Add a Transformation Chain (#257)

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

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)