From c7eb1ff05cc2db01751cc6a7a2d439fc1a8aacfa Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Andr=C3=A9s=20Delfino?= <adelfino@gmail.com>
Date: Tue, 3 Nov 2026 21:40:01 -0300
Subject: [PATCH] gh-161289: Respect fillvalue length when truncating in
 reprlib.Repr

repr_str(), repr_int() and repr_instance() computed how many characters
to keep around the fill value assuming it is always three characters
long (the default '...'). With a custom Repr.fillvalue of another
length, the result exceeded or fell short of maxstring, maxlong and
maxother. Use len(self.fillvalue) instead.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
---
 Lib/reprlib.py                                  | 12 ++++++------
 Lib/test/test_reprlib.py                        | 17 +++++++++++++++++
 ...26-11-03-22-19-40.gh-issue-161289.P62n_9.rst |  4 ++++
 3 files changed, 27 insertions(+), 6 deletions(-)
 create mode 100644 Misc/NEWS.d/next/Library/2026-11-03-22-19-40.gh-issue-161289.P62n_9.rst

diff --git a/Lib/reprlib.py b/Lib/reprlib.py
index ab18247682b69a..f2af3c991c0e9f 100644
--- a/Lib/reprlib.py
+++ b/Lib/reprlib.py
@@ -174,8 +174,8 @@ def repr_dict(self, x, level):
     def repr_str(self, x, level):
         s = builtins.repr(x[:self.maxstring])
         if len(s) > self.maxstring:
-            i = max(0, (self.maxstring-3)//2)
-            j = max(0, self.maxstring-3-i)
+            i = max(0, (self.maxstring-len(self.fillvalue))//2)
+            j = max(0, self.maxstring-len(self.fillvalue)-i)
             s = builtins.repr(x[:i] + x[len(x)-j:])
             s = s[:i] + self.fillvalue + s[len(s)-j:]
         return s
@@ -198,8 +198,8 @@ def repr_int(self, x, level):
             return (f'<{x.__class__.__name__} instance with roughly {k} '
                     f'digits (limit at {max_digits}) at 0x{id(x):x}>')
         if len(s) > self.maxlong:
-            i = max(0, (self.maxlong-3)//2)
-            j = max(0, self.maxlong-3-i)
+            i = max(0, (self.maxlong-len(self.fillvalue))//2)
+            j = max(0, self.maxlong-len(self.fillvalue)-i)
             s = s[:i] + self.fillvalue + s[len(s)-j:]
         return s
 
@@ -211,8 +211,8 @@ def repr_instance(self, x, level):
         except Exception:
             return '<%s instance at %#x>' % (x.__class__.__name__, id(x))
         if len(s) > self.maxother:
-            i = max(0, (self.maxother-3)//2)
-            j = max(0, self.maxother-3-i)
+            i = max(0, (self.maxother-len(self.fillvalue))//2)
+            j = max(0, self.maxother-len(self.fillvalue)-i)
             s = s[:i] + self.fillvalue + s[len(s)-j:]
         return s
 
diff --git a/Lib/test/test_reprlib.py b/Lib/test/test_reprlib.py
index 5a95a05c6496a2..7deb4979dd172f 100644
--- a/Lib/test/test_reprlib.py
+++ b/Lib/test/test_reprlib.py
@@ -201,6 +201,23 @@ def test_instance(self):
         self.assertEndsWith(s, ">")
         self.assertIn(s.find("..."), [12, 13])
 
+    def test_truncation_with_custom_fillvalue(self):
+        # The truncated repr must fit the limit whatever the fillvalue length.
+        eq = self.assertEqual
+        s = 'abcdefghijklmnopqrstuvwxyz'
+        n = 12345678901234567890
+        i = ClassWithRepr('x' * 100)
+
+        r1 = Repr(maxstring=10, maxlong=10, maxother=10, fillvalue='…')
+        eq(r1.repr(s), "'abc…wxyz'")
+        eq(r1.repr(n), '1234…67890')
+        eq(r1.repr(i), "Clas…xxx')")
+
+        r2 = Repr(maxstring=10, maxlong=10, maxother=10, fillvalue='<snip>')
+        eq(r2.repr(s), "'a<snip>z'")
+        eq(r2.repr(n), '12<snip>90')
+        eq(r2.repr(i), "Cl<snip>')")
+
     def test_lambda(self):
         r = repr(lambda x: x)
         self.assertStartsWith(r, "<function ReprTests.test_lambda.<locals>.<lambda")
diff --git a/Misc/NEWS.d/next/Library/2026-11-03-22-19-40.gh-issue-161289.P62n_9.rst b/Misc/NEWS.d/next/Library/2026-11-03-22-19-40.gh-issue-161289.P62n_9.rst
new file mode 100644
index 00000000000000..c077686745b332
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2026-11-03-22-19-40.gh-issue-161289.P62n_9.rst
@@ -0,0 +1,4 @@
+Fix :class:`reprlib.Repr` truncation of strings, integers and other objects
+when a custom :attr:`~reprlib.Repr.fillvalue` is not three characters long.
+The result now fits :attr:`~reprlib.Repr.maxstring`,
+:attr:`~reprlib.Repr.maxlong` and :attr:`~reprlib.Repr.maxother`.