1 files changed, 21 insertions, 0 deletions
diff --git a/Lib/importlib/metadata/_adapters.py b/Lib/importlib/metadata/_adapters.py
index aa460d3..6aed69a 100644
--- a/Lib/importlib/metadata/_adapters.py
+++ b/Lib/importlib/metadata/_adapters.py
@@ -1,3 +1,5 @@
+import functools
+import warnings
 import re
 import textwrap
 import email.message
@@ -5,6 +7,15 @@ import email.message
 from ._text import FoldedCase
 
 
+# Do not remove prior to 2024-01-01 or Python 3.14
+_warn = functools.partial(
+    warnings.warn,
+    "Implicit None on return values is deprecated and will raise KeyErrors.",
+    DeprecationWarning,
+    stacklevel=2,
+)
+
+
 class Message(email.message.Message):
     multiple_use_keys = set(
         map(
@@ -39,6 +50,16 @@ class Message(email.message.Message):
     def __iter__(self):
         return super().__iter__()
 
+    def __getitem__(self, item):
+        """
+        Warn users that a ``KeyError`` can be expected when a
+        mising key is supplied. Ref python/importlib_metadata#371.
+        """
+        res = super().__getitem__(item)
+        if res is None:
+            _warn()
+        return res
+
     def _repair_headers(self):
         def redent(value):
             "Correct for RFC822 indentation"