fix: mimetype encoding for tables (#1658)

* fix: mimetype encoding for tables

* add test

marimo/_plugins/core/json_encoder.py

@@ -15,9 +15,6 @@ class WebComponentEncoder(JSONEncoder):
 
     def default(self, o: Any) -> Any:
         obj = o
-        if dataclasses.is_dataclass(obj):
-            return dataclasses.asdict(obj)
-
         # Handle numpy objects
         if DependencyManager.has_numpy():
             import numpy as np
@@ -70,6 +67,10 @@ def default(self, o: Any) -> Any:
             (mimetype, data) = obj._mime_()
             return {"mimetype": mimetype, "data": data}
 
+        # Must come after MIME objects
+        if dataclasses.is_dataclass(obj):
+            return dataclasses.asdict(obj)
+
         # Handle bytes objects
         if isinstance(obj, bytes):
             return obj.decode("utf-8")

tests/_plugins/core/test_json_encoder.py

@@ -2,6 +2,7 @@
 from __future__ import annotations
 
 import json
+from dataclasses import dataclass
 
 import pytest
 
@@ -85,6 +86,7 @@ def test_pandas_encoding() -> None:
     assert encoded_other == '["a", "b", "c"]'
 
 
+@dataclass
 class MockMIMEObject:
     def _mime_(self) -> tuple[str, str]:
         return "text/plain", "data"
@@ -96,6 +98,18 @@ def test_mime_encoding() -> None:
     assert encoded == '{"mimetype": "text/plain", "data": "data"}'
 
 
+@dataclass
+class MockDataclass:
+    a: int
+    b: str
+
+
+def test_dataclass_encoding() -> None:
+    dataclass_obj = MockDataclass(1, "hello")
+    encoded = json.dumps(dataclass_obj, cls=WebComponentEncoder)
+    assert encoded == '{"a": 1, "b": "hello"}'
+
+
 def test_bytes_encoding() -> None:
     bytes_obj = b"hello"
     encoded = json.dumps(bytes_obj, cls=WebComponentEncoder)