community: support Databricks Unity Catalog functions as LangChain tools (#22555)

This PR adds support for using Databricks Unity Catalog functions as
LangChain tools, which runs inside a Databricks SQL warehouse.

* An example notebook is provided.

libs/community/langchain_community/tools/databricks/_execution.py

@@ -0,0 +1,172 @@
+import json
+from dataclasses import dataclass
+from io import StringIO
+from typing import TYPE_CHECKING, Any, Dict, List, Literal, Optional
+
+if TYPE_CHECKING:
+    from databricks.sdk import WorkspaceClient
+    from databricks.sdk.service.catalog import FunctionInfo
+    from databricks.sdk.service.sql import StatementParameterListItem
+
+
+def is_scalar(function: "FunctionInfo") -> bool:
+    from databricks.sdk.service.catalog import ColumnTypeName
+
+    return function.data_type != ColumnTypeName.TABLE_TYPE
+
+
+@dataclass
+class ParameterizedStatement:
+    statement: str
+    parameters: List["StatementParameterListItem"]
+
+
+@dataclass
+class FunctionExecutionResult:
+    """
+    Result of executing a function.
+    We always use a string to present the result value for AI model to consume.
+    """
+
+    error: Optional[str] = None
+    format: Optional[Literal["SCALAR", "CSV"]] = None
+    value: Optional[str] = None
+    truncated: Optional[bool] = None
+
+    def to_json(self) -> str:
+        data = {k: v for (k, v) in self.__dict__.items() if v is not None}
+        return json.dumps(data)
+
+
+def get_execute_function_sql_stmt(
+    function: "FunctionInfo", json_params: Dict[str, Any]
+) -> ParameterizedStatement:
+    from databricks.sdk.service.catalog import ColumnTypeName
+    from databricks.sdk.service.sql import StatementParameterListItem
+
+    parts = []
+    output_params = []
+    if is_scalar(function):
+        # TODO: IDENTIFIER(:function) did not work
+        parts.append(f"SELECT {function.full_name}(")
+    else:
+        parts.append(f"SELECT * FROM {function.full_name}(")
+    if function.input_params is None or function.input_params.parameters is None:
+        assert (
+            not json_params
+        ), "Function has no parameters but parameters were provided."
+    else:
+        args = []
+        use_named_args = False
+        for p in function.input_params.parameters:
+            if p.name not in json_params:
+                if p.parameter_default is not None:
+                    use_named_args = True
+                else:
+                    raise ValueError(
+                        f"Parameter {p.name} is required but not provided."
+                    )
+            else:
+                arg_clause = ""
+                if use_named_args:
+                    arg_clause += f"{p.name} => "
+                json_value = json_params[p.name]
+                if p.type_name in (
+                    ColumnTypeName.ARRAY,
+                    ColumnTypeName.MAP,
+                    ColumnTypeName.STRUCT,
+                ):
+                    # Use from_json to restore values of complex types.
+                    json_value_str = json.dumps(json_value)
+                    # TODO: parametrize type
+                    arg_clause += f"from_json(:{p.name}, '{p.type_text}')"
+                    output_params.append(
+                        StatementParameterListItem(name=p.name, value=json_value_str)
+                    )
+                elif p.type_name == ColumnTypeName.BINARY:
+                    # Use ubbase64 to restore binary values.
+                    arg_clause += f"unbase64(:{p.name})"
+                    output_params.append(
+                        StatementParameterListItem(name=p.name, value=json_value)
+                    )
+                else:
+                    arg_clause += f":{p.name}"
+                    output_params.append(
+                        StatementParameterListItem(
+                            name=p.name, value=json_value, type=p.type_text
+                        )
+                    )
+                args.append(arg_clause)
+        parts.append(",".join(args))
+    parts.append(")")
+    # TODO: check extra params in kwargs
+    statement = "".join(parts)
+    return ParameterizedStatement(statement=statement, parameters=output_params)
+
+
+def execute_function(
+    ws: "WorkspaceClient",
+    warehouse_id: str,
+    function: "FunctionInfo",
+    parameters: Dict[str, Any],
+) -> FunctionExecutionResult:
+    """
+    Execute a function with the given arguments and return the result.
+    """
+    try:
+        import pandas as pd
+    except ImportError as e:
+        raise ImportError(
+            "Could not import pandas python package. "
+            "Please install it with `pip install pandas`."
+        ) from e
+    from databricks.sdk.service.sql import StatementState
+
+    # TODO: async so we can run functions in parallel
+    parametrized_statement = get_execute_function_sql_stmt(function, parameters)
+    # TODO: configurable limits
+    response = ws.statement_execution.execute_statement(
+        statement=parametrized_statement.statement,
+        warehouse_id=warehouse_id,
+        parameters=parametrized_statement.parameters,
+        wait_timeout="30s",
+        row_limit=100,
+        byte_limit=4096,
+    )
+    status = response.status
+    assert status is not None, f"Statement execution failed: {response}"
+    if status.state != StatementState.SUCCEEDED:
+        error = status.error
+        assert (
+            error is not None
+        ), "Statement execution failed but no error message was provided."
+        return FunctionExecutionResult(error=f"{error.error_code}: {error.message}")
+    manifest = response.manifest
+    assert manifest is not None
+    truncated = manifest.truncated
+    result = response.result
+    assert (
+        result is not None
+    ), "Statement execution succeeded but no result was provided."
+    data_array = result.data_array
+    if is_scalar(function):
+        value = None
+        if data_array and len(data_array) > 0 and len(data_array[0]) > 0:
+            value = str(data_array[0][0])  # type: ignore
+        return FunctionExecutionResult(
+            format="SCALAR", value=value, truncated=truncated
+        )
+    else:
+        schema = manifest.schema
+        assert (
+            schema is not None and schema.columns is not None
+        ), "Statement execution succeeded but no schema was provided."
+        columns = [c.name for c in schema.columns]
+        if data_array is None:
+            data_array = []
+        pdf = pd.DataFrame.from_records(data_array, columns=columns)
+        csv_buffer = StringIO()
+        pdf.to_csv(csv_buffer, index=False)
+        return FunctionExecutionResult(
+            format="CSV", value=csv_buffer.getvalue(), truncated=truncated
+        )