py/runtime: Add MP_BINARY_OP_CONTAINS as reverse of MP_BINARY_OP_IN.

Before this patch MP_BINARY_OP_IN had two meanings: coming from bytecode it
meant that the args needed to be swapped, but coming from within the
runtime meant that the args were already in the correct order.  This lead
to some confusion in the code and comments stating how args were reversed.
It also lead to 2 bugs: 1) containment for a subclass of a native type
didn't work; 2) the expression "{True} in True" would illegally succeed and
return True.  In both of these cases it was because the args to
MP_BINARY_OP_IN ended up being reversed twice.

To fix these things this patch introduces MP_BINARY_OP_CONTAINS which
corresponds exactly to the __contains__ special method, and this is the
operator that built-in types should implement.  MP_BINARY_OP_IN is now only
emitted by the compiler and is converted to MP_BINARY_OP_CONTAINS by
swapping the arguments.

1 files changed, 19 insertions, 23 deletions

diff --git a/py/runtime.c b/py/runtime.c
index 08a35c2e6..c7fe39367 100644
--- a/py/runtime.c
+++ b/py/runtime.c
@@ -523,30 +523,12 @@ mp_obj_t mp_binary_op(mp_binary_op_t op, mp_obj_t lhs, mp_obj_t rhs) {
         }
     }
 
-    /* deal with `in`
-     *
-     * NOTE `a in b` is `b.__contains__(a)`, hence why the generic dispatch
-     * needs to go below with swapped arguments
-     */
+    // Convert MP_BINARY_OP_IN to MP_BINARY_OP_CONTAINS with swapped args.
     if (op == MP_BINARY_OP_IN) {
-        mp_obj_type_t *type = mp_obj_get_type(rhs);
-        if (type->binary_op != NULL) {
-            mp_obj_t res = type->binary_op(op, rhs, lhs);
-            if (res != MP_OBJ_NULL) {
-                return res;
-            }
-        }
-
-        // final attempt, walk the iterator (will raise if rhs is not iterable)
-        mp_obj_iter_buf_t iter_buf;
-        mp_obj_t iter = mp_getiter(rhs, &iter_buf);
-        mp_obj_t next;
-        while ((next = mp_iternext(iter)) != MP_OBJ_STOP_ITERATION) {
-            if (mp_obj_equal(next, lhs)) {
-                return mp_const_true;
-            }
-        }
-        return mp_const_false;
+        op = MP_BINARY_OP_CONTAINS;
+        mp_obj_t temp = lhs;
+        lhs = rhs;
+        rhs = temp;
     }
 
     // generic binary_op supplied by type
@@ -575,6 +557,20 @@ generic_binary_op:
     }
 #endif
 
+    if (op == MP_BINARY_OP_CONTAINS) {
+        // If type didn't support containment then explicitly walk the iterator.
+        // mp_getiter will raise the appropriate exception if lhs is not iterable.
+        mp_obj_iter_buf_t iter_buf;
+        mp_obj_t iter = mp_getiter(lhs, &iter_buf);
+        mp_obj_t next;
+        while ((next = mp_iternext(iter)) != MP_OBJ_STOP_ITERATION) {
+            if (mp_obj_equal(next, rhs)) {
+                return mp_const_true;
+            }
+        }
+        return mp_const_false;
+    }
+
 unsupported_op:
     if (MICROPY_ERROR_REPORTING == MICROPY_ERROR_REPORTING_TERSE) {
         mp_raise_TypeError("unsupported type for operator");