From 7777ac73753eb442c1655f197c0d93cb8066112e Mon Sep 17 00:00:00 2001
From: paul fisher <paul.fisher@tiledb.com>
Date: Fri, 6 Oct 2023 21:16:04 -0400
Subject: [PATCH] Add a note about why the fields of Slice are `@property`s.
 (#175)

---
 python-spec/src/somacore/types.py | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/python-spec/src/somacore/types.py b/python-spec/src/somacore/types.py
index 3d5e8422..63606298 100644
--- a/python-spec/src/somacore/types.py
+++ b/python-spec/src/somacore/types.py
@@ -35,6 +35,18 @@ class Slice(Protocol[_T_co]):
     ``start``/``stop``/``step`` and would match, but are *not* slices.
     """
 
+    # We use @property here to indicate that these fields are read-only;
+    # just saying::
+    #
+    #     start: Optional[_T_co]
+    #
+    # would imply that doing::
+    #
+    #     some_slice.start = a_new_value
+    #
+    # was valid, thus making mypy whine (correctly!) that _T_co should be
+    # invariant rather than covariant.
+
     @property
     def start(self) -> Optional[_T_co]:
         ...