API: fix serialization of union types

Due to this bug in serde.py, the run_generators API id not work for the frontend

pkgs/clan-cli/clan_lib/api/serde.py

@@ -30,6 +30,7 @@ Note: This module assumes the presence of other modules and classes such as `Cla
 
 import dataclasses
 import inspect
+import traceback
 from dataclasses import dataclass, fields, is_dataclass
 from enum import Enum
 from pathlib import Path
@@ -40,6 +41,7 @@ from typing import (
     Literal,
     TypeVar,
     Union,
+    cast,
     get_args,
     get_origin,
     is_typeddict,
@@ -180,6 +182,19 @@ def unwrap_none_type(type_hint: type | UnionType) -> type:
     return type_hint  # type: ignore
 
 
+def unwrap_union_type(type_hint: type | UnionType) -> list[type]:
+    """Takes a type union and returns the first non-None type.
+    None | str
+    =>
+    str
+    """
+    if is_union_type(type_hint):
+        # Return the first non-None type
+        return list(get_args(type_hint))
+
+    return [type_hint]  # type: ignore
+
+
 JsonValue = str | float | dict[str, Any] | list[Any] | None
 
 
@@ -259,9 +274,19 @@ def construct_value(
     # Union types construct the first non-None type
     if is_union_type(t):
         # Unwrap the union type
-        inner = unwrap_none_type(t)
+        inner_types = unwrap_union_type(t)
         # Construct the field value
-        return construct_value(inner, field_value)
+        errors = []
+        for t in inner_types:
+            try:
+                return construct_value(t, field_value, loc)
+            except ClanError as exc:
+                errors.append(exc)
+                continue
+        msg = f"Cannot construct field of type {t} while constructing a union type from value: {field_value}"
+        for e in errors:
+            traceback.print_exception(e)
+        raise ClanError(msg, location=f"{loc}")
 
     # Nested types
     # list
@@ -348,7 +373,6 @@ def construct_dataclass[T: Any](
             continue
         # The first type in a Union
         # str <- None | str | Path
-        field_type: type[Any] = unwrap_none_type(field.type)  # type: ignore
         data_field_name = field.metadata.get("alias", field.name)
 
         if (
@@ -367,7 +391,9 @@ def construct_dataclass[T: Any](
             ):
                 field_values[field.name] = None
             else:
-                field_values[field.name] = construct_value(field_type, field_value)
+                field_values[field.name] = construct_value(
+                    cast(type, field.type), field_value
+                )
 
     # Check that all required field are present.
     for field_name in required:

pkgs/clan-cli/clan_lib/api/serde_deserialize_test.py

@@ -387,3 +387,22 @@ def test_unknown_serialize() -> None:
 
     person = dataclass_to_dict(data)
     assert person == {"name": ["a", "b"]}
+
+
+def test_union_dataclass() -> None:
+    @dataclass
+    class A:
+        val: str | list[str] | None = None
+
+    data1 = {"val": "hello"}
+    expected1 = A(val="hello")
+    assert from_dict(A, data1) == expected1
+    data2 = {"val": ["a", "b"]}
+    expected2 = A(val=["a", "b"])
+    assert from_dict(A, data2) == expected2
+    data3 = {"val": None}
+    expected3 = A(val=None)
+    assert from_dict(A, data3) == expected3
+    data4: dict[str, object] = {}
+    expected4 = A(val=None)
+    assert from_dict(A, data4) == expected4