From c3ece693363351b1835a0fcd88cd2abef27950c2 Mon Sep 17 00:00:00 2001
From: aaron-fl <43595516+aaron-fl@users.noreply.github.com>
Date: Thu, 13 Aug 2026 18:28:51 +0900
Subject: [PATCH] gh-102967: Don't pass None to seek(), simply truncate from
 the current position (GH-102968) (cherry picked from commit
 7586115ce3adeb9133717f0a721fb05621dd6700)

Co-authored-by: aaron-fl <43595516+aaron-fl@users.noreply.github.com>
---
 Lib/doctest.py                                                 | 3 ++-
 Misc/ACKS                                                      | 1 +
 .../Library/2023-03-23-19-47-05.gh-issue-102967.gfIYkN.rst     | 2 ++
 3 files changed, 5 insertions(+), 1 deletion(-)
 create mode 100644 Misc/NEWS.d/next/Library/2023-03-23-19-47-05.gh-issue-102967.gfIYkN.rst

diff --git a/Lib/doctest.py b/Lib/doctest.py
index 84951f46bf52bf4..071124d060a5204 100644
--- a/Lib/doctest.py
+++ b/Lib/doctest.py
@@ -291,7 +291,8 @@ def getvalue(self):
         return result
 
     def truncate(self, size=None):
-        self.seek(size)
+        if size is not None:
+            self.seek(size)
         StringIO.truncate(self)
 
 # Worst-case linear-time ellipsis matching.
diff --git a/Misc/ACKS b/Misc/ACKS
index c39e91c853e9e3a..04401cf87ca6e47 100644
--- a/Misc/ACKS
+++ b/Misc/ACKS
@@ -419,6 +419,7 @@ Lars Damerow
 Evan Dandrea
 Eric Daniel
 Scott David Daniels
+Aaron Davidson
 Derzsi Dániel
 Lawrence D'Anna
 Ben Darnell
diff --git a/Misc/NEWS.d/next/Library/2023-03-23-19-47-05.gh-issue-102967.gfIYkN.rst b/Misc/NEWS.d/next/Library/2023-03-23-19-47-05.gh-issue-102967.gfIYkN.rst
new file mode 100644
index 000000000000000..677b8ee86e6c8f6
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2023-03-23-19-47-05.gh-issue-102967.gfIYkN.rst
@@ -0,0 +1,2 @@
+A bug in :func:`!doctest._SpoofOut.truncate` was causing None to be passed to :func:`!StringIO.seek` when no size was given.
+A simple fix skips the seek call when no size is given so the buffer can be truncated from the current position.