--- a/mercurial/ui.py	Wed Dec 07 20:12:23 2022 +0100
+++ b/mercurial/ui.py	Fri Nov 25 18:39:47 2022 -0500
@@ -1795,6 +1795,9 @@
         # choices containing spaces, ASCII, or basically anything
         # except an ampersand followed by a character.
         m = re.match(br'(?s)(.+?)\$\$([^$]*&[^ $].*)', prompt)
+
+        assert m is not None  # help pytype
+
         msg = m.group(1)
         choices = [p.strip(b' ') for p in m.group(2).split(b'$$')]