Prototype: safe-guard against mutable default values

mesop/dataclass_utils/dataclass_utils.py

@@ -1,6 +1,6 @@
 import base64
 import json
-from dataclasses import asdict, dataclass, field, is_dataclass
+from dataclasses import Field, asdict, dataclass, field, is_dataclass
 from datetime import datetime
 from io import StringIO
 from typing import Any, Type, TypeVar, cast, get_origin, get_type_hints
@@ -32,15 +32,28 @@ def _check_has_pandas():
 _has_pandas = _check_has_pandas()
 
 
+def is_primitive(obj: Any):
+  return isinstance(obj, (int, float, bool, str, type(None), complex, bytes))
+
+
 def dataclass_with_defaults(cls: Type[C]) -> Type[C]:
   """
   Provides defaults for every attribute in a dataclass (recursively) so
   Mesop developers don't need to manually set default values
   """
-  pass
+  for name in cls.__dict__:
+    if name.startswith("__") and name.endswith("__"):
+      continue
+    # If default is already set, make sure it's a primitive value
+    if not isinstance(cls.__dict__[name], Field) and not is_primitive(
+      cls.__dict__[name]
+    ):
+      raise Exception(
+        "Detected potentially mutable type | name", name, "type", type.__name__
+      )
   annotations = get_type_hints(cls)
   for name, type_hint in annotations.items():
-    if name not in cls.__dict__:  # Skip if default already set
+    if name not in cls.__dict__:
       if type_hint == int:
         setattr(cls, name, field(default=0))
       elif type_hint == float:

mesop/dataclass_utils/dataclass_utils_test.py

@@ -5,6 +5,7 @@
 import pandas as pd
 import pytest
 
+import mesop.protos.ui_pb2 as pb
 from mesop.dataclass_utils.dataclass_utils import (
   dataclass_with_defaults,
   serialize_dataclass,
@@ -286,5 +287,45 @@ def test_serialize_deserialize_state_with_list_dict():
   assert new_state == state
 
 
+def test_raises_exception_for_mutable_default_value_default_list():
+  with pytest.raises(Exception) as exc_info:
+
+    @dataclass_with_defaults
+    class StateWithMutableDefaultValue:
+      mutable_list: list[str] = []
+
+  assert "Detected potentially mutable type" in str(exc_info.value)
+
+
+def test_raises_exception_for_mutable_default_value_default_list_no_type_annotation():
+  with pytest.raises(Exception) as exc_info:
+
+    @dataclass_with_defaults
+    class StateWithMutableDefaultValue:
+      mutable_list = []
+
+  assert "Detected potentially mutable type" in str(exc_info.value)
+
+
+def test_raises_exception_for_mutable_default_value_proto():
+  with pytest.raises(Exception) as exc_info:
+
+    @dataclass_with_defaults
+    class StateWithMutableProto:
+      proto: pb.Style = pb.Style()
+
+  assert "Detected potentially mutable type" in str(exc_info.value)
+
+
+def test_proto_works():
+  with pytest.raises(Exception) as exc_info:
+
+    @dataclass_with_defaults
+    class StateWithMutableProto:
+      proto: pb.Style
+
+  assert "FAIL" in str(exc_info.value)
+
+
 if __name__ == "__main__":
   raise SystemExit(pytest.main([__file__]))

mesop/examples/__init__.py

@@ -19,6 +19,7 @@
 from mesop.examples import index as index
 from mesop.examples import integrations as integrations
 from mesop.examples import many_checkboxes as many_checkboxes
+from mesop.examples import mutable_state as mutable_state
 from mesop.examples import navigate_advanced as navigate_advanced
 from mesop.examples import nested as nested
 from mesop.examples import on_load as on_load

mesop/examples/mutable_state.py

@@ -0,0 +1,41 @@
+import mesop as me
+import mesop.components.button.button_pb2 as button_pb
+
+
+class Bar:
+  val: str
+  pass
+
+
+class Foo:
+  val: str
+
+
+b = button_pb.ButtonType()
+b.color = "abc"
+print("~ b", b)
+c = button_pb.ButtonType()
+print("~ c", c)
+
+
+@me.stateclass
+class State:
+  button: button_pb.ButtonType
+  a: str
+  b: Bar
+
+
+s = State()
+s.button.color = "foo"
+print("s.button (1)", s.button)
+s.b.val = "hi"
+print(s)
+print("---")
+b = State()
+print("b.button (2)", b.button)
+print(b)
+
+
+@me.page()
+def page():
+  me.text("mutable state")