Return at most DEFAULT_BUFFER_SIZE bytes if size is 0/-1/unspecified

Author: marcelm

Doc/library/io.rst

@@ -775,7 +775,7 @@ than raw I/O does.
 
       Return a copy of the buffer from the current position onwards without advancing the position.
 
-      If *size* is zero or omitted, the returned :class:`bytes` object extends to EOF.
+      If *size* is less than one or omitted, at most `DEFAULT_BUFFER_SIZE` bytes are returned.
       Otherwise, at most *size* bytes are returned.
       Return an empty :class:`bytes` object at EOF.
 

Lib/_pyio.py

@@ -1004,7 +1004,7 @@ 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 + io.DEFAULT_BUFFER_SIZE]
         return self._buffer[self._pos:self._pos + size]
 
     def truncate(self, pos=None):

Lib/test/test_io/test_memoryio.py

@@ -616,8 +616,11 @@ def test_peek(self):
         # Length greater than DEFAULT_BUFFER_SIZE
         buf = self.buftype("1234567890" * io.DEFAULT_BUFFER_SIZE)
         with self.ioclass(buf) as memio:
-            self.assertEqual(memio.peek(), buf)
+            self.assertEqual(memio.peek(), buf[:io.DEFAULT_BUFFER_SIZE])
+            self.assertEqual(memio.peek(0), buf[:io.DEFAULT_BUFFER_SIZE])
+            self.assertEqual(memio.peek(-1), buf[:io.DEFAULT_BUFFER_SIZE])
             self.assertEqual(memio.peek(io.DEFAULT_BUFFER_SIZE + 100), buf[:io.DEFAULT_BUFFER_SIZE + 100])
+            self.assertEqual(memio.peek(io.DEFAULT_BUFFER_SIZE * 100), buf)
 
         # Current position beyond buffer end
         with self.ioclass(buf) as memio:

Modules/_io/bytesio.c

@@ -527,10 +527,13 @@ _io_BytesIO_peek_impl(bytesio *self, Py_ssize_t size)
 {
     CHECK_CLOSED(self);
 
+    if (size < 1) {
+        size = DEFAULT_BUFFER_SIZE;
+    }
+
     /* adjust invalid sizes */
     Py_ssize_t n = self->string_size - self->pos;
-
-    if (size < 1 || size > n) {
+    if (size > n) {
         size = n;
         /* n can be negative after truncate() or seek() */
         if (size < 0) {