From b833b832f8e236eefe5632f338667eff4809c6cf 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 01/30] Implement BytesIO.peek()

---
 Lib/_pyio.py                      |  8 +++++++
 Lib/test/test_io/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 3306c8a274760b..0fac2aa230cc1d 100644
--- a/Lib/_pyio.py
+++ b/Lib/_pyio.py
@@ -996,6 +996,14 @@ 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_io/test_memoryio.py b/Lib/test/test_io/test_memoryio.py
index 482b183da23ffa..ca5b9880ee58d4 100644
--- a/Lib/test/test_io/test_memoryio.py
+++ b/Lib/test/test_io/test_memoryio.py
@@ -566,6 +566,23 @@ def test_issue141311(self):
         buf = bytearray(2)
         self.assertEqual(0, memio.readinto(buf))
 
+    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 d088bb0efac797..8761766dc54fbb 100644
--- a/Modules/_io/bytesio.c
+++ b/Modules/_io/bytesio.c
@@ -499,6 +499,42 @@ _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]
 @critical_section
 _io.BytesIO.readline
@@ -1135,6 +1171,7 @@ 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 6595dc937bbcf0..d767bf6f02b3e9 100644
--- a/Modules/_io/clinic/bytesio.c.h
+++ b/Modules/_io/clinic/bytesio.c.h
@@ -287,6 +287,42 @@ _io_BytesIO_read1(PyObject *self, PyObject *const *args, Py_ssize_t nargs)
     return return_value;
 }
 
+PyDoc_STRVAR(_io_BytesIO_peek__doc__,
+"peek($self, size=-1, /)\n"
+"--\n"
+"\n"
+"Return bytes from the stream without advancing the position.\n"
+"\n"
+"Return an empty bytes object at EOF.");
+
+#define _IO_BYTESIO_PEEK_METHODDEF    \
+    {"peek", _PyCFunction_CAST(_io_BytesIO_peek), METH_FASTCALL, _io_BytesIO_peek__doc__},
+
+static PyObject *
+_io_BytesIO_peek_impl(bytesio *self, Py_ssize_t size);
+
+static PyObject *
+_io_BytesIO_peek(PyObject *self, PyObject *const *args, Py_ssize_t nargs)
+{
+    PyObject *return_value = NULL;
+    Py_ssize_t size = -1;
+
+    if (!_PyArg_CheckPositional("peek", nargs, 0, 1)) {
+        goto exit;
+    }
+    if (nargs < 1) {
+        goto skip_optional;
+    }
+    if (!_Py_convert_optional_to_ssize_t(args[0], &size)) {
+        goto exit;
+    }
+skip_optional:
+    return_value = _io_BytesIO_peek_impl((bytesio *)self, size);
+
+exit:
+    return return_value;
+}
+
 PyDoc_STRVAR(_io_BytesIO_readline__doc__,
 "readline($self, size=-1, /)\n"
 "--\n"
@@ -637,4 +673,4 @@ _io_BytesIO___init__(PyObject *self, PyObject *args, PyObject *kwargs)
 exit:
     return return_value;
 }
-/*[clinic end generated code: output=daa81dfdae5ccc57 input=a9049054013a1b77]*/
+/*[clinic end generated code: output=c21a2170e9f7efa5 input=a9049054013a1b77]*/

From 50a2cfbe60ee646c31ce2947a27ed4fe64286bde 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 02/30] =?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 eaa76723a2f8801489bc1f2bc1afce447bf51243 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 03/30] 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 494e57fe1c0474..6114495a66d457 100644
--- a/Doc/library/io.rst
+++ b/Doc/library/io.rst
@@ -739,6 +739,12 @@ 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 00457ae459ba4174f2038bb7fdf6d7b12379f97f 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 04/30] 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 8761766dc54fbb..a97ada39451ceb 100644
--- a/Modules/_io/bytesio.c
+++ b/Modules/_io/bytesio.c
@@ -526,11 +526,11 @@ _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_lock_held(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 882579d16d6bd70396bdbc61fab2ed3bb907f3ab 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 05/30] =?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/3.15.rst | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/Doc/whatsnew/3.15.rst b/Doc/whatsnew/3.15.rst
index 927d6035c8c4bc..ae07ac094cf586 100644
--- a/Doc/whatsnew/3.15.rst
+++ b/Doc/whatsnew/3.15.rst
@@ -840,6 +840,14 @@ inspect
   for :func:`~inspect.getdoc`.
   (Contributed by Serhiy Storchaka in :gh:`132686`.)
 
+
+io
+--
+
+* Add :meth:`~io.BytesIO.peek`.
+  (Contributed by Marcel Martin in :gh:`90533`.)
+
+
 json
 ----
 

From c1eed72ef5ebbe6e157e78491fc778f1681193fe 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 06/30] 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 6114495a66d457..5abc6e86f2e438 100644
--- a/Doc/library/io.rst
+++ b/Doc/library/io.rst
@@ -741,7 +741,7 @@ 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 afc200cbd589e3fa1c85e30faaa893e1f34982de 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 07/30] 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 a97ada39451ceb..f7b31d05aff701 100644
--- a/Modules/_io/bytesio.c
+++ b/Modules/_io/bytesio.c
@@ -515,7 +515,6 @@ _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 79ab9a4886db9bd4e5d6306087b5c84e1b1872b1 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 08/30] Test tell() after peek()

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

diff --git a/Lib/test/test_io/test_memoryio.py b/Lib/test/test_io/test_memoryio.py
index ca5b9880ee58d4..3de42297a9a13e 100644
--- a/Lib/test/test_io/test_memoryio.py
+++ b/Lib/test/test_io/test_memoryio.py
@@ -569,11 +569,12 @@ def test_issue141311(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 b4939148aeb51c2c7b4cd42cf3a64d9905224b57 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 09/30] 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_io/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 5abc6e86f2e438..837cfdc7acfa9a 100644
--- a/Doc/library/io.rst
+++ b/Doc/library/io.rst
@@ -739,12 +739,15 @@ 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:: 3.15
 
    .. method:: read1(size=-1, /)
 
diff --git a/Lib/_pyio.py b/Lib/_pyio.py
index 0fac2aa230cc1d..1b38614420061d 100644
--- a/Lib/_pyio.py
+++ b/Lib/_pyio.py
@@ -996,13 +996,12 @@ 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_io/test_memoryio.py b/Lib/test/test_io/test_memoryio.py
index 3de42297a9a13e..4d6d731bd7c36b 100644
--- a/Lib/test/test_io/test_memoryio.py
+++ b/Lib/test/test_io/test_memoryio.py
@@ -572,12 +572,13 @@ 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 f7b31d05aff701..fe0ee7e416fe95 100644
--- a/Modules/_io/bytesio.c
+++ b/Modules/_io/bytesio.c
@@ -420,8 +420,9 @@ _io_BytesIO_tell_impl(bytesio *self)
     return PyLong_FromSsize_t(self->pos);
 }
 
+// Read without advancing position
 static PyObject *
-read_bytes_lock_held(bytesio *self, Py_ssize_t size)
+peek_bytes_lock_held(bytesio *self, Py_ssize_t size)
 {
     _Py_CRITICAL_SECTION_ASSERT_OBJECT_LOCKED(self);
 
@@ -432,7 +433,6 @@ read_bytes_lock_held(bytesio *self, Py_ssize_t size)
     if (size > 1 &&
         self->pos == 0 && size == PyBytes_GET_SIZE(self->buf) &&
         FT_ATOMIC_LOAD_SSIZE_RELAXED(self->exports) == 0) {
-        self->pos += size;
         return Py_NewRef(self->buf);
     }
 
@@ -444,10 +444,16 @@ read_bytes_lock_held(bytesio *self, Py_ssize_t size)
     }
 
     output = PyBytes_AS_STRING(self->buf) + self->pos;
-    self->pos += size;
     return PyBytes_FromStringAndSize(output, size);
 }
 
+static PyObject *
+read_bytes_lock_held(bytesio *self, Py_ssize_t size) {
+    PyObject *bytes = peek_bytes_lock_held(self, size);
+    self->pos += size;
+    return bytes;
+}
+
 /*[clinic input]
 @critical_section
 _io.BytesIO.read
@@ -502,17 +508,18 @@ _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;
 
@@ -525,11 +532,7 @@ _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_lock_held(self, size);
-    self->pos = prev_pos;
-
-    return result;
+    return peek_bytes_lock_held(self, size);
 }
 
 
diff --git a/Modules/_io/clinic/bytesio.c.h b/Modules/_io/clinic/bytesio.c.h
index d767bf6f02b3e9..fe3ec2f3bcad0b 100644
--- a/Modules/_io/clinic/bytesio.c.h
+++ b/Modules/_io/clinic/bytesio.c.h
@@ -288,11 +288,12 @@ _io_BytesIO_read1(PyObject *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    \
@@ -305,7 +306,7 @@ static PyObject *
 _io_BytesIO_peek(PyObject *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;
@@ -313,8 +314,17 @@ _io_BytesIO_peek(PyObject *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((bytesio *)self, size);
@@ -673,4 +683,4 @@ _io_BytesIO___init__(PyObject *self, PyObject *args, PyObject *kwargs)
 exit:
     return return_value;
 }
-/*[clinic end generated code: output=c21a2170e9f7efa5 input=a9049054013a1b77]*/
+/*[clinic end generated code: output=02299ed558ba722f input=a9049054013a1b77]*/

From 2a1c85c513f6c83ff3b48181239be2e679738908 Mon Sep 17 00:00:00 2001
From: Marcel Martin <marcel.martin@scilifelab.se>
Date: Fri, 29 Sep 2023 00:49:27 +0200
Subject: [PATCH 10/30] 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 d398717b1b6907376ef42e4a8ac8060af8e13ed1 Mon Sep 17 00:00:00 2001
From: Marcel Martin <marcel.martin@scilifelab.se>
Date: Fri, 29 Sep 2023 00:50:46 +0200
Subject: [PATCH 11/30] 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 fe0ee7e416fe95..4106806bef8047 100644
--- a/Modules/_io/bytesio.c
+++ b/Modules/_io/bytesio.c
@@ -448,7 +448,8 @@ peek_bytes_lock_held(bytesio *self, Py_ssize_t size)
 }
 
 static PyObject *
-read_bytes_lock_held(bytesio *self, Py_ssize_t size) {
+read_bytes_lock_held(bytesio *self, Py_ssize_t size)
+{
     PyObject *bytes = peek_bytes_lock_held(self, size);
     self->pos += size;
     return bytes;

From 26d1e81ed12601132b2fc8500b372dc357fa8ae2 Mon Sep 17 00:00:00 2001
From: Marcel Martin <marcel.martin@scilifelab.se>
Date: Fri, 29 Sep 2023 00:51:19 +0200
Subject: [PATCH 12/30] 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 4106806bef8047..e610acf107770f 100644
--- a/Modules/_io/bytesio.c
+++ b/Modules/_io/bytesio.c
@@ -530,8 +530,9 @@ _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_lock_held(self, size);
 }

From 9a19ff95ef56b2cdba4165cf451b97e1539f7bc7 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 13/30] 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 837cfdc7acfa9a..a6b25b52b59770 100644
--- a/Doc/library/io.rst
+++ b/Doc/library/io.rst
@@ -741,10 +741,9 @@ 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:: 3.15

From 9300ade195c04f02b62aaf03f315ccecb3781532 Mon Sep 17 00:00:00 2001
From: Marcel Martin <marcel.martin@scilifelab.se>
Date: Fri, 29 Sep 2023 00:54:10 +0200
Subject: [PATCH 14/30] Update Doc/whatsnew/3.13.rst

Co-authored-by: Erlend E. Aasland <erlend.aasland@protonmail.com>
---
 Doc/whatsnew/3.15.rst | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/Doc/whatsnew/3.15.rst b/Doc/whatsnew/3.15.rst
index ae07ac094cf586..0bb823b5062a80 100644
--- a/Doc/whatsnew/3.15.rst
+++ b/Doc/whatsnew/3.15.rst
@@ -844,7 +844,7 @@ inspect
 io
 --
 
-* Add :meth:`~io.BytesIO.peek`.
+* Add :meth:`io.BytesIO.peek`.
   (Contributed by Marcel Martin in :gh:`90533`.)
 
 

From d214089c1da33dd33a1b5d75362d54ff6bcc24a6 Mon Sep 17 00:00:00 2001
From: Marcel Martin <marcel.martin@scilifelab.se>
Date: Fri, 29 Sep 2023 00:55:39 +0200
Subject: [PATCH 15/30] Apply suggestions from code review

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

diff --git a/Lib/test/test_io/test_memoryio.py b/Lib/test/test_io/test_memoryio.py
index 4d6d731bd7c36b..f50e7e1246f4e2 100644
--- a/Lib/test/test_io/test_memoryio.py
+++ b/Lib/test/test_io/test_memoryio.py
@@ -569,12 +569,12 @@ def test_issue141311(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 d6691b81b2f49504b16b26bab4a4af76d7ac1ade 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 16/30] Use a context manager around memio in test_peek

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

diff --git a/Lib/test/test_io/test_memoryio.py b/Lib/test/test_io/test_memoryio.py
index f50e7e1246f4e2..bd52237d841aff 100644
--- a/Lib/test/test_io/test_memoryio.py
+++ b/Lib/test/test_io/test_memoryio.py
@@ -568,21 +568,20 @@ def test_issue141311(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 3e51adbb4a9b20d24665d6988e3e83887fb554f5 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 17/30] Add more tests for tell() after peek()

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

diff --git a/Lib/test/test_io/test_memoryio.py b/Lib/test/test_io/test_memoryio.py
index bd52237d841aff..8fc4751525ef35 100644
--- a/Lib/test/test_io/test_memoryio.py
+++ b/Lib/test/test_io/test_memoryio.py
@@ -576,12 +576,16 @@ 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 3661b65866114bbbe3b23a253d6ef95123d6bb88 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 18/30] 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 e610acf107770f..ea24a86ed359eb 100644
--- a/Modules/_io/bytesio.c
+++ b/Modules/_io/bytesio.c
@@ -530,6 +530,7 @@ _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 cd40d77ba90c67ad8c3f32b6c327dcb1d268df4b Mon Sep 17 00:00:00 2001
From: Marcel Martin <marcel.martin@scilifelab.se>
Date: Fri, 29 Sep 2023 09:18:17 +0200
Subject: [PATCH 19/30] 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 ea24a86ed359eb..c94ab03a76e186 100644
--- a/Modules/_io/bytesio.c
+++ b/Modules/_io/bytesio.c
@@ -522,12 +522,10 @@ 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 04372bd9f79f9e80eb1fca88c5c112dd0b13240f 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 20/30] 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 c94ab03a76e186..03d0594920b907 100644
--- a/Modules/_io/bytesio.c
+++ b/Modules/_io/bytesio.c
@@ -451,7 +451,9 @@ static PyObject *
 read_bytes_lock_held(bytesio *self, Py_ssize_t size)
 {
     PyObject *bytes = peek_bytes_lock_held(self, size);
-    self->pos += size;
+    if (bytes != NULL) {
+        self->pos += size;
+    }
     return bytes;
 }
 

From 6b9ae8c47c800b135618a6ae4811497b96bcae75 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 21/30] 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 03d0594920b907..e4da995b7ff485 100644
--- a/Modules/_io/bytesio.c
+++ b/Modules/_io/bytesio.c
@@ -530,7 +530,7 @@ _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 f7406f676cbdf3f9f9c628fa5a4770745289663f 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 22/30] Test with size<0 and size>len(buf)

---
 Lib/test/test_io/test_memoryio.py | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/Lib/test/test_io/test_memoryio.py b/Lib/test/test_io/test_memoryio.py
index 8fc4751525ef35..4d07ed0cc87c48 100644
--- a/Lib/test/test_io/test_memoryio.py
+++ b/Lib/test/test_io/test_memoryio.py
@@ -574,13 +574,16 @@ 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(), 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 d9528e24987baad7f2f9cc7a29b90b0fee9de72b 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 23/30] Test peek() after write()

---
 Lib/test/test_io/test_memoryio.py | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/Lib/test/test_io/test_memoryio.py b/Lib/test/test_io/test_memoryio.py
index 4d07ed0cc87c48..6b471d4b3cef6a 100644
--- a/Lib/test/test_io/test_memoryio.py
+++ b/Lib/test/test_io/test_memoryio.py
@@ -589,6 +589,16 @@ 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 bc8134bf8173718995c2ebaefdfc188eb5d61c73 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 24/30] Document BufferedReader.peek and BytesIO.peek similarly

---
 Doc/library/io.rst | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/Doc/library/io.rst b/Doc/library/io.rst
index a6b25b52b59770..481bc0071294da 100644
--- a/Doc/library/io.rst
+++ b/Doc/library/io.rst
@@ -742,9 +742,10 @@ 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:: 3.15
 
@@ -780,8 +781,12 @@ than raw I/O does.
 
    .. method:: peek(size=0, /)
 
-      Return bytes from the stream without advancing the position. The number of
-      bytes returned may be less or more than requested. If the underlying raw
+      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). If the underlying raw
       stream is non-blocking and the operation would block, returns empty bytes.
 
    .. method:: read(size=-1, /)

From b6ffca868b55b0a82e2e65504d41bd74c63689b3 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 25/30] Comment

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

diff --git a/Lib/_pyio.py b/Lib/_pyio.py
index 1b38614420061d..185b2aac1af025 100644
--- a/Lib/_pyio.py
+++ b/Lib/_pyio.py
@@ -999,6 +999,9 @@ 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 5fe5645ca38caf43d1aa3bd2ffeae304b6d83a15 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 26/30] Make it more explicit that size is ignored

---
 Doc/library/io.rst | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/Doc/library/io.rst b/Doc/library/io.rst
index 481bc0071294da..a44ca640d71fd2 100644
--- a/Doc/library/io.rst
+++ b/Doc/library/io.rst
@@ -785,8 +785,9 @@ 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). If the underlying raw
+      The *size* argument is ignored.
+      The number of read bytes depends on the buffer size and the current position in the internal buffer.
+      If the underlying raw
       stream is non-blocking and the operation would block, returns empty bytes.
 
    .. method:: read(size=-1, /)

From 4126a64eb0add5e5f361184839710797c444e99b 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 27/30] Return an empty bytes object for size=0

---
 Doc/library/io.rst                | 2 +-
 Lib/_pyio.py                      | 2 +-
 Lib/test/test_io/test_memoryio.py | 5 +++--
 Modules/_io/bytesio.c             | 6 +++---
 Modules/_io/clinic/bytesio.c.h    | 2 +-
 5 files changed, 9 insertions(+), 8 deletions(-)

diff --git a/Doc/library/io.rst b/Doc/library/io.rst
index a44ca640d71fd2..88180cb9fba844 100644
--- a/Doc/library/io.rst
+++ b/Doc/library/io.rst
@@ -744,7 +744,7 @@ 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:: 3.15
diff --git a/Lib/_pyio.py b/Lib/_pyio.py
index 185b2aac1af025..bbf572b5e578a3 100644
--- a/Lib/_pyio.py
+++ b/Lib/_pyio.py
@@ -1002,7 +1002,7 @@ 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_io/test_memoryio.py b/Lib/test/test_io/test_memoryio.py
index 6b471d4b3cef6a..afcf69f003e6d7 100644
--- a/Lib/test/test_io/test_memoryio.py
+++ b/Lib/test/test_io/test_memoryio.py
@@ -573,7 +573,7 @@ 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)
@@ -581,13 +581,14 @@ def test_peek(self):
             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(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 e4da995b7ff485..84f0693f4626e7 100644
--- a/Modules/_io/bytesio.c
+++ b/Modules/_io/bytesio.c
@@ -516,7 +516,7 @@ _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]*/
 
@@ -528,9 +528,9 @@ _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;
         }
diff --git a/Modules/_io/clinic/bytesio.c.h b/Modules/_io/clinic/bytesio.c.h
index fe3ec2f3bcad0b..195eaca5ca14a4 100644
--- a/Modules/_io/clinic/bytesio.c.h
+++ b/Modules/_io/clinic/bytesio.c.h
@@ -293,7 +293,7 @@ PyDoc_STRVAR(_io_BytesIO_peek__doc__,
 "\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"
+"If the size argument is negative, read until EOF is reached.\n"
 "Return an empty bytes object at EOF.");
 
 #define _IO_BYTESIO_PEEK_METHODDEF    \

From 1ea40c26f359628f6d33d329b5b8501297bca860 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 28/30] Simplify

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

diff --git a/Lib/_pyio.py b/Lib/_pyio.py
index bbf572b5e578a3..fc6da3ac701fdc 100644
--- a/Lib/_pyio.py
+++ b/Lib/_pyio.py
@@ -999,12 +999,9 @@ 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 77e04d6bb53ea12a94797cc62e9b45dcbf4a1be9 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 29/30] Test peek(3) and peek(5)

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

diff --git a/Lib/test/test_io/test_memoryio.py b/Lib/test/test_io/test_memoryio.py
index afcf69f003e6d7..4393ed074fae24 100644
--- a/Lib/test/test_io/test_memoryio.py
+++ b/Lib/test/test_io/test_memoryio.py
@@ -573,6 +573,8 @@ 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(3), buf[:3])
+            self.assertEqual(memio.peek(5), buf[:5])
             self.assertEqual(memio.peek(0), b"")
             self.assertEqual(memio.peek(len(buf) + 100), buf)
             self.assertEqual(memio.peek(-1), buf)
@@ -581,6 +583,8 @@ def test_peek(self):
             self.assertEqual(memio.tell(), 1)
             self.assertEqual(memio.peek(1), buf[1:2])
             self.assertEqual(memio.peek(), buf[1:2])
+            self.assertEqual(memio.peek(3), buf[1:4])
+            self.assertEqual(memio.peek(5), buf[1:6])
             self.assertEqual(memio.peek(0), b"")
             self.assertEqual(memio.peek(len(buf) + 100), buf[1:])
             self.assertEqual(memio.peek(-1), buf[1:])
@@ -588,6 +592,8 @@ 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

From 4d2f2dd147ab6813fae5ef03ef1377ed1dcf9893 Mon Sep 17 00:00:00 2001
From: Marcel Martin <marcel.martin@scilifelab.se>
Date: Fri, 10 Apr 2026 23:41:33 +0200
Subject: [PATCH 30/30] Run clinic.py

---
 Modules/_io/bytesio.c          | 2 +-
 Modules/_io/clinic/bytesio.c.h | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/Modules/_io/bytesio.c b/Modules/_io/bytesio.c
index 84f0693f4626e7..d859104d547cf9 100644
--- a/Modules/_io/bytesio.c
+++ b/Modules/_io/bytesio.c
@@ -522,7 +522,7 @@ Return an empty bytes object at EOF.
 
 static PyObject *
 _io_BytesIO_peek_impl(bytesio *self, Py_ssize_t size)
-/*[clinic end generated code: output=fa4d8ce28b35db9b input=cb06614a3ed0496e]*/
+/*[clinic end generated code: output=fa4d8ce28b35db9b input=1510f0fcf77c0048]*/
 {
     CHECK_CLOSED(self);
 
diff --git a/Modules/_io/clinic/bytesio.c.h b/Modules/_io/clinic/bytesio.c.h
index 195eaca5ca14a4..ac7dd8733b6fff 100644
--- a/Modules/_io/clinic/bytesio.c.h
+++ b/Modules/_io/clinic/bytesio.c.h
@@ -683,4 +683,4 @@ _io_BytesIO___init__(PyObject *self, PyObject *args, PyObject *kwargs)
 exit:
     return return_value;
 }
-/*[clinic end generated code: output=02299ed558ba722f input=a9049054013a1b77]*/
+/*[clinic end generated code: output=396250eb9869a0b4 input=a9049054013a1b77]*/