adding sentinel metaclass example

15-more-types/mysum.py

@@ -0,0 +1,13 @@
+from functools import reduce  # <1>
+from operator import add
+from typing import overload, Iterable, Union, TypeVar
+
+T = TypeVar('T')
+S = TypeVar('S')  # <2>
+
+@overload
+def sum(it: Iterable[T]) -> Union[T, int]: ...  # <3>
+@overload
+def sum(it: Iterable[T], /, start: S) -> Union[T, S]: ...  # <4>
+def sum(it, /, start=0):  # <5>
+    return reduce(add, it, start)