From 181df51a0c77ac520f7fc4ac7d2cde4591686ec3 Mon Sep 17 00:00:00 2001
From: Saiyam Jain <jimisierra2@gmail.com>
Date: Sun, 25 Oct 2026 20:56:00 +0530
Subject: [PATCH 1/2] gh-160106: Fix csv.Dialect.__replace__() losing
 slot-backed attributes

copy.replace() on a csv.Dialect subclass that stores dialect options in
__slots__ would silently drop those attributes.  __replace__() only copied
self.__dict__, so any attribute stored in a slot was missing from the new
object, causing csv.reader/writer to fall back to the default value.

Fix by iterating the MRO for __slots__ entries and copying any dialect
attribute that was not already set via __dict__.
---
 Lib/csv.py | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/Lib/csv.py b/Lib/csv.py
index b406d82efffd9f3..1bf15177aa0d3a5 100644
--- a/Lib/csv.py
+++ b/Lib/csv.py
@@ -125,6 +125,15 @@ def __replace__(self, /, **changes):
                             f'argument {min(unexpected)!r}')
         new = object.__new__(self.__class__)
         new.__dict__.update(self.__dict__)
+        # Copy slot-backed dialect attributes from the instance's MRO,
+        # since __dict__ only captures attributes stored in __dict__.
+        for cls in type(self).__mro__:
+            for attr in getattr(cls, '__slots__', ()):
+                if attr in _dialect_attributes and attr not in new.__dict__:
+                    try:
+                        new.__dict__[attr] = getattr(self, attr)
+                    except AttributeError:
+                        pass
         new.__dict__.update(changes)
         new._validate()
         return new

From 376074f8051049b05e447d241ac3b2f8f97d42be Mon Sep 17 00:00:00 2001
From: "blurb-it[bot]" <43283697+blurb-it[bot]@users.noreply.github.com>
Date: Sun, 25 Oct 2026 15:33:29 +0000
Subject: [PATCH 2/2] =?UTF-8?q?=F0=9F=93=9C=F0=9F=A4=96=20Added=20by=20blu?=
 =?UTF-8?q?rb=5Fit.?=
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

---
 .../next/Library/2026-10-25-15-33-28.gh-issue-160106.G7eA2V.rst  | 1 +
 1 file changed, 1 insertion(+)
 create mode 100644 Misc/NEWS.d/next/Library/2026-10-25-15-33-28.gh-issue-160106.G7eA2V.rst

diff --git a/Misc/NEWS.d/next/Library/2026-10-25-15-33-28.gh-issue-160106.G7eA2V.rst b/Misc/NEWS.d/next/Library/2026-10-25-15-33-28.gh-issue-160106.G7eA2V.rst
new file mode 100644
index 000000000000000..ff1d43731b4a3d4
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2026-10-25-15-33-28.gh-issue-160106.G7eA2V.rst
@@ -0,0 +1 @@
+Fix :func:`copy.replace` on :class:`csv.Dialect` subclasses losing slot-backed attributes.