Use default factory to in notes for markdown demo (#393)

* Use default factory to in notes for markdown demo

* Fix type checking issue

demo/markdown_editor.py

@@ -1,7 +1,13 @@
-from dataclasses import dataclass
+from dataclasses import dataclass, field
 
 import mesop as me
 
+_INTRO_TEXT = """
+# Mesop Markdown Editor Example
+
+This is example shows how to make a simple markdown editor.
+""".strip()
+
 
 @dataclass(kw_only=True)
 class Note:
@@ -12,9 +18,9 @@ class Note:
 
 @me.stateclass
 class State:
-  notes: list[Note]
-  selected_note_index: int
-  selected_note_content: str
+  notes: list[Note] = field(default_factory=lambda: [Note(content=_INTRO_TEXT)])
+  selected_note_index: int = 0
+  selected_note_content: str = _INTRO_TEXT
   show_preview: bool = True
 
 
@@ -28,12 +34,6 @@ class State:
 def page():
   state = me.state(State)
 
-  # On load, add a starter note.
-  if not state.notes:
-    state.notes.append(Note(content=_INTRO_TEXT))
-    state.selected_note_content = state.notes[0].content
-    state.selected_note_index = 0
-
   with me.box(style=_style_container(state.show_preview)):
     # Note list column
     with me.box(style=_STYLE_NOTES_NAV):
@@ -83,13 +83,6 @@ def _render_note_excerpt(content: str) -> str:
   return content[:_EXCERPT_CHAR_LIMIT] + "..."
 
 
-_INTRO_TEXT = """
-# Mesop Markdown Editor Example
-
-This is example shows how to make a simple markdown editor.
-""".strip()
-
-
 # EVENT HANDLERS