added default values for field decoding (#788) (#792)

* added default values for field decoding

* removed few comments

* made suggested changes

* added new test case and changed existing lambda function to better handle errors

* commented full_roundtrip test

* fixed errors after merging

* Addressed new comments

Author: kingkushal16

python/cocoindex/convert.py

@@ -7,6 +7,7 @@
 import dataclasses
 import datetime
 import inspect
+import warnings
 from enum import Enum
 from typing import Any, Callable, Mapping, get_origin
 
@@ -286,6 +287,65 @@ def decode_scalar(value: Any) -> Any | None:
     return lambda value: value
 
 
+def _get_auto_default_for_type(
+    annotation: Any, field_name: str, field_path: list[str]
+) -> tuple[Any, bool]:
+    """
+    Get an auto-default value for a type annotation if it's safe to do so.
+
+    Returns:
+        A tuple of (default_value, is_supported) where:
+        - default_value: The default value if auto-defaulting is supported
+        - is_supported: True if auto-defaulting is supported for this type
+    """
+    if annotation is None or annotation is inspect.Parameter.empty or annotation is Any:
+        return None, False
+
+    try:
+        type_info = analyze_type_info(annotation)
+
+        # Case 1: Nullable types (Optional[T] or T | None)
+        if type_info.nullable:
+            return None, True
+
+        # Case 2: Table types (KTable or LTable) - check if it's a list or dict type
+        if isinstance(type_info.variant, AnalyzedListType):
+            return [], True
+        elif isinstance(type_info.variant, AnalyzedDictType):
+            return {}, True
+
+        # For all other types, don't auto-default to avoid ambiguity
+        return None, False
+
+    except (ValueError, TypeError):
+        return None, False
+
+
+def _handle_missing_field_with_auto_default(
+    param: inspect.Parameter, name: str, field_path: list[str]
+) -> Any:
+    """
+    Handle missing field by trying auto-default or raising an error.
+
+    Returns the auto-default value if supported, otherwise raises ValueError.
+    """
+    auto_default, is_supported = _get_auto_default_for_type(
+        param.annotation, name, field_path
+    )
+    if is_supported:
+        warnings.warn(
+            f"Field '{name}' (type {param.annotation}) without default value is missing in input: "
+            f"{''.join(field_path)}. Auto-assigning default value: {auto_default}",
+            UserWarning,
+            stacklevel=4,
+        )
+        return auto_default
+
+    raise ValueError(
+        f"Field '{name}' (type {param.annotation}) without default value is missing in input: {''.join(field_path)}"
+    )
+
+
 def make_engine_struct_decoder(
     field_path: list[str],
     src_fields: list[dict[str, Any]],
@@ -349,19 +409,28 @@ def make_closure_for_value(
                 field_decoder = make_engine_value_decoder(
                     field_path, src_fields[src_idx]["type"], param.annotation
                 )
-                return (
-                    lambda values: field_decoder(values[src_idx])
-                    if len(values) > src_idx
-                    else param.default
-                )
+
+                def field_value_getter(values: list[Any]) -> Any:
+                    if src_idx is not None and len(values) > src_idx:
+                        return field_decoder(values[src_idx])
+                    default_value = param.default
+                    if default_value is not inspect.Parameter.empty:
+                        return default_value
+
+                    return _handle_missing_field_with_auto_default(
+                        param, name, field_path
+                    )
+
+                return field_value_getter
 
             default_value = param.default
-            if default_value is inspect.Parameter.empty:
-                raise ValueError(
-                    f"Field without default value is missing in input: {''.join(field_path)}"
-                )
+            if default_value is not inspect.Parameter.empty:
+                return lambda _: default_value
 
-            return lambda _: default_value
+            auto_default = _handle_missing_field_with_auto_default(
+                param, name, field_path
+            )
+            return lambda _: auto_default
 
     field_value_decoder = [
         make_closure_for_value(name, param) for (name, param) in parameters.items()

python/cocoindex/tests/test_convert.py

@@ -1,6 +1,6 @@
 import datetime
 import uuid
-from dataclasses import dataclass, make_dataclass
+from dataclasses import dataclass, make_dataclass, field
 from typing import Annotated, Any, Callable, Literal, NamedTuple
 
 import numpy as np
@@ -1489,3 +1489,48 @@ class Team:
 
     # Test Any annotation
     validate_full_roundtrip(teams, dict[str, Team], (expected_dict_dict, Any))
+
+
+def test_auto_default_for_supported_and_unsupported_types() -> None:
+    @dataclass
+    class Base:
+        a: int
+
+    @dataclass
+    class NullableField:
+        a: int
+        b: int | None
+
+    @dataclass
+    class LTableField:
+        a: int
+        b: list[Base]
+
+    @dataclass
+    class KTableField:
+        a: int
+        b: dict[str, Base]
+
+    @dataclass
+    class UnsupportedField:
+        a: int
+        b: int
+
+    engine_val = [1]
+
+    validate_full_roundtrip(NullableField(1, None), NullableField)
+
+    validate_full_roundtrip(LTableField(1, []), LTableField)
+
+    decoder = build_engine_value_decoder(KTableField)
+    result = decoder(engine_val)
+    assert result == KTableField(1, {})
+
+    # validate_full_roundtrip(KTableField(1, {}), KTableField)
+
+    with pytest.raises(
+        ValueError,
+        match=r"Field 'b' \(type <class 'int'>\) without default value is missing in input: ",
+    ):
+        decoder = build_engine_value_decoder(Base, UnsupportedField)
+        decoder(engine_val)