gh-149119: docs: mention that sentinel does not support subclassing (#149120)

Add note about sentinel objects not supporting subclassing

Clarify that sentinel objects do not support subclassing.

diff --git a/Doc/library/functions.rst b/Doc/library/functions.rst
index aa99d198e436d575a05bd4517a38c3242fa38a62..06fd5cdc7be2a63cd2015d3ca438de801cfe4f4e 100644 (file)
--- a/Doc/library/functions.rst
+++ b/Doc/library/functions.rst
@@ -1839,6 +1839,8 @@ are always available.  They are listed here in alphabetical order.
    Sentinel objects are truthy and compare equal only to themselves.  They are
    intended to be compared with the :keyword:`is` operator.
 
+   ``sentinel`` does not support subclassing.
+
    Shallow and deep copies of a sentinel object return the object itself.
 
    Sentinels are conventionally assigned to a variable with a matching name.