Default to size=0; cap to DEFAULT_BUFFER_SIZE

Author: marcelm

Doc/library/io.rst

@@ -739,13 +739,13 @@ than raw I/O does.
 
       Return :class:`bytes` containing the entire contents of the buffer.
 
-   .. method:: peek(size=1, /)
+   .. method:: peek(size=0, /)
 
       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.
+      The number of read bytes depends on the buffer size
+      and the current position in the internal buffer.
 
       .. versionadded:: 3.15
 

Lib/_pyio.py

@@ -996,12 +996,12 @@ def tell(self):
             raise ValueError("tell on closed file")
         return self._pos
 
-    def peek(self, size=1):
+    def peek(self, size=0):
         if self.closed:
             raise ValueError("peek on closed file")
         if size < 1:
-            return self._buffer[self._pos:]
-        return self._buffer[self._pos:self._pos + size]
+            return self._buffer[self._pos:self._pos + DEFAULT_BUFFER_SIZE]
+        return self._buffer[self._pos:self._pos + min(size, DEFAULT_BUFFER_SIZE)]
 
     def truncate(self, pos=None):
         if self.closed:

Lib/test/test_io/test_memoryio.py

@@ -572,7 +572,7 @@ 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(), buf[:1])
+            self.assertEqual(memio.peek(), buf)
             self.assertEqual(memio.peek(3), buf[:3])
             self.assertEqual(memio.peek(5), buf[:5])
             self.assertEqual(memio.peek(0), buf)
@@ -583,7 +583,7 @@ def test_peek(self):
             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(), buf[1:])
             self.assertEqual(memio.peek(3), buf[1:4])
             self.assertEqual(memio.peek(5), buf[1:6])
             self.assertEqual(memio.peek(0), buf[1:])
@@ -604,11 +604,17 @@ def test_peek(self):
             memio.write(abc)
             self.assertEqual(memio.peek(), self.EOF)
             memio.seek(len(buf))
-            self.assertEqual(memio.peek(), abc[:1])
+            self.assertEqual(memio.peek(), abc)
             self.assertEqual(memio.peek(-1), abc)
             self.assertEqual(memio.peek(len(abc) + 100), abc)
             self.assertEqual(memio.tell(), len(buf))
 
+        # Length of returned bytes object is capped to DEFAULT_BUFFER_SIZE
+        buf = self.buftype("a" * (io.DEFAULT_BUFFER_SIZE + 10))
+        with self.ioclass(buf) as memio:
+            self.assertEqual(len(memio.peek()), io.DEFAULT_BUFFER_SIZE)
+            self.assertEqual(len(memio.peek(io.DEFAULT_BUFFER_SIZE + 1)), io.DEFAULT_BUFFER_SIZE)
+
         with self.ioclass(buf) as memio:
             memio.seek(len(buf))
             self.assertEqual(memio.peek(), self.EOF)

Modules/_io/bytesio.c

@@ -513,26 +513,28 @@ _io_BytesIO_read1_impl(bytesio *self, Py_ssize_t size)
 /*[clinic input]
 @critical_section
 _io.BytesIO.peek
-    size: Py_ssize_t = 1
+    size: Py_ssize_t = 0
     /
 
 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=36dc9805d333fe24]*/
+/*[clinic end generated code: output=fa4d8ce28b35db9b input=2ce74234b10aec3e]*/
 {
     CHECK_CLOSED(self);
 
     /* adjust invalid sizes */
     Py_ssize_t n = self->string_size - self->pos;
+    if (n > DEFAULT_BUFFER_SIZE) {
+        n = DEFAULT_BUFFER_SIZE;
+    }
     if (size < 1 || 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;
         }