lf.QueryInvocation.output to return lf.MappingError when OOP failed.

Previously accessing `output` for bad LLM response will raise.

PiperOrigin-RevId: 708037894

langfun/core/structured/querying.py

@@ -583,7 +583,16 @@ def lm_request(self) -> lf.Message:
 
   @functools.cached_property
   def output(self) -> Any:
-    return query_output(self.lm_response, self.schema)
+    """The output of `lf.query`. If it failed, returns the `MappingError`."""
+    try:
+      return query_output(self.lm_response, self.schema)
+    except mapping.MappingError as e:
+      return e
+
+  @property
+  def has_error(self) -> bool:
+    """Returns True if the query failed to generate a valid output."""
+    return isinstance(self.output, BaseException)
 
   @property
   def elapse(self) -> float:

langfun/core/structured/querying_test.py

@@ -1051,6 +1051,16 @@ def test_query(self):
 
 class QueryInvocationTest(unittest.TestCase):
 
+  def test_basics(self):
+    lm = fake.StaticSequence([
+        'Activity(description="hi"',
+    ])
+    with querying.track_queries() as queries:
+      querying.query('foo', Activity, default=None, lm=lm)
+
+    self.assertTrue(queries[0].has_error)
+    self.assertIsInstance(queries[0].output, mapping.MappingError)
+
   def test_to_html(self):
     lm = fake.StaticSequence([
         'Activity(description="hi")',