diff --git a/python/cocoindex/flow.py b/python/cocoindex/flow.py
index 5d1de9a1..605e771d 100644
--- a/python/cocoindex/flow.py
+++ b/python/cocoindex/flow.py
@@ -5,25 +5,27 @@
 from __future__ import annotations
 
 import asyncio
-import re
-import inspect
 import datetime
 import functools
+import inspect
+import re
+
+from dataclasses import dataclass
+from enum import Enum
+from threading import Lock
 from typing import (
     Any,
     Callable,
+    Generic,
+    NamedTuple,
     Sequence,
     TypeVar,
-    Generic,
+    cast,
     get_args,
     get_origin,
-    NamedTuple,
-    cast,
     Iterable,
 )
-from threading import Lock
-from enum import Enum
-from dataclasses import dataclass
+
 from rich.text import Text
 from rich.tree import Tree
 
@@ -32,9 +34,10 @@
 from . import op
 from . import setting
 from .convert import dump_engine_object, encode_engine_value, make_engine_value_decoder
-from .typing import encode_enriched_type
+from .op import FunctionSpec
 from .runtime import execution_context
 from .setup import SetupChangeBundle
+from .typing import encode_enriched_type
 
 
 class _NameBuilder:
@@ -92,6 +95,30 @@ def _spec_kind(spec: Any) -> str:
     return cast(str, spec.__class__.__name__)
 
 
+def _transform_helper(
+    flow_builder_state: _FlowBuilderState,
+    fn_spec: FunctionSpec,
+    transform_args: list[tuple[Any, str | None]],
+    name: str | None = None,
+) -> DataSlice[Any]:
+    if not isinstance(fn_spec, FunctionSpec):
+        raise ValueError("transform() can only be called on a CocoIndex function")
+
+    return _create_data_slice(
+        flow_builder_state,
+        lambda target_scope, name: flow_builder_state.engine_flow_builder.transform(
+            _spec_kind(fn_spec),
+            dump_engine_object(fn_spec),
+            transform_args,
+            target_scope,
+            flow_builder_state.field_name_builder.build_name(
+                name, prefix=_to_snake_case(_spec_kind(fn_spec)) + "_"
+            ),
+        ),
+        name,
+    )
+
+
 T = TypeVar("T")
 S = TypeVar("S")
 
@@ -191,31 +218,19 @@ def transform(
         """
         Apply a function to the data slice.
         """
-        if not isinstance(fn_spec, op.FunctionSpec):
-            raise ValueError("transform() can only be called on a CocoIndex function")
-
-        transform_args: list[tuple[Any, str | None]]
-        transform_args = [(self._state.engine_data_slice, None)]
+        transform_args: list[tuple[Any, str | None]] = [
+            (self._state.engine_data_slice, None)
+        ]
         transform_args += [
             (self._state.flow_builder_state.get_data_slice(v), None) for v in args
         ]
         transform_args += [
             (self._state.flow_builder_state.get_data_slice(v), k)
-            for (k, v) in kwargs.items()
+            for k, v in kwargs.items()
         ]
 
-        flow_builder_state = self._state.flow_builder_state
-        return _create_data_slice(
-            flow_builder_state,
-            lambda target_scope, name: flow_builder_state.engine_flow_builder.transform(
-                _spec_kind(fn_spec),
-                dump_engine_object(fn_spec),
-                transform_args,
-                target_scope,
-                flow_builder_state.field_name_builder.build_name(
-                    name, prefix=_to_snake_case(_spec_kind(fn_spec)) + "_"
-                ),
-            ),
+        return _transform_helper(
+            self._state.flow_builder_state, fn_spec, transform_args
         )
 
     def call(self, func: Callable[..., S], *args: Any, **kwargs: Any) -> S:
@@ -446,6 +461,24 @@ def add_source(
             name,
         )
 
+    def transform(
+        self, fn_spec: FunctionSpec, *args: Any, **kwargs: Any
+    ) -> DataSlice[Any]:
+        """
+        Apply a function to inputs, returning a DataSlice.
+        """
+        transform_args: list[tuple[Any, str | None]] = [
+            (self._state.get_data_slice(v), None) for v in args
+        ]
+        transform_args += [
+            (self._state.get_data_slice(v), k) for k, v in kwargs.items()
+        ]
+
+        if not transform_args:
+            raise ValueError("At least one input is required for transformation")
+
+        return _transform_helper(self._state, fn_spec, transform_args)
+
     def declare(self, spec: op.DeclarationSpec) -> None:
         """
         Add a declaration to the flow.
diff --git a/python/cocoindex/op.py b/python/cocoindex/op.py
index 140d6fd8..a53e337d 100644
--- a/python/cocoindex/op.py
+++ b/python/cocoindex/op.py
@@ -5,13 +5,12 @@
 import asyncio
 import dataclasses
 import inspect
-
-from typing import Protocol, Any, Callable, Awaitable, dataclass_transform
 from enum import Enum
+from typing import Any, Awaitable, Callable, Protocol, dataclass_transform
 
-from .typing import encode_enriched_type, resolve_forward_ref
-from .convert import encode_engine_value, make_engine_value_decoder
 from . import _engine  # type: ignore
+from .convert import encode_engine_value, make_engine_value_decoder
+from .typing import encode_enriched_type, resolve_forward_ref
 
 
 class OpCategory(Enum):