Make object serializability extensible

Now, Python classes just need to implement a for_json method, so that
objects can be sent across processes via the Appose JSON protocol.

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: ctrueden

src/appose/shm.py

@@ -105,6 +105,13 @@ def dispose(self) -> None:
         else:
             self.close()
 
+    def for_json(self):
+        return {
+            "appose_type": "shm",
+            "name": self.name,
+            "rsize": self.rsize,
+        }
+
     def __enter__(self) -> "SharedMemory":
         return self
 
@@ -162,6 +169,14 @@ def ndarray(self):
         except ModuleNotFoundError:
             raise ImportError("NumPy is not available.")
 
+    def for_json(self):
+        return {
+            "appose_type": "ndarray",
+            "dtype": self.dtype,
+            "shape": self.shape,
+            "shm": self.shm,
+        }
+
     def __enter__(self) -> "NDArray":
         return self
 

src/appose/util/message.py

@@ -36,19 +36,8 @@ def decode(the_json: str) -> Args:
 
 class _ApposeJSONEncoder(json.JSONEncoder):
     def default(self, obj):
-        if isinstance(obj, SharedMemory):
-            return {
-                "appose_type": "shm",
-                "name": obj.name,
-                "rsize": obj.rsize,
-            }
-        if isinstance(obj, NDArray):
-            return {
-                "appose_type": "ndarray",
-                "dtype": obj.dtype,
-                "shape": obj.shape,
-                "shm": obj.shm,
-            }
+        if hasattr(obj, "for_json"):
+            return obj.for_json()
 
         # If in worker mode and object is not JSON-serializable,
         # auto-export it and return a worker_object reference.