From 1e5e3ab69de09f72f877b8eb72ac10a646c9d373 Mon Sep 17 00:00:00 2001
From: Marcel Martin <marcel.martin@scilifelab.se>
Date: Sat, 22 Jan 2022 22:21:41 +0100
Subject: [PATCH 01/29] Implement BytesIO.peek()

---
 Lib/_pyio.py                   |  8 +++++++
 Lib/test/test_memoryio.py      | 17 +++++++++++++++
 Modules/_io/bytesio.c          | 37 +++++++++++++++++++++++++++++++++
 Modules/_io/clinic/bytesio.c.h | 38 +++++++++++++++++++++++++++++++++-
 4 files changed, 99 insertions(+), 1 deletion(-)

diff --git a/Lib/_pyio.py b/Lib/_pyio.py
index 32698abac78d25..339d1aa7e13084 100644
--- a/Lib/_pyio.py
+++ b/Lib/_pyio.py
@@ -978,6 +978,14 @@ def tell(self):
             raise ValueError("tell on closed file")
         return self._pos
 
+    def peek(self, size=-1):
+        pos = self.tell()
+        if size == 0:
+            size = -1
+        b = self.read(size)
+        self.seek(pos)
+        return b
+
     def truncate(self, pos=None):
         if self.closed:
             raise ValueError("truncate on closed file")
diff --git a/Lib/test/test_memoryio.py b/Lib/test/test_memoryio.py
index cd2faba1791c77..f77a99f1bbe14d 100644
--- a/Lib/test/test_memoryio.py
+++ b/Lib/test/test_memoryio.py
@@ -517,6 +517,23 @@ def test_relative_seek(self):
         memio.seek(1, 1)
         self.assertEqual(memio.read(), buf[1:])
 
+    def test_peek(self):
+        buf = self.buftype("1234567890")
+        memio = self.ioclass(buf)
+
+        self.assertEqual(memio.peek(1), buf[:1])
+        self.assertEqual(memio.peek(1), buf[:1])
+        self.assertEqual(memio.peek(), buf)
+        self.assertEqual(memio.peek(0), buf)
+        memio.read(1)
+        self.assertEqual(memio.peek(1), buf[1:2])
+        self.assertEqual(memio.peek(), buf[1:])
+        self.assertEqual(memio.peek(42), buf[1:])
+        memio.read()
+        self.assertEqual(memio.peek(1), self.EOF)
+        memio.close()
+        self.assertRaises(ValueError, memio.peek)
+
     def test_unicode(self):
         memio = self.ioclass()
 
diff --git a/Modules/_io/bytesio.c b/Modules/_io/bytesio.c
index f3074203f54ea2..51297f55eee8a1 100644
--- a/Modules/_io/bytesio.c
+++ b/Modules/_io/bytesio.c
@@ -462,6 +462,42 @@ _io_BytesIO_read1_impl(bytesio *self, Py_ssize_t size)
     return _io_BytesIO_read_impl(self, size);
 }
 
+
+/*[clinic input]
+_io.BytesIO.peek
+    size: Py_ssize_t(accept={int, NoneType}) = -1
+    /
+
+Return bytes from the stream without advancing the position.
+
+Return an empty bytes object at EOF.
+[clinic start generated code]*/
+
+static PyObject *
+_io_BytesIO_peek_impl(bytesio *self, Py_ssize_t size)
+/*[clinic end generated code: output=fa4d8ce28b35db9b input=afc80e71b37e7c59]*/
+{
+    Py_ssize_t n;
+    const char *output;
+
+    CHECK_CLOSED(self);
+
+    /* adjust invalid sizes */
+    n = self->string_size - self->pos;
+    if (size < 1 || size > n) {
+        size = n;
+        if (size < 0)
+            size = 0;
+    }
+
+    assert(self->buf != NULL);
+    assert(size <= self->string_size);
+    output = PyBytes_AS_STRING(self->buf) + self->pos;
+    return PyBytes_FromStringAndSize(output, size);
+}
+
+
+
 /*[clinic input]
 _io.BytesIO.readline
     size: Py_ssize_t(accept={int, NoneType}) = -1
@@ -1019,6 +1055,7 @@ static struct PyMethodDef bytesio_methods[] = {
     _IO_BYTESIO_READLINE_METHODDEF
     _IO_BYTESIO_READLINES_METHODDEF
     _IO_BYTESIO_READ_METHODDEF
+    _IO_BYTESIO_PEEK_METHODDEF
     _IO_BYTESIO_GETBUFFER_METHODDEF
     _IO_BYTESIO_GETVALUE_METHODDEF
     _IO_BYTESIO_SEEK_METHODDEF
diff --git a/Modules/_io/clinic/bytesio.c.h b/Modules/_io/clinic/bytesio.c.h
index d42ab48cef2859..ae647d9d809673 100644
--- a/Modules/_io/clinic/bytesio.c.h
+++ b/Modules/_io/clinic/bytesio.c.h
@@ -232,6 +232,42 @@ _io_BytesIO_read1(bytesio *self, PyObject *const *args, Py_ssize_t nargs)
     return return_value;
 }
 
+PyDoc_STRVAR(_io_BytesIO_peek__doc__,
+"peek($self, size=-1, /)\n"
+"--\n"
+"\n"
+"Return bytes from the stream without advancing the position.\n"
+"\n"
+"Return an empty bytes object at EOF.");
+
+#define _IO_BYTESIO_PEEK_METHODDEF    \
+    {"peek", _PyCFunction_CAST(_io_BytesIO_peek), METH_FASTCALL, _io_BytesIO_peek__doc__},
+
+static PyObject *
+_io_BytesIO_peek_impl(bytesio *self, Py_ssize_t size);
+
+static PyObject *
+_io_BytesIO_peek(bytesio *self, PyObject *const *args, Py_ssize_t nargs)
+{
+    PyObject *return_value = NULL;
+    Py_ssize_t size = -1;
+
+    if (!_PyArg_CheckPositional("peek", nargs, 0, 1)) {
+        goto exit;
+    }
+    if (nargs < 1) {
+        goto skip_optional;
+    }
+    if (!_Py_convert_optional_to_ssize_t(args[0], &size)) {
+        goto exit;
+    }
+skip_optional:
+    return_value = _io_BytesIO_peek_impl(self, size);
+
+exit:
+    return return_value;
+}
+
 PyDoc_STRVAR(_io_BytesIO_readline__doc__,
 "readline($self, size=-1, /)\n"
 "--\n"
@@ -537,4 +573,4 @@ _io_BytesIO___init__(PyObject *self, PyObject *args, PyObject *kwargs)
 exit:
     return return_value;
 }
-/*[clinic end generated code: output=b753fdf1ba36c461 input=a9049054013a1b77]*/
+/*[clinic end generated code: output=106d7918b585497c input=a9049054013a1b77]*/

From acdfe2eaba72680abc0dccea2f158860038ff218 Mon Sep 17 00:00:00 2001
From: "blurb-it[bot]" <43283697+blurb-it[bot]@users.noreply.github.com>
Date: Sun, 10 Apr 2022 20:11:00 +0000
Subject: [PATCH 02/29] =?UTF-8?q?=F0=9F=93=9C=F0=9F=A4=96=20Added=20by=20b?=
 =?UTF-8?q?lurb=5Fit.?=
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

---
 .../NEWS.d/next/Library/2022-04-10-20-10-59.bpo-46375.8j1ogZ.rst | 1 +
 1 file changed, 1 insertion(+)
 create mode 100644 Misc/NEWS.d/next/Library/2022-04-10-20-10-59.bpo-46375.8j1ogZ.rst

diff --git a/Misc/NEWS.d/next/Library/2022-04-10-20-10-59.bpo-46375.8j1ogZ.rst b/Misc/NEWS.d/next/Library/2022-04-10-20-10-59.bpo-46375.8j1ogZ.rst
new file mode 100644
index 00000000000000..530381efef8746
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2022-04-10-20-10-59.bpo-46375.8j1ogZ.rst
@@ -0,0 +1 @@
+Add a ``peek()`` method to :class:`io.BytesIO`.

From b3f3c3d108b31e801c20443d64f3a88d5dabdf41 Mon Sep 17 00:00:00 2001
From: Marcel Martin <marcel.martin@scilifelab.se>
Date: Wed, 9 Nov 2022 22:13:41 +0100
Subject: [PATCH 03/29] Document BytesIO.peek()

---
 Doc/library/io.rst | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/Doc/library/io.rst b/Doc/library/io.rst
index 01088879218cb4..6327d32a4d82c7 100644
--- a/Doc/library/io.rst
+++ b/Doc/library/io.rst
@@ -728,6 +728,12 @@ than raw I/O does.
 
       Return :class:`bytes` containing the entire contents of the buffer.
 
+   .. method:: peek(size=0, /)
+
+      .. versionadded:: 3.12
+
+      Return bytes from the current position onwards but without advancing the
+      position. The number of bytes returned may be less or more than requested.
 
    .. method:: read1(size=-1, /)
 

From 25069677314c276f26c0c0eaa18c30840c3e81cd Mon Sep 17 00:00:00 2001
From: Marcel Martin <marcel.martin@scilifelab.se>
Date: Wed, 9 Nov 2022 22:18:46 +0100
Subject: [PATCH 04/29] Implement with the help of read_bytes()

This allows peek() to use the same optimization that read_bytes() has of
returning a reference to the buffer when possible (without copying).
---
 Modules/_io/bytesio.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/Modules/_io/bytesio.c b/Modules/_io/bytesio.c
index 51297f55eee8a1..e6c6634037d752 100644
--- a/Modules/_io/bytesio.c
+++ b/Modules/_io/bytesio.c
@@ -489,11 +489,11 @@ _io_BytesIO_peek_impl(bytesio *self, Py_ssize_t size)
         if (size < 0)
             size = 0;
     }
+    Py_ssize_t prev_pos = self->pos;
+    PyObject* result = read_bytes(self, size);
+    self->pos = prev_pos;
 
-    assert(self->buf != NULL);
-    assert(size <= self->string_size);
-    output = PyBytes_AS_STRING(self->buf) + self->pos;
-    return PyBytes_FromStringAndSize(output, size);
+    return result;
 }
 
 

From a5ac6012019065f19c474c7d9bf258480e55734f Mon Sep 17 00:00:00 2001
From: Marcel Martin <marcel.martin@scilifelab.se>
Date: Sat, 8 Jul 2023 14:29:35 +0200
Subject: [PATCH 05/29] =?UTF-8?q?Add=20to=20What=E2=80=99s=20New?=
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

---
 Doc/whatsnew/3.13.rst | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/Doc/whatsnew/3.13.rst b/Doc/whatsnew/3.13.rst
index fa24dc072ddefd..23d7007d03ecda 100644
--- a/Doc/whatsnew/3.13.rst
+++ b/Doc/whatsnew/3.13.rst
@@ -151,6 +151,8 @@ and only logged in :ref:`Python Development Mode <devmode>` or on :ref:`Python
 built on debug mode <debug-build>`.
 (Contributed by Victor Stinner in :gh:`62948`.)
 
+* Add :meth:`~io.BytesIO.peek`. (Contributed by Marcel Martin in :gh:`90533`.)
+
 opcode
 ------
 

From 9da7f9ff712faded9a6c3a67f0b1de4302397d37 Mon Sep 17 00:00:00 2001
From: Marcel Martin <marcel.martin@scilifelab.se>
Date: Sat, 8 Jul 2023 14:33:54 +0200
Subject: [PATCH 06/29] versionadded: 3.12 -> 3.13

---
 Doc/library/io.rst | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/Doc/library/io.rst b/Doc/library/io.rst
index 6327d32a4d82c7..d89c892e0256c9 100644
--- a/Doc/library/io.rst
+++ b/Doc/library/io.rst
@@ -730,7 +730,7 @@ than raw I/O does.
 
    .. method:: peek(size=0, /)
 
-      .. versionadded:: 3.12
+      .. versionadded:: 3.13
 
       Return bytes from the current position onwards but without advancing the
       position. The number of bytes returned may be less or more than requested.

From 79d503224309596aa2dd54c4bfb2f12f9e390b64 Mon Sep 17 00:00:00 2001
From: Marcel Martin <marcel.martin@scilifelab.se>
Date: Sat, 8 Jul 2023 14:35:22 +0200
Subject: [PATCH 07/29] Remove unused variable

---
 Modules/_io/bytesio.c | 1 -
 1 file changed, 1 deletion(-)

diff --git a/Modules/_io/bytesio.c b/Modules/_io/bytesio.c
index e6c6634037d752..8ecf5e5b0719b4 100644
--- a/Modules/_io/bytesio.c
+++ b/Modules/_io/bytesio.c
@@ -478,7 +478,6 @@ _io_BytesIO_peek_impl(bytesio *self, Py_ssize_t size)
 /*[clinic end generated code: output=fa4d8ce28b35db9b input=afc80e71b37e7c59]*/
 {
     Py_ssize_t n;
-    const char *output;
 
     CHECK_CLOSED(self);
 

From 2963dab13fc4fa8ee70ecea16a6d3b8f8906a212 Mon Sep 17 00:00:00 2001
From: Marcel Martin <marcel.martin@scilifelab.se>
Date: Fri, 22 Sep 2023 09:49:24 +0200
Subject: [PATCH 08/29] Test tell() after peek()

---
 Lib/test/test_memoryio.py | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/Lib/test/test_memoryio.py b/Lib/test/test_memoryio.py
index f77a99f1bbe14d..d1d351000fe63b 100644
--- a/Lib/test/test_memoryio.py
+++ b/Lib/test/test_memoryio.py
@@ -520,11 +520,12 @@ def test_relative_seek(self):
     def test_peek(self):
         buf = self.buftype("1234567890")
         memio = self.ioclass(buf)
-
+        pos = memio.tell()
         self.assertEqual(memio.peek(1), buf[:1])
         self.assertEqual(memio.peek(1), buf[:1])
         self.assertEqual(memio.peek(), buf)
         self.assertEqual(memio.peek(0), buf)
+        self.assertEqual(memio.tell(), pos)
         memio.read(1)
         self.assertEqual(memio.peek(1), buf[1:2])
         self.assertEqual(memio.peek(), buf[1:])

From 7d8793a8e7c7c8d91139d350280a9a93d958153b Mon Sep 17 00:00:00 2001
From: Marcel Martin <marcel.martin@scilifelab.se>
Date: Fri, 22 Sep 2023 09:49:55 +0200
Subject: [PATCH 09/29] Update docs, factor out peek_bytes, semantics

Semantic change: The default argument for peek is now size=1.
---
 Doc/library/io.rst             | 11 +++++++----
 Lib/_pyio.py                   | 13 ++++++-------
 Lib/test/test_memoryio.py      |  5 +++--
 Modules/_io/bytesio.c          | 23 +++++++++++++----------
 Modules/_io/clinic/bytesio.c.h | 20 +++++++++++++++-----
 5 files changed, 44 insertions(+), 28 deletions(-)

diff --git a/Doc/library/io.rst b/Doc/library/io.rst
index d89c892e0256c9..9c6ad31a7b7092 100644
--- a/Doc/library/io.rst
+++ b/Doc/library/io.rst
@@ -728,12 +728,15 @@ than raw I/O does.
 
       Return :class:`bytes` containing the entire contents of the buffer.
 
-   .. method:: peek(size=0, /)
-
-      .. versionadded:: 3.13
+   .. method:: peek(size=1, /)
 
       Return bytes from the current position onwards but without advancing the
-      position. The number of bytes returned may be less or more than requested.
+      position. If the size argument is less than one or larger than the number
+      of available bytes, a copy of the buffer from the current position until
+      the end is returned.
+      Return an empty bytes object at EOF.
+
+      .. versionadded:: 3.13
 
    .. method:: read1(size=-1, /)
 
diff --git a/Lib/_pyio.py b/Lib/_pyio.py
index 339d1aa7e13084..e76c404f470f79 100644
--- a/Lib/_pyio.py
+++ b/Lib/_pyio.py
@@ -978,13 +978,12 @@ def tell(self):
             raise ValueError("tell on closed file")
         return self._pos
 
-    def peek(self, size=-1):
-        pos = self.tell()
-        if size == 0:
-            size = -1
-        b = self.read(size)
-        self.seek(pos)
-        return b
+    def peek(self, size=1):
+        if self.closed:
+            raise ValueError("peek on closed file")
+        if size < 1:
+            size = len(self._buffer) - self._pos
+        return self._buffer[self._pos : self._pos + size]
 
     def truncate(self, pos=None):
         if self.closed:
diff --git a/Lib/test/test_memoryio.py b/Lib/test/test_memoryio.py
index d1d351000fe63b..bb5164ae0067da 100644
--- a/Lib/test/test_memoryio.py
+++ b/Lib/test/test_memoryio.py
@@ -523,12 +523,13 @@ def test_peek(self):
         pos = memio.tell()
         self.assertEqual(memio.peek(1), buf[:1])
         self.assertEqual(memio.peek(1), buf[:1])
-        self.assertEqual(memio.peek(), buf)
+        self.assertEqual(memio.peek(), buf[:1])
         self.assertEqual(memio.peek(0), buf)
         self.assertEqual(memio.tell(), pos)
         memio.read(1)
         self.assertEqual(memio.peek(1), buf[1:2])
-        self.assertEqual(memio.peek(), buf[1:])
+        self.assertEqual(memio.peek(), buf[1:2])
+        self.assertEqual(memio.peek(0), buf[1:])
         self.assertEqual(memio.peek(42), buf[1:])
         memio.read()
         self.assertEqual(memio.peek(1), self.EOF)
diff --git a/Modules/_io/bytesio.c b/Modules/_io/bytesio.c
index 8ecf5e5b0719b4..1b6c7364a0d8ac 100644
--- a/Modules/_io/bytesio.c
+++ b/Modules/_io/bytesio.c
@@ -394,8 +394,9 @@ _io_BytesIO_tell_impl(bytesio *self)
     return PyLong_FromSsize_t(self->pos);
 }
 
+// Read without advancing position
 static PyObject *
-read_bytes(bytesio *self, Py_ssize_t size)
+peek_bytes(bytesio *self, Py_ssize_t size)
 {
     const char *output;
 
@@ -404,15 +405,20 @@ read_bytes(bytesio *self, Py_ssize_t size)
     if (size > 1 &&
         self->pos == 0 && size == PyBytes_GET_SIZE(self->buf) &&
         self->exports == 0) {
-        self->pos += size;
         return Py_NewRef(self->buf);
     }
 
     output = PyBytes_AS_STRING(self->buf) + self->pos;
-    self->pos += size;
     return PyBytes_FromStringAndSize(output, size);
 }
 
+static PyObject *
+read_bytes(bytesio *self, Py_ssize_t size) {
+    PyObject *bytes = peek_bytes(self, size);
+    self->pos += size;
+    return bytes;
+}
+
 /*[clinic input]
 _io.BytesIO.read
     size: Py_ssize_t(accept={int, NoneType}) = -1
@@ -465,17 +471,18 @@ _io_BytesIO_read1_impl(bytesio *self, Py_ssize_t size)
 
 /*[clinic input]
 _io.BytesIO.peek
-    size: Py_ssize_t(accept={int, NoneType}) = -1
+    size: Py_ssize_t = 1
     /
 
 Return bytes from the stream without advancing the position.
 
+If the size argument is zero or negative, read until EOF is reached.
 Return an empty bytes object at EOF.
 [clinic start generated code]*/
 
 static PyObject *
 _io_BytesIO_peek_impl(bytesio *self, Py_ssize_t size)
-/*[clinic end generated code: output=fa4d8ce28b35db9b input=afc80e71b37e7c59]*/
+/*[clinic end generated code: output=fa4d8ce28b35db9b input=cb06614a3ed0496e]*/
 {
     Py_ssize_t n;
 
@@ -488,11 +495,7 @@ _io_BytesIO_peek_impl(bytesio *self, Py_ssize_t size)
         if (size < 0)
             size = 0;
     }
-    Py_ssize_t prev_pos = self->pos;
-    PyObject* result = read_bytes(self, size);
-    self->pos = prev_pos;
-
-    return result;
+    return peek_bytes(self, size);
 }
 
 
diff --git a/Modules/_io/clinic/bytesio.c.h b/Modules/_io/clinic/bytesio.c.h
index ae647d9d809673..d384c74d84aa15 100644
--- a/Modules/_io/clinic/bytesio.c.h
+++ b/Modules/_io/clinic/bytesio.c.h
@@ -233,11 +233,12 @@ _io_BytesIO_read1(bytesio *self, PyObject *const *args, Py_ssize_t nargs)
 }
 
 PyDoc_STRVAR(_io_BytesIO_peek__doc__,
-"peek($self, size=-1, /)\n"
+"peek($self, size=1, /)\n"
 "--\n"
 "\n"
 "Return bytes from the stream without advancing the position.\n"
 "\n"
+"If the size argument is zero or negative, read until EOF is reached.\n"
 "Return an empty bytes object at EOF.");
 
 #define _IO_BYTESIO_PEEK_METHODDEF    \
@@ -250,7 +251,7 @@ static PyObject *
 _io_BytesIO_peek(bytesio *self, PyObject *const *args, Py_ssize_t nargs)
 {
     PyObject *return_value = NULL;
-    Py_ssize_t size = -1;
+    Py_ssize_t size = 1;
 
     if (!_PyArg_CheckPositional("peek", nargs, 0, 1)) {
         goto exit;
@@ -258,8 +259,17 @@ _io_BytesIO_peek(bytesio *self, PyObject *const *args, Py_ssize_t nargs)
     if (nargs < 1) {
         goto skip_optional;
     }
-    if (!_Py_convert_optional_to_ssize_t(args[0], &size)) {
-        goto exit;
+    {
+        Py_ssize_t ival = -1;
+        PyObject *iobj = _PyNumber_Index(args[0]);
+        if (iobj != NULL) {
+            ival = PyLong_AsSsize_t(iobj);
+            Py_DECREF(iobj);
+        }
+        if (ival == -1 && PyErr_Occurred()) {
+            goto exit;
+        }
+        size = ival;
     }
 skip_optional:
     return_value = _io_BytesIO_peek_impl(self, size);
@@ -573,4 +583,4 @@ _io_BytesIO___init__(PyObject *self, PyObject *args, PyObject *kwargs)
 exit:
     return return_value;
 }
-/*[clinic end generated code: output=106d7918b585497c input=a9049054013a1b77]*/
+/*[clinic end generated code: output=9fe27d8d0d4d695b input=a9049054013a1b77]*/

From d4d5a55ec7e6d1daa7d20c28c8308b086a1e5f26 Mon Sep 17 00:00:00 2001
From: Marcel Martin <mail@marcelm.net>
Date: Fri, 29 Sep 2023 00:49:27 +0200
Subject: [PATCH 10/29] Update
 Misc/NEWS.d/next/Library/2022-04-10-20-10-59.bpo-46375.8j1ogZ.rst

Co-authored-by: Erlend E. Aasland <erlend.aasland@protonmail.com>
---
 .../next/Library/2022-04-10-20-10-59.bpo-46375.8j1ogZ.rst       | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/Misc/NEWS.d/next/Library/2022-04-10-20-10-59.bpo-46375.8j1ogZ.rst b/Misc/NEWS.d/next/Library/2022-04-10-20-10-59.bpo-46375.8j1ogZ.rst
index 530381efef8746..c11f101b749c17 100644
--- a/Misc/NEWS.d/next/Library/2022-04-10-20-10-59.bpo-46375.8j1ogZ.rst
+++ b/Misc/NEWS.d/next/Library/2022-04-10-20-10-59.bpo-46375.8j1ogZ.rst
@@ -1 +1 @@
-Add a ``peek()`` method to :class:`io.BytesIO`.
+Add :meth:`io.BytesIO.peek`.

From 9cdd2319fed63fcc6749d5a327f2b2396b39c7da Mon Sep 17 00:00:00 2001
From: Marcel Martin <mail@marcelm.net>
Date: Fri, 29 Sep 2023 00:50:46 +0200
Subject: [PATCH 11/29] Update Modules/_io/bytesio.c

Co-authored-by: Erlend E. Aasland <erlend.aasland@protonmail.com>
---
 Modules/_io/bytesio.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/Modules/_io/bytesio.c b/Modules/_io/bytesio.c
index 1b6c7364a0d8ac..92f79b1290f78d 100644
--- a/Modules/_io/bytesio.c
+++ b/Modules/_io/bytesio.c
@@ -413,7 +413,8 @@ peek_bytes(bytesio *self, Py_ssize_t size)
 }
 
 static PyObject *
-read_bytes(bytesio *self, Py_ssize_t size) {
+read_bytes(bytesio *self, Py_ssize_t size)
+{
     PyObject *bytes = peek_bytes(self, size);
     self->pos += size;
     return bytes;

From d9948c86cd74a2bf564afd958088bbd4251ee107 Mon Sep 17 00:00:00 2001
From: Marcel Martin <mail@marcelm.net>
Date: Fri, 29 Sep 2023 00:51:19 +0200
Subject: [PATCH 12/29] Update Modules/_io/bytesio.c

Co-authored-by: Erlend E. Aasland <erlend.aasland@protonmail.com>
---
 Modules/_io/bytesio.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/Modules/_io/bytesio.c b/Modules/_io/bytesio.c
index 92f79b1290f78d..3a70987ead9817 100644
--- a/Modules/_io/bytesio.c
+++ b/Modules/_io/bytesio.c
@@ -493,8 +493,9 @@ _io_BytesIO_peek_impl(bytesio *self, Py_ssize_t size)
     n = self->string_size - self->pos;
     if (size < 1 || size > n) {
         size = n;
-        if (size < 0)
+        if (size < 0) {
             size = 0;
+        }
     }
     return peek_bytes(self, size);
 }

From 69ddb4f5e9351c41af5033de6b0b42be375ff9b2 Mon Sep 17 00:00:00 2001
From: Marcel Martin <marcel.martin@scilifelab.se>
Date: Fri, 29 Sep 2023 00:25:06 +0200
Subject: [PATCH 13/29] Use SemBr

Co-authored-by: Erlend E. Aasland <erlend.aasland@protonmail.com>
---
 Doc/library/io.rst | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/Doc/library/io.rst b/Doc/library/io.rst
index 9c6ad31a7b7092..07627712828cbe 100644
--- a/Doc/library/io.rst
+++ b/Doc/library/io.rst
@@ -730,10 +730,9 @@ than raw I/O does.
 
    .. method:: peek(size=1, /)
 
-      Return bytes from the current position onwards but without advancing the
-      position. If the size argument is less than one or larger than the number
-      of available bytes, a copy of the buffer from the current position until
-      the end is returned.
+      Return bytes from the current position onwards without advancing the position.
+      If the size argument is less than one or larger than the number of available bytes,
+      a copy of the buffer from the current position until the end is returned.
       Return an empty bytes object at EOF.
 
       .. versionadded:: 3.13

From 58a5d580081e0b9ef82daf69140a3c20a2458c44 Mon Sep 17 00:00:00 2001
From: Marcel Martin <mail@marcelm.net>
Date: Fri, 29 Sep 2023 00:54:10 +0200
Subject: [PATCH 14/29] Update Doc/whatsnew/3.13.rst

Co-authored-by: Erlend E. Aasland <erlend.aasland@protonmail.com>
---
 Doc/whatsnew/3.13.rst | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/Doc/whatsnew/3.13.rst b/Doc/whatsnew/3.13.rst
index 8cfd74df46f32a..e741bc3656c2f5 100644
--- a/Doc/whatsnew/3.13.rst
+++ b/Doc/whatsnew/3.13.rst
@@ -151,7 +151,7 @@ and only logged in :ref:`Python Development Mode <devmode>` or on :ref:`Python
 built on debug mode <debug-build>`.
 (Contributed by Victor Stinner in :gh:`62948`.)
 
-* Add :meth:`~io.BytesIO.peek`. (Contributed by Marcel Martin in :gh:`90533`.)
+* Add :meth:`io.BytesIO.peek`. (Contributed by Marcel Martin in :gh:`90533`.)
 
 opcode
 ------

From 3d21011f573ff2f5a07ebf88549b66ad43fd1143 Mon Sep 17 00:00:00 2001
From: Marcel Martin <mail@marcelm.net>
Date: Fri, 29 Sep 2023 00:55:39 +0200
Subject: [PATCH 15/29] Apply suggestions from code review

Co-authored-by: Victor Stinner <vstinner@python.org>
---
 Lib/test/test_memoryio.py | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/Lib/test/test_memoryio.py b/Lib/test/test_memoryio.py
index bb5164ae0067da..a9bf8c08f827f2 100644
--- a/Lib/test/test_memoryio.py
+++ b/Lib/test/test_memoryio.py
@@ -520,12 +520,12 @@ def test_relative_seek(self):
     def test_peek(self):
         buf = self.buftype("1234567890")
         memio = self.ioclass(buf)
-        pos = memio.tell()
+        self.assertEqual(memio.tell(), 0)
         self.assertEqual(memio.peek(1), buf[:1])
         self.assertEqual(memio.peek(1), buf[:1])
         self.assertEqual(memio.peek(), buf[:1])
         self.assertEqual(memio.peek(0), buf)
-        self.assertEqual(memio.tell(), pos)
+        self.assertEqual(memio.tell(), 0)
         memio.read(1)
         self.assertEqual(memio.peek(1), buf[1:2])
         self.assertEqual(memio.peek(), buf[1:2])

From 4f4999fb3059225dd1728c4eb99d20c13c38c4c4 Mon Sep 17 00:00:00 2001
From: Marcel Martin <marcel.martin@scilifelab.se>
Date: Fri, 29 Sep 2023 00:58:42 +0200
Subject: [PATCH 16/29] Use a context manager around memio in test_peek

---
 Lib/test/test_memoryio.py | 29 ++++++++++++++---------------
 1 file changed, 14 insertions(+), 15 deletions(-)

diff --git a/Lib/test/test_memoryio.py b/Lib/test/test_memoryio.py
index a9bf8c08f827f2..1b852d31091299 100644
--- a/Lib/test/test_memoryio.py
+++ b/Lib/test/test_memoryio.py
@@ -519,21 +519,20 @@ def test_relative_seek(self):
 
     def test_peek(self):
         buf = self.buftype("1234567890")
-        memio = self.ioclass(buf)
-        self.assertEqual(memio.tell(), 0)
-        self.assertEqual(memio.peek(1), buf[:1])
-        self.assertEqual(memio.peek(1), buf[:1])
-        self.assertEqual(memio.peek(), buf[:1])
-        self.assertEqual(memio.peek(0), buf)
-        self.assertEqual(memio.tell(), 0)
-        memio.read(1)
-        self.assertEqual(memio.peek(1), buf[1:2])
-        self.assertEqual(memio.peek(), buf[1:2])
-        self.assertEqual(memio.peek(0), buf[1:])
-        self.assertEqual(memio.peek(42), buf[1:])
-        memio.read()
-        self.assertEqual(memio.peek(1), self.EOF)
-        memio.close()
+        with self.ioclass(buf) as memio:
+            self.assertEqual(memio.tell(), 0)
+            self.assertEqual(memio.peek(1), buf[:1])
+            self.assertEqual(memio.peek(1), buf[:1])
+            self.assertEqual(memio.peek(), buf[:1])
+            self.assertEqual(memio.peek(0), buf)
+            self.assertEqual(memio.tell(), 0)
+            memio.read(1)
+            self.assertEqual(memio.peek(1), buf[1:2])
+            self.assertEqual(memio.peek(), buf[1:2])
+            self.assertEqual(memio.peek(0), buf[1:])
+            self.assertEqual(memio.peek(42), buf[1:])
+            memio.read()
+            self.assertEqual(memio.peek(1), self.EOF)
         self.assertRaises(ValueError, memio.peek)
 
     def test_unicode(self):

From 56fbee311c5febe44005ce1ee1871e11f3d173ea Mon Sep 17 00:00:00 2001
From: Marcel Martin <marcel.martin@scilifelab.se>
Date: Fri, 29 Sep 2023 01:01:41 +0200
Subject: [PATCH 17/29] Add more tests for tell() after peek()

---
 Lib/test/test_memoryio.py | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/Lib/test/test_memoryio.py b/Lib/test/test_memoryio.py
index 1b852d31091299..df8e11b9e702c2 100644
--- a/Lib/test/test_memoryio.py
+++ b/Lib/test/test_memoryio.py
@@ -527,12 +527,16 @@ def test_peek(self):
             self.assertEqual(memio.peek(0), buf)
             self.assertEqual(memio.tell(), 0)
             memio.read(1)
+            self.assertEqual(memio.tell(), 1)
             self.assertEqual(memio.peek(1), buf[1:2])
             self.assertEqual(memio.peek(), buf[1:2])
             self.assertEqual(memio.peek(0), buf[1:])
             self.assertEqual(memio.peek(42), buf[1:])
+            self.assertEqual(memio.tell(), 1)
             memio.read()
+            self.assertEqual(memio.tell(), len(buf))
             self.assertEqual(memio.peek(1), self.EOF)
+            self.assertEqual(memio.tell(), len(buf))
         self.assertRaises(ValueError, memio.peek)
 
     def test_unicode(self):

From 4c3c9083aad4bfd97b49bf4b013879b92daeecf0 Mon Sep 17 00:00:00 2001
From: Marcel Martin <marcel.martin@scilifelab.se>
Date: Fri, 29 Sep 2023 01:05:17 +0200
Subject: [PATCH 18/29] Document why size < 0 can happen

---
 Modules/_io/bytesio.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/Modules/_io/bytesio.c b/Modules/_io/bytesio.c
index 3a70987ead9817..94bdd69ef3a1aa 100644
--- a/Modules/_io/bytesio.c
+++ b/Modules/_io/bytesio.c
@@ -493,6 +493,7 @@ _io_BytesIO_peek_impl(bytesio *self, Py_ssize_t size)
     n = self->string_size - self->pos;
     if (size < 1 || size > n) {
         size = n;
+        /* size can be negative after truncate() */
         if (size < 0) {
             size = 0;
         }

From 1f2b5c54fdad8047773ebc65dc0b896195ead3e3 Mon Sep 17 00:00:00 2001
From: Marcel Martin <mail@marcelm.net>
Date: Fri, 29 Sep 2023 09:18:17 +0200
Subject: [PATCH 19/29] Update Modules/_io/bytesio.c

Co-authored-by: Erlend E. Aasland <erlend.aasland@protonmail.com>
---
 Modules/_io/bytesio.c | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/Modules/_io/bytesio.c b/Modules/_io/bytesio.c
index 94bdd69ef3a1aa..071c8694bcd843 100644
--- a/Modules/_io/bytesio.c
+++ b/Modules/_io/bytesio.c
@@ -485,12 +485,10 @@ static PyObject *
 _io_BytesIO_peek_impl(bytesio *self, Py_ssize_t size)
 /*[clinic end generated code: output=fa4d8ce28b35db9b input=cb06614a3ed0496e]*/
 {
-    Py_ssize_t n;
-
     CHECK_CLOSED(self);
 
     /* adjust invalid sizes */
-    n = self->string_size - self->pos;
+    Py_ssize_t n = self->string_size - self->pos;
     if (size < 1 || size > n) {
         size = n;
         /* size can be negative after truncate() */

From a1504a7474f38b8d1e9ec91002361928827c6a42 Mon Sep 17 00:00:00 2001
From: Marcel Martin <marcel.martin@scilifelab.se>
Date: Fri, 29 Sep 2023 09:17:09 +0200
Subject: [PATCH 20/29] Do not update pos if peek_bytes failed

---
 Modules/_io/bytesio.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/Modules/_io/bytesio.c b/Modules/_io/bytesio.c
index 071c8694bcd843..e90fedd2d8ae0b 100644
--- a/Modules/_io/bytesio.c
+++ b/Modules/_io/bytesio.c
@@ -416,7 +416,9 @@ static PyObject *
 read_bytes(bytesio *self, Py_ssize_t size)
 {
     PyObject *bytes = peek_bytes(self, size);
-    self->pos += size;
+    if (bytes != NULL) {
+        self->pos += size;
+    }
     return bytes;
 }
 

From 827a785a926f57755d65af458381c76469cd8b74 Mon Sep 17 00:00:00 2001
From: Marcel Martin <marcel.martin@scilifelab.se>
Date: Fri, 29 Sep 2023 09:17:42 +0200
Subject: [PATCH 21/29] Size can be negative after truncate or seek

---
 Modules/_io/bytesio.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/Modules/_io/bytesio.c b/Modules/_io/bytesio.c
index e90fedd2d8ae0b..2ba9676056f46e 100644
--- a/Modules/_io/bytesio.c
+++ b/Modules/_io/bytesio.c
@@ -493,7 +493,7 @@ _io_BytesIO_peek_impl(bytesio *self, Py_ssize_t size)
     Py_ssize_t n = self->string_size - self->pos;
     if (size < 1 || size > n) {
         size = n;
-        /* size can be negative after truncate() */
+        /* size can be negative after truncate() or seek() */
         if (size < 0) {
             size = 0;
         }

From eebd2894f07320a71bb061be3680878ccb6d29c3 Mon Sep 17 00:00:00 2001
From: Marcel Martin <marcel.martin@scilifelab.se>
Date: Fri, 29 Sep 2023 10:39:34 +0200
Subject: [PATCH 22/29] Test with size<0 and size>len(buf)

---
 Lib/test/test_memoryio.py | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/Lib/test/test_memoryio.py b/Lib/test/test_memoryio.py
index df8e11b9e702c2..1eaf85a7e18ff1 100644
--- a/Lib/test/test_memoryio.py
+++ b/Lib/test/test_memoryio.py
@@ -525,13 +525,17 @@ def test_peek(self):
             self.assertEqual(memio.peek(1), buf[:1])
             self.assertEqual(memio.peek(), buf[:1])
             self.assertEqual(memio.peek(0), buf)
+            self.assertEqual(memio.peek(len(buf) + 100), buf)
+            self.assertEqual(memio.peek(-1), buf)
             self.assertEqual(memio.tell(), 0)
             memio.read(1)
             self.assertEqual(memio.tell(), 1)
             self.assertEqual(memio.peek(1), buf[1:2])
+            self.assertEqual(memio.peek(1), buf[1:2])
             self.assertEqual(memio.peek(), buf[1:2])
             self.assertEqual(memio.peek(0), buf[1:])
-            self.assertEqual(memio.peek(42), buf[1:])
+            self.assertEqual(memio.peek(len(buf) + 100), buf[1:])
+            self.assertEqual(memio.peek(-1), buf[1:])
             self.assertEqual(memio.tell(), 1)
             memio.read()
             self.assertEqual(memio.tell(), len(buf))

From 48551896ac03b105b95327accc4b75128b2afa80 Mon Sep 17 00:00:00 2001
From: Marcel Martin <marcel.martin@scilifelab.se>
Date: Fri, 29 Sep 2023 11:48:57 +0200
Subject: [PATCH 23/29] Test peek() after write()

---
 Lib/test/test_memoryio.py | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/Lib/test/test_memoryio.py b/Lib/test/test_memoryio.py
index 1eaf85a7e18ff1..3ddf1b769f41a6 100644
--- a/Lib/test/test_memoryio.py
+++ b/Lib/test/test_memoryio.py
@@ -541,6 +541,15 @@ def test_peek(self):
             self.assertEqual(memio.tell(), len(buf))
             self.assertEqual(memio.peek(1), self.EOF)
             self.assertEqual(memio.tell(), len(buf))
+            # Peeking works after writing
+            abc = self.buftype("abc")
+            memio.write(abc)
+            self.assertEqual(memio.peek(), self.EOF)
+            memio.seek(len(buf))
+            self.assertEqual(memio.peek(), abc[:1])
+            self.assertEqual(memio.peek(-1), abc)
+            self.assertEqual(memio.peek(len(abc) + 100), abc)
+            self.assertEqual(memio.tell(), len(buf))
         self.assertRaises(ValueError, memio.peek)
 
     def test_unicode(self):

From fd85b46830f0015a39d41965ecbe42cb83f7fc54 Mon Sep 17 00:00:00 2001
From: Marcel Martin <marcel.martin@scilifelab.se>
Date: Fri, 29 Sep 2023 15:53:11 +0200
Subject: [PATCH 24/29] Document BufferedReader.peek and BytesIO.peek similarly

---
 Doc/library/io.rst | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/Doc/library/io.rst b/Doc/library/io.rst
index 07627712828cbe..991b41a4335453 100644
--- a/Doc/library/io.rst
+++ b/Doc/library/io.rst
@@ -731,9 +731,10 @@ than raw I/O does.
    .. method:: peek(size=1, /)
 
       Return bytes from the current position onwards without advancing the position.
+      At least one byte of data is returned if not at EOF.
+      Return an empty :class:`bytes` object at EOF.
       If the size argument is less than one or larger than the number of available bytes,
       a copy of the buffer from the current position until the end is returned.
-      Return an empty bytes object at EOF.
 
       .. versionadded:: 3.13
 
@@ -769,9 +770,12 @@ than raw I/O does.
 
    .. method:: peek(size=0, /)
 
-      Return bytes from the stream without advancing the position.  At most one
-      single read on the raw stream is done to satisfy the call. The number of
-      bytes returned may be less or more than requested.
+      Return bytes from the current position onwards without advancing the position.
+      At least one byte of data is returned if not at EOF.
+      Return an empty :class:`bytes` object at EOF.
+      At most one single read on the underlying raw stream is done to satisfy the call.
+      The exact number of bytes returned is unspecified
+      (*size* is ignored).
 
    .. method:: read(size=-1, /)
 

From 5733d5a5b285926945510f4398a612ef5ce3b95d Mon Sep 17 00:00:00 2001
From: Marcel Martin <marcel.martin@scilifelab.se>
Date: Fri, 29 Sep 2023 16:02:09 +0200
Subject: [PATCH 25/29] Comment

---
 Lib/_pyio.py | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/Lib/_pyio.py b/Lib/_pyio.py
index e76c404f470f79..0fa89983a99bee 100644
--- a/Lib/_pyio.py
+++ b/Lib/_pyio.py
@@ -981,6 +981,9 @@ def tell(self):
     def peek(self, size=1):
         if self.closed:
             raise ValueError("peek on closed file")
+        # Due to slicing semantics, this works correctly
+        # even if the size is greater than the buffer length or
+        # the position is beyond the end of the buffer
         if size < 1:
             size = len(self._buffer) - self._pos
         return self._buffer[self._pos : self._pos + size]

From 4d832e0576d79f495e641c144868eae18d17c5d7 Mon Sep 17 00:00:00 2001
From: Marcel Martin <marcel.martin@scilifelab.se>
Date: Fri, 29 Sep 2023 16:45:54 +0200
Subject: [PATCH 26/29] More it more explicit that size is ignored

---
 Doc/library/io.rst | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/Doc/library/io.rst b/Doc/library/io.rst
index 991b41a4335453..c7c067b52ff24f 100644
--- a/Doc/library/io.rst
+++ b/Doc/library/io.rst
@@ -774,8 +774,8 @@ than raw I/O does.
       At least one byte of data is returned if not at EOF.
       Return an empty :class:`bytes` object at EOF.
       At most one single read on the underlying raw stream is done to satisfy the call.
-      The exact number of bytes returned is unspecified
-      (*size* is ignored).
+      The *size* argument is ignored.
+      The number of read bytes depends on the buffer size and the current position in the internal buffer.
 
    .. method:: read(size=-1, /)
 

From 1d793e3df9b072053ed3df79505bdc62b16448a0 Mon Sep 17 00:00:00 2001
From: Marcel Martin <marcel.martin@scilifelab.se>
Date: Mon, 23 Oct 2023 08:47:15 +0200
Subject: [PATCH 27/29] Return an empty bytes object for size=0

---
 Doc/library/io.rst        | 2 +-
 Lib/_pyio.py              | 2 +-
 Lib/test/test_memoryio.py | 5 +++--
 Modules/_io/bytesio.c     | 6 +++---
 4 files changed, 8 insertions(+), 7 deletions(-)

diff --git a/Doc/library/io.rst b/Doc/library/io.rst
index 4f7c980585215b..d1e7dfdc471b6c 100644
--- a/Doc/library/io.rst
+++ b/Doc/library/io.rst
@@ -733,7 +733,7 @@ than raw I/O does.
       Return bytes from the current position onwards without advancing the position.
       At least one byte of data is returned if not at EOF.
       Return an empty :class:`bytes` object at EOF.
-      If the size argument is less than one or larger than the number of available bytes,
+      If the size argument is negative or larger than the number of available bytes,
       a copy of the buffer from the current position until the end is returned.
 
       .. versionadded:: 3.13
diff --git a/Lib/_pyio.py b/Lib/_pyio.py
index 0fa89983a99bee..f4d3d11b791ffe 100644
--- a/Lib/_pyio.py
+++ b/Lib/_pyio.py
@@ -984,7 +984,7 @@ def peek(self, size=1):
         # Due to slicing semantics, this works correctly
         # even if the size is greater than the buffer length or
         # the position is beyond the end of the buffer
-        if size < 1:
+        if size < 0:
             size = len(self._buffer) - self._pos
         return self._buffer[self._pos : self._pos + size]
 
diff --git a/Lib/test/test_memoryio.py b/Lib/test/test_memoryio.py
index 3ddf1b769f41a6..55d08d952cc9b8 100644
--- a/Lib/test/test_memoryio.py
+++ b/Lib/test/test_memoryio.py
@@ -524,7 +524,7 @@ def test_peek(self):
             self.assertEqual(memio.peek(1), buf[:1])
             self.assertEqual(memio.peek(1), buf[:1])
             self.assertEqual(memio.peek(), buf[:1])
-            self.assertEqual(memio.peek(0), buf)
+            self.assertEqual(memio.peek(0), b"")
             self.assertEqual(memio.peek(len(buf) + 100), buf)
             self.assertEqual(memio.peek(-1), buf)
             self.assertEqual(memio.tell(), 0)
@@ -533,13 +533,14 @@ def test_peek(self):
             self.assertEqual(memio.peek(1), buf[1:2])
             self.assertEqual(memio.peek(1), buf[1:2])
             self.assertEqual(memio.peek(), buf[1:2])
-            self.assertEqual(memio.peek(0), buf[1:])
+            self.assertEqual(memio.peek(0), b"")
             self.assertEqual(memio.peek(len(buf) + 100), buf[1:])
             self.assertEqual(memio.peek(-1), buf[1:])
             self.assertEqual(memio.tell(), 1)
             memio.read()
             self.assertEqual(memio.tell(), len(buf))
             self.assertEqual(memio.peek(1), self.EOF)
+            self.assertEqual(memio.peek(0), b"")
             self.assertEqual(memio.tell(), len(buf))
             # Peeking works after writing
             abc = self.buftype("abc")
diff --git a/Modules/_io/bytesio.c b/Modules/_io/bytesio.c
index 2ba9676056f46e..5c7d436416b387 100644
--- a/Modules/_io/bytesio.c
+++ b/Modules/_io/bytesio.c
@@ -479,7 +479,7 @@ _io.BytesIO.peek
 
 Return bytes from the stream without advancing the position.
 
-If the size argument is zero or negative, read until EOF is reached.
+If the size argument is negative, read until EOF is reached.
 Return an empty bytes object at EOF.
 [clinic start generated code]*/
 
@@ -491,9 +491,9 @@ _io_BytesIO_peek_impl(bytesio *self, Py_ssize_t size)
 
     /* adjust invalid sizes */
     Py_ssize_t n = self->string_size - self->pos;
-    if (size < 1 || size > n) {
+    if (size < 0 || size > n) {
         size = n;
-        /* size can be negative after truncate() or seek() */
+        /* n can be negative after truncate() or seek() */
         if (size < 0) {
             size = 0;
         }

From 9b0b04fce52316fff2ab91f57280f703d8395921 Mon Sep 17 00:00:00 2001
From: Marcel Martin <marcel.martin@scilifelab.se>
Date: Mon, 23 Oct 2023 09:10:13 +0200
Subject: [PATCH 28/29] Simplify

---
 Lib/_pyio.py | 7 ++-----
 1 file changed, 2 insertions(+), 5 deletions(-)

diff --git a/Lib/_pyio.py b/Lib/_pyio.py
index f4d3d11b791ffe..801f0447447a21 100644
--- a/Lib/_pyio.py
+++ b/Lib/_pyio.py
@@ -981,12 +981,9 @@ def tell(self):
     def peek(self, size=1):
         if self.closed:
             raise ValueError("peek on closed file")
-        # Due to slicing semantics, this works correctly
-        # even if the size is greater than the buffer length or
-        # the position is beyond the end of the buffer
         if size < 0:
-            size = len(self._buffer) - self._pos
-        return self._buffer[self._pos : self._pos + size]
+            return self._buffer[self._pos:]
+        return self._buffer[self._pos:self._pos + size]
 
     def truncate(self, pos=None):
         if self.closed:

From bb6447d860e81234594ea5e0615a175eaa898901 Mon Sep 17 00:00:00 2001
From: Marcel Martin <marcel.martin@scilifelab.se>
Date: Mon, 23 Oct 2023 09:11:33 +0200
Subject: [PATCH 29/29] Test peek(3) and peek(5)

---
 Lib/test/test_memoryio.py | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/Lib/test/test_memoryio.py b/Lib/test/test_memoryio.py
index 55d08d952cc9b8..9564f85cfa5689 100644
--- a/Lib/test/test_memoryio.py
+++ b/Lib/test/test_memoryio.py
@@ -523,6 +523,8 @@ def test_peek(self):
             self.assertEqual(memio.tell(), 0)
             self.assertEqual(memio.peek(1), buf[:1])
             self.assertEqual(memio.peek(1), buf[:1])
+            self.assertEqual(memio.peek(3), buf[:3])
+            self.assertEqual(memio.peek(5), buf[:5])
             self.assertEqual(memio.peek(), buf[:1])
             self.assertEqual(memio.peek(0), b"")
             self.assertEqual(memio.peek(len(buf) + 100), buf)
@@ -532,6 +534,8 @@ def test_peek(self):
             self.assertEqual(memio.tell(), 1)
             self.assertEqual(memio.peek(1), buf[1:2])
             self.assertEqual(memio.peek(1), buf[1:2])
+            self.assertEqual(memio.peek(3), buf[1:4])
+            self.assertEqual(memio.peek(5), buf[1:6])
             self.assertEqual(memio.peek(), buf[1:2])
             self.assertEqual(memio.peek(0), b"")
             self.assertEqual(memio.peek(len(buf) + 100), buf[1:])
@@ -540,6 +544,8 @@ def test_peek(self):
             memio.read()
             self.assertEqual(memio.tell(), len(buf))
             self.assertEqual(memio.peek(1), self.EOF)
+            self.assertEqual(memio.peek(3), self.EOF)
+            self.assertEqual(memio.peek(5), self.EOF)
             self.assertEqual(memio.peek(0), b"")
             self.assertEqual(memio.tell(), len(buf))
             # Peeking works after writing