RunnableLambda: create afunc instance from func when not provided (#13408)

Fixes #13407.

This workaround consists in letting the RunnableLambda create its
self.afunc from its self.func when self.afunc is not provided; the
change has no dependency.

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---------

Co-authored-by: Erick Friis <erick@langchain.dev>
Co-authored-by: Nuno Campos <nuno@langchain.dev>

libs/core/langchain_core/runnables/base.py

@@ -2477,8 +2477,19 @@ class RunnableLambda(Runnable[Input, Output]):
         config: RunnableConfig,
         **kwargs: Any,
     ) -> Output:
+        if hasattr(self, "afunc"):
+            afunc = self.afunc
+        else:
+
+            async def f(*args, **kwargs):  # type: ignore[no-untyped-def]
+                return await asyncio.get_running_loop().run_in_executor(
+                    None, partial(self.func, **kwargs), *args
+                )
+
+            afunc = f
+
         output = await acall_func_with_variable_args(
-            self.afunc, input, config, run_manager, **kwargs
+            afunc, input, config, run_manager, **kwargs
         )
         # If the output is a runnable, invoke it
         if isinstance(output, Runnable):
@@ -2539,17 +2550,13 @@ class RunnableLambda(Runnable[Input, Output]):
         **kwargs: Optional[Any],
     ) -> Output:
         """Invoke this runnable asynchronously."""
-        if hasattr(self, "afunc"):
-            return await self._acall_with_config(
-                self._ainvoke,
-                input,
-                self._config(config, self.afunc),
-                **kwargs,
-            )
-        else:
-            # Delegating to super implementation of ainvoke.
-            # Uses asyncio executor to run the sync version (invoke)
-            return await super().ainvoke(input, config)
+        the_func = self.afunc if hasattr(self, "afunc") else self.func
+        return await self._acall_with_config(
+            self._ainvoke,
+            input,
+            self._config(config, the_func),
+            **kwargs,
+        )
 
 
 class RunnableEachBase(RunnableSerializable[List[Input], List[Output]]):

libs/core/tests/unit_tests/runnables/test_runnable.py

@@ -4061,3 +4061,22 @@ def test_with_config_callbacks() -> None:
     # ConfigError: field "callbacks" not yet prepared so type is still a ForwardRef,
     # you might need to call RunnableConfig.update_forward_refs().
     assert isinstance(result, RunnableBinding)
+
+
+@pytest.mark.asyncio
+async def test_ainvoke_on_returned_runnable() -> None:
+    """Verify that a runnable returned by a sync runnable in the async path will
+    be runthroughaasync path (issue #13407)"""
+
+    def idchain_sync(__input: dict) -> bool:
+        return False
+
+    async def idchain_async(__input: dict) -> bool:
+        return True
+
+    idchain = RunnableLambda(func=idchain_sync, afunc=idchain_async)
+
+    def func(__input: dict) -> Runnable:
+        return idchain
+
+    assert await RunnableLambda(func).ainvoke({})