Add not_none

static_tests/test_not_none.py

@@ -0,0 +1,12 @@
+from __future__ import annotations
+
+from typing import Any
+from typing_extensions import assert_type
+
+from useful_types import not_none
+
+
+def test(x: Any, y: str | None, z: str) -> None:
+    assert_type(not_none(x), Any)
+    assert_type(not_none(y), str)
+    assert_type(not_none(z), str)

useful_types/__init__.py

@@ -321,3 +321,17 @@ def __getitem__(self, __i: int) -> int:
 
 class SizedBuffer(Sized, Buffer, Protocol):
     ...
+
+
+def not_none(obj: _T | None, /, message: str | None = None) -> _T:
+    """Raise TypeError if obj is None, otherwise return obj.
+
+    Useful for safely casting away optional types.
+
+    """
+    if obj is None:
+        if message is not None:
+            raise TypeError(message)
+        else:
+            raise TypeError("Object is unexpectedly None")
+    return obj