Consider errors from earlier indices (in instances) to be better matches

Improves `best_match` and generally error messages in the presence of
`anyOf` / `oneOf` in cases where the errors are at the same level of
depth within the instance but occur earlier.

In other words:

    {"anyOf": [{"items": {"const": 37}]}

now behaves like simply `{"items": {"const": 37}}`.

Closes: #1250

Author: Julian

CHANGELOG.rst

@@ -1,3 +1,9 @@
+v4.22.0
+=======
+
+* Improve ``best_match`` (and thereby error messages from ``jsonschema.validate``) in cases where there are multiple *sibling* errors from applying ``anyOf`` / ``allOf`` -- i.e. when multiple elements of a JSON array have errors, we now do prefer showing errors from earlier elements rather than simply showing an error for the full array (#1250).
+* (Micro-)optimize equality checks when comparing for JSON Schema equality by first checking for object identity, as ``==`` would.
+
 v4.21.1
 =======
 

jsonschema/exceptions.py

@@ -395,12 +395,13 @@ def by_relevance(weak=WEAK_MATCHES, strong=STRONG_MATCHES):
 
     def relevance(error):
         validator = error.validator
-        return (
-            -len(error.path),
-            validator not in weak,
-            validator in strong,
-            not error._matches_type(),
-        )
+        return (                        # prefer errors which are ...
+            -len(error.path),           # 'deeper' and thereby more specific
+            error.path,                 # earlier (for sibling errors)
+            validator not in weak,      # for a non-low-priority keyword
+            validator in strong,        # for a high priority keyword
+            not error._matches_type(),  # at least match the instance's type
+        )                               # otherwise we'll treat them the same
 
     return relevance
 

jsonschema/tests/test_exceptions.py

@@ -100,6 +100,35 @@ def test_anyOf_traversal_for_single_equally_relevant_error(self):
         best = self.best_match_of(instance=[], schema=schema)
         self.assertEqual(best.validator, "type")
 
+    def test_anyOf_traversal_for_single_sibling_errors(self):
+        """
+        We *do* traverse anyOf with a single subschema that fails multiple
+        times (e.g. on multiple items).
+        """
+
+        schema = {
+            "anyOf": [
+                {"items": {"const": 37}},
+            ],
+        }
+        best = self.best_match_of(instance=[12, 12], schema=schema)
+        self.assertEqual(best.validator, "const")
+
+    def test_anyOf_traversal_for_non_type_matching_sibling_errors(self):
+        """
+        We *do* traverse anyOf with multiple subschemas when one does not type
+        match.
+        """
+
+        schema = {
+            "anyOf": [
+                {"type": "object"},
+                {"items": {"const": 37}},
+            ],
+        }
+        best = self.best_match_of(instance=[12, 12], schema=schema)
+        self.assertEqual(best.validator, "const")
+
     def test_if_the_most_relevant_error_is_oneOf_it_is_traversed(self):
         """
         If the most relevant error is an oneOf, then we traverse its context
@@ -153,6 +182,35 @@ def test_oneOf_traversal_for_single_equally_relevant_error(self):
         best = self.best_match_of(instance=[], schema=schema)
         self.assertEqual(best.validator, "type")
 
+    def test_oneOf_traversal_for_single_sibling_errors(self):
+        """
+        We *do* traverse oneOf with a single subschema that fails multiple
+        times (e.g. on multiple items).
+        """
+
+        schema = {
+            "oneOf": [
+                {"items": {"const": 37}},
+            ],
+        }
+        best = self.best_match_of(instance=[12, 12], schema=schema)
+        self.assertEqual(best.validator, "const")
+
+    def test_oneOf_traversal_for_non_type_matching_sibling_errors(self):
+        """
+        We *do* traverse oneOf with multiple subschemas when one does not type
+        match.
+        """
+
+        schema = {
+            "oneOf": [
+                {"type": "object"},
+                {"items": {"const": 37}},
+            ],
+        }
+        best = self.best_match_of(instance=[12, 12], schema=schema)
+        self.assertEqual(best.validator, "const")
+
     def test_if_the_most_relevant_error_is_allOf_it_is_traversed(self):
         """
         Now, if the error is allOf, we traverse but select the *most* relevant