Index: Include/object.h
===================================================================
--- Include/object.h	(revision 65931)
+++ Include/object.h	(working copy)
@@ -423,6 +423,7 @@
 PyAPI_FUNC(PyObject *) PyObject_Repr(PyObject *);
 PyAPI_FUNC(PyObject *) PyObject_Str(PyObject *);
 PyAPI_FUNC(PyObject *) PyObject_ASCII(PyObject *);
+PyAPI_FUNC(PyObject *) PyObject_Bytes(PyObject *);
 PyAPI_FUNC(int) PyObject_Compare(PyObject *, PyObject *);
 PyAPI_FUNC(PyObject *) PyObject_RichCompare(PyObject *, PyObject *, int);
 PyAPI_FUNC(int) PyObject_RichCompareBool(PyObject *, PyObject *, int);
Index: Include/bytesobject.h
===================================================================
--- Include/bytesobject.h	(revision 65931)
+++ Include/bytesobject.h	(working copy)
@@ -48,6 +48,7 @@
 
 PyAPI_FUNC(PyObject *) PyBytes_FromStringAndSize(const char *, Py_ssize_t);
 PyAPI_FUNC(PyObject *) PyBytes_FromString(const char *);
+PyAPI_FUNC(PyObject *) PyBytes_FromObject(PyObject *);
 PyAPI_FUNC(PyObject *) PyBytes_FromFormatV(const char*, va_list)
 				Py_GCC_ATTRIBUTE((format(printf, 1, 0)));
 PyAPI_FUNC(PyObject *) PyBytes_FromFormat(const char*, ...)
Index: Objects/object.c
===================================================================
--- Objects/object.c	(revision 65931)
+++ Objects/object.c	(working copy)
@@ -453,6 +453,45 @@
 	return res;
 }
 
+PyObject *
+PyObject_Bytes(PyObject *v)
+{
+	PyObject *bytesmeth, *result, *func;
+	static PyObject *bytesstring = NULL;
+
+	if (bytesstring == NULL) {
+		bytesstring = PyUnicode_InternFromString("__bytes__");
+		if (bytesstring == NULL)
+			return NULL;
+	}
+
+	if (v == NULL)
+		return PyBytes_FromString("<NULL>");
+
+	if (PyBytes_CheckExact(v)) {
+		Py_INCREF(v);
+		return v;
+	}
+
+        /* Doesn't create a reference */
+	func = _PyType_Lookup(Py_TYPE(v), bytesstring);
+	if (func != NULL) {
+            result = PyObject_CallFunctionObjArgs(func, v, NULL);
+            if (result == NULL)
+		return NULL;
+            if (!PyBytes_Check(result)) {
+		PyErr_Format(PyExc_TypeError,
+			     "__bytes__ returned non-bytes (type %.200s)",
+			     Py_TYPE(result)->tp_name);
+		Py_DECREF(result);
+		return NULL;
+            }
+            return result;
+	}
+        PyErr_Clear();
+	return PyBytes_FromObject(v);
+}
+
 /* The new comparison philosophy is: we completely separate three-way
    comparison from rich comparison.  That is, PyObject_Compare() and
    PyObject_Cmp() *just* use the tp_compare slot.  And PyObject_RichCompare()
Index: Objects/bytesobject.c
===================================================================
--- Objects/bytesobject.c	(revision 65931)
+++ Objects/bytesobject.c	(working copy)
@@ -2870,11 +2870,10 @@
 static PyObject *
 string_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
 {
-	PyObject *x = NULL, *it;
+	PyObject *x = NULL;
 	const char *encoding = NULL;
 	const char *errors = NULL;
 	PyObject *new = NULL;
-	Py_ssize_t i, size;
 	static char *kwlist[] = {"source", "encoding", "errors", 0};
 
 	if (type != &PyBytes_Type)
@@ -2912,7 +2911,15 @@
 			"encoding or errors without a string argument");
 		return NULL;
 	}
+        return PyObject_Bytes(x);
+}
 
+PyObject *
+PyBytes_FromObject(PyObject *x)
+{
+	PyObject *new, *it;
+	Py_ssize_t i, size;
+
 	/* Is it an int? */
 	size = PyNumber_AsSsize_t(x, PyExc_ValueError);
 	if (size == -1 && PyErr_Occurred()) {
Index: Lib/test/test_bytes.py
===================================================================
--- Lib/test/test_bytes.py	(revision 65931)
+++ Lib/test/test_bytes.py	(working copy)
@@ -458,7 +458,19 @@
         with open(fd, "rb", buffering=0) as f:
             self.assertRaises(TypeError, f.readinto, b"")
 
+    def test_custom(self):
+        class A:
+            def __bytes__(self):
+                return b'abc'
+        self.assertEqual(bytes(A()), b'abc')
+        class A: pass
+        self.assertRaises(TypeError, bytes, A())
+        class A:
+            def __bytes__(self):
+                return None
+        self.assertRaises(TypeError, bytes, A())
 
+
 class ByteArrayTest(BaseBytesTest):
     type2test = bytearray