fix: reconstruct dataclasses with derived fields

CHANGELOG.md

@@ -9,6 +9,9 @@ adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 FIXED
 
+- Dataclasses with `init=False` fields now reconstruct as their declared type
+instead of falling back to a raw dictionary. Derived fields are initialized by
+the dataclass constructor rather than passed as unsupported keyword arguments.
 - Timer callbacks no longer schedule additional long-timer chunks, retry
 activities or sub-orchestrations, or resume orchestrator code after completion,
 failure, termination, or continue-as-new. Work scheduled before the terminal

durabletask/serialization.py

@@ -504,7 +504,9 @@ def _build_dataclass(cls: Any, data: dict[str, Any],
     globalns = _type_namespace(cls)
     kwargs: dict[str, Any] = {}
     for field in dataclasses.fields(cls):
-        if field.name not in data:
+        # Derived fields are serialized, but are initialized by the dataclass
+        # itself (for example in __post_init__), not by constructor arguments.
+        if not field.init or field.name not in data:
             continue
         # ``get_type_hints`` on Python 3.10 does not deep-resolve forward
         # references nested inside container args (e.g. the ``"TreeNode"`` in

tests/durabletask/test_data_converter.py

@@ -5,7 +5,7 @@
 
 import json
 import logging
-from dataclasses import dataclass
+from dataclasses import dataclass, field
 from typing import Any
 
 from durabletask.serialization import (
@@ -21,6 +21,42 @@ class Order:
     quantity: int
 
 
+@dataclass(frozen=True)
+class PricedOrder:
+    quantity: int
+    unit_price: int
+    total: int = field(init=False)
+
+    def __post_init__(self):
+        object.__setattr__(self, "total", self.quantity * self.unit_price)
+
+
+@dataclass
+class Shipment:
+    order: PricedOrder
+
+
+def test_round_trip_dataclass_with_derived_field():
+    converter = JsonDataConverter()
+    order = PricedOrder(3, 10)
+    encoded = converter.serialize(order)
+    assert json.loads(encoded) == {"quantity": 3, "unit_price": 10, "total": 30}
+    assert converter.deserialize(encoded, PricedOrder) == order
+
+
+def test_coerce_dataclass_recomputes_derived_field():
+    converter = JsonDataConverter()
+    result = converter.coerce({"quantity": 3, "unit_price": 10, "total": 999}, PricedOrder)
+    assert result == PricedOrder(3, 10)
+    assert result.total == 30
+
+
+def test_round_trip_nested_dataclass_with_derived_field():
+    converter = JsonDataConverter()
+    shipment = Shipment(PricedOrder(3, 10))
+    assert converter.deserialize(converter.serialize(shipment), Shipment) == shipment
+
+
 # ----- JsonDataConverter -----