diff -r ce637db93b36 pypy/interpreter/function.py
--- a/pypy/interpreter/function.py	Mon Dec 03 17:42:13 2018 +0100
+++ b/pypy/interpreter/function.py	Mon Dec 03 22:12:07 2018 +0200
@@ -39,8 +39,6 @@
                           'qualname?',
                           'w_kw_defs?']
 
-    w_kw_defs = None
-
     def __init__(self, space, code, w_globals=None, defs_w=[], kw_defs_w=None,
                  closure=None, w_ann=None, forcename=None, qualname=None):
         self.space = space
@@ -54,6 +52,7 @@
         self.w_func_dict = None # filled out below if needed
         self.w_module = None
         self.w_ann = w_ann
+        self.w_kw_defs = None
         #
         if kw_defs_w is not None:
             self.init_kwdefaults_dict(kw_defs_w)
@@ -406,10 +405,22 @@
     def init_kwdefaults_dict(self, kw_defs_w):
         # use the moduledict logic to get normally-constant entries
         space = self.space
-        w_dict = space.newdict(module=True)
-        for w_name, w_value in kw_defs_w:
-            space.setitem(w_dict, w_name, w_value)
-        self.w_kw_defs = w_dict
+        if hasattr(space, 'w_dict'):  # normal path
+            w_dict = space.newdict(module=True)
+            for w_name, w_value in kw_defs_w:
+                space.setitem(w_dict, w_name, w_value)
+            self.w_kw_defs = w_dict
+        else:
+            del self.w_kw_defs        # delayed path during initialization
+            self._saved_kw_defs_w = kw_defs_w
+
+    def __getattr__(self, attr):
+        if attr == 'w_kw_defs' and hasattr(self, '_saved_kw_defs_w'):
+            assert hasattr(self.space, 'w_dict')
+            self.init_kwdefaults_dict(self._saved_kw_defs_w)
+            del self._saved_kw_defs_w
+            return self.w_kw_defs
+        raise AttributeError
 
     def fget_func_doc(self, space):
         if self.w_doc is None:
diff -r ce637db93b36 pypy/objspace/std/intobject.py
--- a/pypy/objspace/std/intobject.py	Mon Dec 03 17:42:13 2018 +0100
+++ b/pypy/objspace/std/intobject.py	Mon Dec 03 22:12:07 2018 +0200
@@ -98,7 +98,7 @@
         return w_obj
 
     @unwrap_spec(length=int, byteorder='text', signed=bool)
-    def descr_to_bytes(self, space, length, byteorder, signed=False):
+    def descr_to_bytes(self, space, length, byteorder, __kwonly__, signed=False):
         """to_bytes(...)
         int.to_bytes(length, byteorder, *, signed=False) -> bytes
 
diff -r ce637db93b36 pypy/objspace/std/test/test_longobject.py
--- a/pypy/objspace/std/test/test_longobject.py	Mon Dec 03 17:42:13 2018 +0100
+++ b/pypy/objspace/std/test/test_longobject.py	Mon Dec 03 22:12:07 2018 +0200
@@ -379,6 +379,7 @@
         raises(OverflowError, (-5).to_bytes, 1, 'big')
         raises(ValueError, (-5).to_bytes, 1, 'foo')
         assert 65535 .to_bytes(length=2, byteorder='big') == b'\xff\xff'
+        raises(TypeError, 65535 .to_bytes, 2, 'big', False)
 
     def test_negative_zero(self):
         x = eval("-self._long(0)")