__reversed__ method not called when calling reversed()

[nilp0inter]

Hi,

I'm trying to implement reverse iteratoration using the dunder method reversed.
I'm having problems because reversed is not called; instead, the interpreted is trying to reverse the iterator using len and getitem.

This is my code and my expected result (I'm using CPython 3.4 here):

>>> class I:                     
    def __iter__(self):
        return (x for x in range(10))
    def __reversed__(self):
        return (x for x in range(9, -1, -1))
...     
>>> list(I())
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
>>> list(reversed(I()))              
[9, 8, 7, 6, 5, 4, 3, 2, 1, 0]
>>> 

Instead of the desired result I'm getting this on Micro Python v1.3.3 PYBv1.0:

>>> list(I())
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
>>> list(reversed(I()))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: object of type 'I' has no len()
>>> 

Thank you.

Best regards,
Roberto

[pfalcon]

Current known workaround is to write list(my_reversed(I())) or list(I().__reversed__()). I doubt you'll need to change more than couple of places in your code. reversed() is used pretty rarely on its own, and even rare on custom types, so it's on low priority to be fully implemented.

[pfalcon]

Oh, and now it's possible to override builtins, so you can implement it in Python and override builtin.

[dpgeorge]

Well, the patch to implement it is 3 semicolon-lines and 1 qstr:

diff --git a/py/objreversed.c b/py/objreversed.c
index b614958..7e84743 100644
--- a/py/objreversed.c
+++ b/py/objreversed.c
@@ -39,6 +39,13 @@ typedef struct _mp_obj_reversed_t {
 STATIC mp_obj_t reversed_make_new(mp_obj_t type_in, mp_uint_t n_args, mp_uint_t n_kw, const mp_obj_t *args) {
     mp_arg_check_num(n_args, n_kw, 1, 1, false);

+    // check if __reversed__ exists, and if so delegate to it
+    mp_obj_t dest[2];
+    mp_load_method_maybe(args[0], MP_QSTR___reversed__, dest);
+    if (dest[0] != MP_OBJ_NULL) {
+        return mp_call_method_n_kw(0, 0, dest);
+    }
+
     mp_obj_reversed_t *o = m_new_obj(mp_obj_reversed_t);
     o->base.type = &mp_type_reversed;
     o->seq = args[0];
diff --git a/py/qstrdefs.h b/py/qstrdefs.h
index 71a523c..86e2918 100644
--- a/py/qstrdefs.h
+++ b/py/qstrdefs.h
@@ -55,6 +55,7 @@ Q(__file__)

 Q(__bool__)
 Q(__contains__)
+Q(__reversed__)
 Q(__enter__)
 Q(__exit__)
 Q(__len__)

[dpgeorge]

Now implemented.