gh-158952: Reject read-only buffers before reading in _pyio

Lib/_pyio.py

@@ -728,6 +728,8 @@ def _readinto(self, b, read1):
         if not isinstance(b, memoryview):
             b = memoryview(b)
         b = b.cast('B')
+        if b.readonly:
+            raise TypeError("readinto() argument must be read-write bytes-like object")
 
         if read1:
             data = self.read1(len(b))

Lib/test/test_io/test_memoryio.py

@@ -568,6 +568,21 @@ def test_readinto(self):
         memio.readinto(b)
         self.assertEqual(b, b"")
 
+    def test_readinto_readonly_buffer(self):
+        data = b"abcdef"
+        for method in ("readinto", "readinto1"):
+            for buffer in (b"xxx", memoryview(b"xxx"),
+                           b"", memoryview(b"")):
+                for position in (0, 4, 10):
+                    with self.subTest(method=method, buffer=buffer,
+                                      position=position):
+                        with self.ioclass(data) as memio:
+                            memio.seek(position)
+                            with self.assertRaises(TypeError):
+                                getattr(memio, method)(buffer)
+                            self.assertEqual(memio.tell(), position)
+                            self.assertEqual(memio.read(), data[position:])
+
     def test_relative_seek(self):
         buf = self.buftype("1234567890")
         memio = self.ioclass(buf)

Misc/NEWS.d/next/Library/2026-10-07-06-28-00.gh-issue-158952.tR7kLp.rst

@@ -0,0 +1,3 @@
+The pure Python implementation of :meth:`io.BufferedIOBase.readinto` and
+:meth:`~io.BufferedIOBase.readinto1` now rejects read-only buffers before
+reading input, without changing the stream position.