python · hashbrowncipher · Jun 27, 2018 · Jun 27, 2018
diff --git a/Doc/library/io.rst b/Doc/library/io.rst
@@ -662,12 +662,15 @@ than raw I/O does.
    :class:`BufferedReader` provides or overrides these methods in addition to
    those from :class:`BufferedIOBase` and :class:`IOBase`:
 
-   .. method:: peek([size])
+   .. method:: peek([size][, allow_read])
 
       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.
 
+      If *allow_read* is supplied and False, only bytes already buffered are
+      returned, and no reads are done on the raw stream.
+
    .. method:: read([size])
 
       Read and return *size* bytes, or if *size* is not given or negative, until

diff --git a/Misc/NEWS.d/next/Core and Builtins/2018-06-27-19-31-01.bpo-32475.pF3sWK.rst b/Misc/NEWS.d/next/Core and Builtins/2018-06-27-19-31-01.bpo-32475.pF3sWK.rst
@@ -0,0 +1,2 @@
+New optional argument to `BufferedReader.peek`: `allow_read` prevents reads of
+the underlying file when set to False.
diff --git a/Modules/_io/bufferedio.c b/Modules/_io/bufferedio.c
@@ -628,7 +628,7 @@ _bufferedreader_reset_buf(buffered *self);
 static void
 _bufferedwriter_reset_buf(buffered *self);
 static PyObject *
-_bufferedreader_peek_unlocked(buffered *self);
+_bufferedreader_peek_unlocked(buffered *self, int);
 static PyObject *
 _bufferedreader_read_all(buffered *self);
 static PyObject *
@@ -843,13 +843,14 @@ buffered_flush(buffered *self, PyObject *args)
 /*[clinic input]
 _io._Buffered.peek
     size: Py_ssize_t = 0
+    allow_read: bool = True
     /
 
 [clinic start generated code]*/
 
 static PyObject *
-_io__Buffered_peek_impl(buffered *self, Py_ssize_t size)
-/*[clinic end generated code: output=ba7a097ca230102b input=37ffb97d06ff4adb]*/
+_io__Buffered_peek_impl(buffered *self, Py_ssize_t size, int allow_read)
+/*[clinic end generated code: output=14a0ef8e7da9806a input=97d65e49c07606d3]*/
 {
     PyObject *res = NULL;
 
@@ -865,7 +866,7 @@ _io__Buffered_peek_impl(buffered *self, Py_ssize_t size)
             goto end;
         Py_CLEAR(res);
     }
-    res = _bufferedreader_peek_unlocked(self);
+    res = _bufferedreader_peek_unlocked(self, allow_read);
 
 end:
     LEAVE_BUFFERED(self)
@@ -1725,7 +1726,7 @@ _bufferedreader_read_generic(buffered *self, Py_ssize_t n)
 }
 
 static PyObject *
-_bufferedreader_peek_unlocked(buffered *self)
+_bufferedreader_peek_unlocked(buffered *self, int allow_read)
 {
     Py_ssize_t have, r;
 
@@ -1736,7 +1737,7 @@ _bufferedreader_peek_unlocked(buffered *self)
           to make some place.
        Therefore, we either return `have` bytes (if > 0), or a full buffer.
     */
-    if (have > 0) {
+    if (have > 0 || !allow_read) {
         return PyBytes_FromStringAndSize(self->buffer + self->pos, have);
     }
 

diff --git a/Modules/_io/clinic/bufferedio.c.h b/Modules/_io/clinic/bufferedio.c.h
@@ -86,27 +86,28 @@ _io__BufferedIOBase_detach(PyObject *self, PyObject *Py_UNUSED(ignored))
 }
 
 PyDoc_STRVAR(_io__Buffered_peek__doc__,
-"peek($self, size=0, /)\n"
+"peek($self, size=0, allow_read=True, /)\n"
 "--\n"
 "\n");
 
 #define _IO__BUFFERED_PEEK_METHODDEF    \
     {"peek", (PyCFunction)_io__Buffered_peek, METH_FASTCALL, _io__Buffered_peek__doc__},
 
 static PyObject *
-_io__Buffered_peek_impl(buffered *self, Py_ssize_t size);
+_io__Buffered_peek_impl(buffered *self, Py_ssize_t size, int allow_read);
 
 static PyObject *
 _io__Buffered_peek(buffered *self, PyObject *const *args, Py_ssize_t nargs)
 {
     PyObject *return_value = NULL;
     Py_ssize_t size = 0;
+    int allow_read = 1;
 
-    if (!_PyArg_ParseStack(args, nargs, "|n:peek",
-        &size)) {
+    if (!_PyArg_ParseStack(args, nargs, "|np:peek",
+        &size, &allow_read)) {
         goto exit;
     }
-    return_value = _io__Buffered_peek_impl(self, size);
+    return_value = _io__Buffered_peek_impl(self, size, allow_read);
 
 exit:
     return return_value;
@@ -476,4 +477,4 @@ _io_BufferedRandom___init__(PyObject *self, PyObject *args, PyObject *kwargs)
 exit:
     return return_value;
 }
-/*[clinic end generated code: output=9a20dd4eaabb5d58 input=a9049054013a1b77]*/
+/*[clinic end generated code: output=b0f91041c4b36d20 input=a9049054013a1b77]*/
Original file line number	Diff line number	Diff line change
		@@ -0,0 +1,2 @@
		New optional argument to `BufferedReader.peek`: `allow_read` prevents reads of
		the underlying file when set to False.