, V], match_description: Description) -> None:
+        super().describe_match(item, match_description)
+
 
 def has_entry(
     key_match: Union[K, Matcher[K]], value_match: Union[V, Matcher[V]]

tests/hamcrest_unit_test/collection/isdict_containing_test.py

@@ -1,8 +1,9 @@
 import unittest
 
+from hamcrest import starts_with
 from hamcrest.core.core.isequal import equal_to
 from hamcrest.library.collection.isdict_containing import has_entry
-from hamcrest_unit_test.matcher_test import MatcherTest
+from hamcrest_unit_test.matcher_test import MatcherTest, assert_mismatch_description
 
 from .quasidict import QuasiDictionary
 
@@ -40,6 +41,18 @@ def testMismatchDescriptionShowsActualArgument(self):
     def testDescribeMismatch(self):
         self.assert_describe_mismatch("was 'bad'", has_entry("a", 1), "bad")
 
+    def test_describe_single_matching_key_mismatching_value(self):
+        assert_mismatch_description("value for 'a' was <2>", has_entry("a", 1), {"a": 2})
+        assert_mismatch_description(
+            "value for 'aa' was <2>", has_entry(starts_with("a"), 1), {"aa": 2}
+        )
+        assert_mismatch_description(
+            "was <{'ab': 2, 'ac': 3}>", has_entry(starts_with("a"), 1), {"ab": 2, "ac": 3}
+        )
+
+    # def test_describe_match(self):
+    #     assert_match_description("value for 'a' was <1>", has_entry("a", 1), {"a": 1})
+
 
 if __name__ == "__main__":
     unittest.main()