gh-161272: Fix off-by-one in _pyio.IOBase.readlines(hint)

Author: tsayin4

Lib/_pyio.py

@@ -576,7 +576,7 @@ def readlines(self, hint=None):
         for line in self:
             lines.append(line)
             n += len(line)
-            if n >= hint:
+            if n > hint:
                 break
         return lines
 

Lib/test/test_io/test_general.py

@@ -319,6 +319,24 @@ def test_readline(self):
         with self.open(os_helper.TESTFN, "r", encoding="utf-8") as f:
             self.assertRaises(TypeError, f.readline, 5.3)
 
+    def test_readlines_hint(self):
+        # gh-161272: reading stops only once the total size of the lines
+        # read *exceeds* hint, not when it is equal to it.
+        all_lines = [b"a\n", b"bb\n", b"ccc\n", b"dddd\n"]
+        with self.open(os_helper.TESTFN, "wb") as f:
+            f.writelines(all_lines)
+        for hint, expected in ((1, all_lines[:1]), (2, all_lines[:2]),
+                               (3, all_lines[:2]), (5, all_lines[:3]),
+                               (9, all_lines), (100, all_lines),
+                               (0, all_lines), (-1, all_lines)):
+            with self.subTest(hint=hint):
+                with self.open(os_helper.TESTFN, "rb") as f:
+                    self.assertEqual(f.readlines(hint), expected)
+        with self.open(os_helper.TESTFN, "rb") as f:
+            self.assertEqual(f.readlines(), all_lines)
+        with self.open(os_helper.TESTFN, "r", encoding="ascii") as f:
+            self.assertEqual(f.readlines(2), ["a\n", "bb\n"])
+
     def test_readline_limit_with_short_read(self):
         class Reader(self.IOBase):
             data = b"abcdef"

Misc/NEWS.d/next/Library/2026-11-03-20-12-00.gh-issue-161272.Zr9Tb4.rst

@@ -0,0 +1,5 @@
+Fix an off-by-one in the pure Python implementation of
+:meth:`io.IOBase.readlines` (``_pyio``): it now stops reading only once the
+total size of the lines read *exceeds* *hint*, as documented and as the C
+implementation does.  Previously one line fewer was returned when that total
+was exactly *hint*.