langchain[patch]: Invoke chain prep_inputs and prep_outputs inside try block to catch validation errors (#16644)

- **Description:** Callback manager can't catch chain input or output
validation errors because `prepare_input` and `prepare_output` are not
part of the try/raise logic, this PR fixes that logic.
 
  - **Issue:** #15954

libs/langchain/langchain/chains/base.py

@@ -146,24 +146,28 @@ class Chain(RunnableSerializable[Dict[str, Any], Dict[str, Any]], ABC):
             self.metadata,
         )
         new_arg_supported = inspect.signature(self._call).parameters.get("run_manager")
+
         run_manager = callback_manager.on_chain_start(
             dumpd(self),
             inputs,
             name=run_name,
         )
         try:
+            self._validate_inputs(inputs)
             outputs = (
                 self._call(inputs, run_manager=run_manager)
                 if new_arg_supported
                 else self._call(inputs)
             )
+
+            final_outputs: Dict[str, Any] = self.prep_outputs(
+                inputs, outputs, return_only_outputs
+            )
         except BaseException as e:
             run_manager.on_chain_error(e)
             raise e
         run_manager.on_chain_end(outputs)
-        final_outputs: Dict[str, Any] = self.prep_outputs(
-            inputs, outputs, return_only_outputs
-        )
+
         if include_run_info:
             final_outputs[RUN_KEY] = RunInfo(run_id=run_manager.run_id)
         return final_outputs
@@ -199,18 +203,20 @@ class Chain(RunnableSerializable[Dict[str, Any], Dict[str, Any]], ABC):
             name=run_name,
         )
         try:
+            self._validate_inputs(inputs)
             outputs = (
                 await self._acall(inputs, run_manager=run_manager)
                 if new_arg_supported
                 else await self._acall(inputs)
             )
+            final_outputs: Dict[str, Any] = self.prep_outputs(
+                inputs, outputs, return_only_outputs
+            )
         except BaseException as e:
             await run_manager.on_chain_error(e)
             raise e
         await run_manager.on_chain_end(outputs)
-        final_outputs: Dict[str, Any] = self.prep_outputs(
-            inputs, outputs, return_only_outputs
-        )
+
         if include_run_info:
             final_outputs[RUN_KEY] = RunInfo(run_id=run_manager.run_id)
         return final_outputs
@@ -259,6 +265,20 @@ class Chain(RunnableSerializable[Dict[str, Any], Dict[str, Any]], ABC):
 
     def _validate_inputs(self, inputs: Dict[str, Any]) -> None:
         """Check that all inputs are present."""
+        if not isinstance(inputs, dict):
+            _input_keys = set(self.input_keys)
+            if self.memory is not None:
+                # If there are multiple input keys, but some get set by memory so that
+                # only one is not set, we can still figure out which key it is.
+                _input_keys = _input_keys.difference(self.memory.memory_variables)
+            if len(_input_keys) != 1:
+                raise ValueError(
+                    f"A single string input was passed in, but this chain expects "
+                    f"multiple inputs ({_input_keys}). When a chain expects "
+                    f"multiple inputs, please call it by passing in a dictionary, "
+                    "eg `chain({'foo': 1, 'bar': 2})`"
+                )
+
         missing_keys = set(self.input_keys).difference(inputs)
         if missing_keys:
             raise ValueError(f"Missing some input keys: {missing_keys}")
@@ -461,18 +481,10 @@ class Chain(RunnableSerializable[Dict[str, Any], Dict[str, Any]], ABC):
                 # If there are multiple input keys, but some get set by memory so that
                 # only one is not set, we can still figure out which key it is.
                 _input_keys = _input_keys.difference(self.memory.memory_variables)
-            if len(_input_keys) != 1:
-                raise ValueError(
-                    f"A single string input was passed in, but this chain expects "
-                    f"multiple inputs ({_input_keys}). When a chain expects "
-                    f"multiple inputs, please call it by passing in a dictionary, "
-                    "eg `chain({'foo': 1, 'bar': 2})`"
-                )
             inputs = {list(_input_keys)[0]: inputs}
         if self.memory is not None:
             external_context = self.memory.load_memory_variables(inputs)
             inputs = dict(inputs, **external_context)
-        self._validate_inputs(inputs)
         return inputs
 
     @property

libs/langchain/tests/unit_tests/chains/test_base.py

@@ -162,3 +162,35 @@ def test_run_with_callback() -> None:
     assert handler.starts == 1
     assert handler.ends == 1
     assert handler.errors == 0
+
+
+def test_run_with_callback_and_input_error() -> None:
+    """Test callback manager catches run validation input error."""
+    handler = FakeCallbackHandler()
+    chain = FakeChain(
+        the_input_keys=["foo", "bar"],
+        callbacks=[handler],
+    )
+
+    with pytest.raises(ValueError):
+        chain({"bar": "foo"})
+
+    assert handler.starts == 1
+    assert handler.ends == 0
+    assert handler.errors == 1
+
+
+def test_run_with_callback_and_output_error() -> None:
+    """Test callback manager catches run validation output error."""
+    handler = FakeCallbackHandler()
+    chain = FakeChain(
+        the_output_keys=["foo", "bar"],
+        callbacks=[handler],
+    )
+
+    with pytest.raises(ValueError):
+        chain("foo")
+
+    assert handler.starts == 1
+    assert handler.ends == 0
+    assert handler.errors == 1