Do not return an empty bytes object for size=0

Author: marcelm

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 negative or larger than the number of available bytes,
+      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.
 
       .. versionadded:: 3.15

Lib/_pyio.py

@@ -999,7 +999,7 @@ def tell(self):
     def peek(self, size=1):
         if self.closed:
             raise ValueError("peek on closed file")
-        if size < 0:
+        if size < 1:
             return self._buffer[self._pos:]
         return self._buffer[self._pos:self._pos + size]
 

Lib/test/test_io/test_memoryio.py

@@ -575,27 +575,30 @@ def test_peek(self):
             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(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(3), buf[1:4])
             self.assertEqual(memio.peek(5), buf[1:6])
-            self.assertEqual(memio.peek(0), b"")
+            self.assertEqual(memio.peek(0), 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))
             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
             abc = self.buftype("abc")
             memio.write(abc)

Modules/_io/bytesio.c

@@ -517,7 +517,7 @@ _io.BytesIO.peek
 
 Return bytes from the stream without advancing the position.
 
-If the size argument is negative, read until EOF is reached.
+If the size argument is zero or negative, read until EOF is reached.
 Return an empty bytes object at EOF.
 [clinic start generated code]*/
 
@@ -529,9 +529,9 @@ _io_BytesIO_peek_impl(bytesio *self, Py_ssize_t size)
 
     /* adjust invalid sizes */
     Py_ssize_t n = self->string_size - self->pos;
-    if (size < 0 || size > n) {
+    if (size < 1 || size > n) {
         size = n;
-        /* n can be negative after truncate() or seek() */
+        /* size can be negative after truncate() or seek() */
         if (size < 0) {
             size = 0;
         }