Correct number of elements in config list in `batch()` and `abatch()` of `BaseLLM` (#12713)

- **Description:** Correct number of elements in config list in
`batch()` and `abatch()` of `BaseLLM` in case `max_concurrency` is not
None.
- **Issue:** #12643
- **Twitter handle:** @akionux

---------

Co-authored-by: Bagatur <baskaryan@gmail.com>

libs/langchain/langchain/llms/base.py

@@ -297,9 +297,12 @@ class BaseLLM(BaseLanguageModel[str], ABC):
             config = [{**c, "max_concurrency": None} for c in config]  # type: ignore[misc]
             return [
                 output
-                for batch in batches
+                for i, batch in enumerate(batches)
                 for output in self.batch(
-                    batch, config=config, return_exceptions=return_exceptions, **kwargs
+                    batch,
+                    config=config[i * max_concurrency : (i + 1) * max_concurrency],
+                    return_exceptions=return_exceptions,
+                    **kwargs,
                 )
             ]
 
@@ -340,9 +343,12 @@ class BaseLLM(BaseLanguageModel[str], ABC):
             config = [{**c, "max_concurrency": None} for c in config]  # type: ignore[misc]
             return [
                 output
-                for batch in batches
+                for i, batch in enumerate(batches)
                 for output in await self.abatch(
-                    batch, config=config, return_exceptions=return_exceptions, **kwargs
+                    batch,
+                    config=config[i * max_concurrency : (i + 1) * max_concurrency],
+                    return_exceptions=return_exceptions,
+                    **kwargs,
                 )
             ]
 

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

@@ -6,6 +6,8 @@ try:
 except ImportError:
     from sqlalchemy.ext.declarative import declarative_base
 
+import pytest
+
 from langchain.cache import InMemoryCache, SQLAlchemyCache
 from langchain.globals import get_llm_cache, set_llm_cache
 from langchain.schema import Generation, LLMResult
@@ -73,3 +75,22 @@ def test_custom_caching() -> None:
         llm_output=None,
     )
     assert output == expected_output
+
+
+def test_batch() -> None:
+    llm = FakeLLM()
+    output = llm.batch(["foo", "bar", "foo"])
+    assert output == ["foo"] * 3
+
+    output = llm.batch(["foo", "bar", "foo"], config={"max_concurrency": 2})
+    assert output == ["foo"] * 3
+
+
+@pytest.mark.asyncio
+async def test_abatch() -> None:
+    llm = FakeLLM()
+    output = await llm.abatch(["foo", "bar", "foo"])
+    assert output == ["foo"] * 3
+
+    output = await llm.abatch(["foo", "bar", "foo"], config={"max_concurrency": 2})
+    assert output == ["foo"] * 3