use ... for undefined

noxfile.py

@@ -3,6 +3,7 @@
 import functools
 import os
 import re
+import sys
 from pathlib import Path
 from typing import Any, Callable, TypeVar
 
@@ -160,7 +161,10 @@ def test_python(session: Session) -> None:
     """Run all Python checks"""
     session.notify("test_python_suite", posargs=session.posargs)
     session.notify("test_python_types")
-    session.notify("test_python_style")
+    if sys.version_info >= (3, 10):
+        session.notify("test_python_style")
+    else:
+        session.log("Skipping type checking - Python<3.10")
     session.notify("test_python_build")
 
 

pyproject.toml

@@ -10,3 +10,9 @@ ensure_newline_before_comments = "True"
 include_trailing_comma = "True"
 line_length = 88
 lines_after_imports = 2
+
+[tool.mypy]
+ignore_missing_imports = "True"
+warn_unused_configs = "True"
+warn_redundant_casts = "True"
+warn_unused_ignores = "True"

setup.cfg

@@ -1,12 +1,6 @@
 [bdist_wheel]
 universal=1
 
-[mypy]
-ignore_missing_imports = True
-warn_unused_configs = True
-warn_redundant_casts = True
-warn_unused_ignores = True
-
 [flake8]
 ignore = E203, E266, E501, W503, F811, N802, N806
 per-file-ignores =

src/idom/_py_compat.py

@@ -0,0 +1,6 @@
+__all__ = ["EllipsisType"]
+
+try:
+    from types import EllipsisType
+except ImportError:
+    EllipsisType = type(Ellipsis)  # pragma: no cover

src/idom/backend/flask.py

@@ -85,7 +85,7 @@ def run_server() -> None:
         if started:
             loop.call_soon_threadsafe(started.set)
         try:
-            server.current.serve_forever()  # type: ignore
+            server.current.serve_forever()
         finally:
             loop.call_soon_threadsafe(stopped.set)
 

src/idom/utils.py

@@ -1,9 +1,12 @@
+from __future__ import annotations
+
 from html.parser import HTMLParser as _HTMLParser
 from typing import Any, Callable, Dict, Generic, List, Optional, Tuple, TypeVar
 
+from ._py_compat import EllipsisType
+
 
 _RefValue = TypeVar("_RefValue")
-_UNDEFINED: Any = object()
 
 
 class Ref(Generic[_RefValue]):
@@ -19,8 +22,8 @@ class Ref(Generic[_RefValue]):
 
     __slots__ = "current"
 
-    def __init__(self, initial_value: _RefValue = _UNDEFINED) -> None:
-        if initial_value is not _UNDEFINED:
+    def __init__(self, initial_value: _RefValue | EllipsisType = ...) -> None:
+        if initial_value is not ...:
             self.current = initial_value
             """The present value"""