Add a Transformation Chain (#257)

Arbitrary transformation chains that can be used to add dictionary
extractions from llms/other chains

langchain/chains/transform.py

@@ -0,0 +1,41 @@
+"""Chain that runs an arbitrary python function."""
+from typing import Callable, Dict, List
+
+from pydantic import BaseModel
+
+from langchain.chains.base import Chain
+
+
+class TransformChain(Chain, BaseModel):
+    """Chain transform chain output.
+
+    Example:
+        .. code-block:: python
+
+            from langchain import TransformChain
+            transform_chain = TransformChain(input_variables=["text"],
+             output_variables["entities"], transform=func())
+    """
+
+    input_variables: List[str]
+    output_variables: List[str]
+    transform: Callable[[Dict[str, str]], Dict[str, str]]
+
+    @property
+    def input_keys(self) -> List[str]:
+        """Expect input keys.
+
+        :meta private:
+        """
+        return self.input_variables
+
+    @property
+    def output_keys(self) -> List[str]:
+        """Return output keys.
+
+        :meta private:
+        """
+        return self.output_variables
+
+    def _call(self, inputs: Dict[str, str]) -> Dict[str, str]:
+        return self.transform(inputs)

tests/unit_tests/chains/test_transform.py

@@ -0,0 +1,40 @@
+"""Test transform chain."""
+from typing import Dict
+
+import pytest
+
+from langchain.chains.transform import TransformChain
+
+
+def dummy_transform(inputs: Dict[str, str]) -> Dict[str, str]:
+    """Transform a dummy input for tests."""
+    outputs = inputs
+    outputs["greeting"] = f"{inputs['first_name']} {inputs['last_name']} says hello"
+    del outputs["first_name"]
+    del outputs["last_name"]
+    return outputs
+
+
+def test_tranform_chain() -> None:
+    """Test basic transform chain."""
+    transform_chain = TransformChain(
+        input_variables=["first_name", "last_name"],
+        output_variables=["greeting"],
+        transform=dummy_transform,
+    )
+    input_dict = {"first_name": "Leroy", "last_name": "Jenkins"}
+    response = transform_chain(input_dict)
+    expected_response = {"greeting": "Leroy Jenkins says hello"}
+    assert response == expected_response
+
+
+def test_transform_chain_bad_inputs() -> None:
+    """Test basic transform chain."""
+    transform_chain = TransformChain(
+        input_variables=["first_name", "last_name"],
+        output_variables=["greeting"],
+        transform=dummy_transform,
+    )
+    input_dict = {"name": "Leroy", "last_name": "Jenkins"}
+    with pytest.raises(ValueError):
+        _ = transform_chain(input_dict)