diff --git a/extmod/modbtree.c b/extmod/modbtree.c
index 5c13115328..8b76885809 100644
--- a/extmod/modbtree.c
+++ b/extmod/modbtree.c
@@ -282,7 +282,7 @@ STATIC mp_obj_t btree_subscr(mp_obj_t self_in, mp_obj_t index, mp_obj_t value) {
 STATIC mp_obj_t btree_binary_op(mp_binary_op_t op, mp_obj_t lhs_in, mp_obj_t rhs_in) {
     mp_obj_btree_t *self = MP_OBJ_TO_PTR(lhs_in);
     switch (op) {
-        case MP_BINARY_OP_IN: {
+        case MP_BINARY_OP_CONTAINS: {
             DBT key, val;
             key.data = (void*)mp_obj_str_get_data(rhs_in, &key.size);
             int res = __bt_get(self->db, &key, &val, 0);
diff --git a/py/objarray.c b/py/objarray.c
index 7003ec9e7d..a35539484b 100644
--- a/py/objarray.c
+++ b/py/objarray.c
@@ -269,8 +269,7 @@ STATIC mp_obj_t array_binary_op(mp_binary_op_t op, mp_obj_t lhs_in, mp_obj_t rhs
             return lhs_in;
         }
 
-        case MP_BINARY_OP_IN: {
-            /* NOTE `a in b` is `b.__contains__(a)` */
+        case MP_BINARY_OP_CONTAINS: {
             mp_buffer_info_t lhs_bufinfo;
             mp_buffer_info_t rhs_bufinfo;
 
diff --git a/py/objdict.c b/py/objdict.c
index 1553a83b46..d0f95e41ad 100644
--- a/py/objdict.c
+++ b/py/objdict.c
@@ -115,7 +115,7 @@ STATIC mp_obj_t dict_unary_op(mp_unary_op_t op, mp_obj_t self_in) {
 STATIC mp_obj_t dict_binary_op(mp_binary_op_t op, mp_obj_t lhs_in, mp_obj_t rhs_in) {
     mp_obj_dict_t *o = MP_OBJ_TO_PTR(lhs_in);
     switch (op) {
-        case MP_BINARY_OP_IN: {
+        case MP_BINARY_OP_CONTAINS: {
             mp_map_elem_t *elem = mp_map_lookup(&o->map, rhs_in, MP_MAP_LOOKUP);
             return mp_obj_new_bool(elem != NULL);
         }
@@ -485,7 +485,7 @@ STATIC mp_obj_t dict_view_binary_op(mp_binary_op_t op, mp_obj_t lhs_in, mp_obj_t
     if (o->kind != MP_DICT_VIEW_KEYS) {
         return MP_OBJ_NULL; // op not supported
     }
-    if (op != MP_BINARY_OP_IN) {
+    if (op != MP_BINARY_OP_CONTAINS) {
         return MP_OBJ_NULL; // op not supported
     }
     return dict_binary_op(op, o->dict, rhs_in);
diff --git a/py/objint_mpz.c b/py/objint_mpz.c
index 7b5cb0b9d4..17e3ee6d24 100644
--- a/py/objint_mpz.c
+++ b/py/objint_mpz.c
@@ -207,7 +207,7 @@ mp_obj_t mp_obj_int_binary_op(mp_binary_op_t op, mp_obj_t lhs_in, mp_obj_t rhs_i
         return mp_obj_new_float(flhs / frhs);
 #endif
 
-    } else if (op >= MP_BINARY_OP_INPLACE_OR) {
+    } else if (op >= MP_BINARY_OP_INPLACE_OR && op < MP_BINARY_OP_CONTAINS) {
         mp_obj_int_t *res = mp_obj_int_new_mpz();
 
         switch (op) {
diff --git a/py/objset.c b/py/objset.c
index 6ed15c7914..3e98c30e8f 100644
--- a/py/objset.c
+++ b/py/objset.c
@@ -461,7 +461,7 @@ STATIC mp_obj_t set_binary_op(mp_binary_op_t op, mp_obj_t lhs, mp_obj_t rhs) {
     #else
     bool update = true;
     #endif
-    if (op != MP_BINARY_OP_IN && !is_set_or_frozenset(rhs)) {
+    if (op != MP_BINARY_OP_CONTAINS && !is_set_or_frozenset(rhs)) {
         // For all ops except containment the RHS must be a set/frozenset
         return MP_OBJ_NULL;
     }
@@ -507,7 +507,7 @@ STATIC mp_obj_t set_binary_op(mp_binary_op_t op, mp_obj_t lhs, mp_obj_t rhs) {
             return set_issubset(lhs, rhs);
         case MP_BINARY_OP_MORE_EQUAL:
             return set_issuperset(lhs, rhs);
-        case MP_BINARY_OP_IN: {
+        case MP_BINARY_OP_CONTAINS: {
             mp_obj_set_t *o = MP_OBJ_TO_PTR(lhs);
             mp_obj_t elem = mp_set_lookup(&o->set, rhs, MP_MAP_LOOKUP);
             return mp_obj_new_bool(elem != MP_OBJ_NULL);
diff --git a/py/objstr.c b/py/objstr.c
index 1ff5132d29..b4f15b38d5 100644
--- a/py/objstr.c
+++ b/py/objstr.c
@@ -384,8 +384,7 @@ mp_obj_t mp_obj_str_binary_op(mp_binary_op_t op, mp_obj_t lhs_in, mp_obj_t rhs_i
             return mp_obj_new_str_from_vstr(lhs_type, &vstr);
         }
 
-        case MP_BINARY_OP_IN:
-            /* NOTE `a in b` is `b.__contains__(a)` */
+        case MP_BINARY_OP_CONTAINS:
             return mp_obj_new_bool(find_subbytes(lhs_data, lhs_len, rhs_data, rhs_len, 1) != NULL);
 
         //case MP_BINARY_OP_NOT_EQUAL: // This is never passed here
diff --git a/py/objtype.c b/py/objtype.c
index 01d248256a..267cae8156 100644
--- a/py/objtype.c
+++ b/py/objtype.c
@@ -424,7 +424,7 @@ const byte mp_binary_op_method_name[MP_BINARY_OP_NUM_RUNTIME] = {
     [MP_BINARY_OP_LESS_EQUAL] = MP_QSTR___le__,
     [MP_BINARY_OP_MORE_EQUAL] = MP_QSTR___ge__,
     // MP_BINARY_OP_NOT_EQUAL, // a != b calls a == b and inverts result
-    [MP_BINARY_OP_IN] = MP_QSTR___contains__,
+    [MP_BINARY_OP_CONTAINS] = MP_QSTR___contains__,
 
     // All inplace methods are optional, and normal methods will be used
     // as a fallback.
diff --git a/py/opmethods.c b/py/opmethods.c
index 31901bb521..247fa5bbc8 100644
--- a/py/opmethods.c
+++ b/py/opmethods.c
@@ -47,6 +47,6 @@ MP_DEFINE_CONST_FUN_OBJ_2(mp_op_delitem_obj, op_delitem);
 
 STATIC mp_obj_t op_contains(mp_obj_t lhs_in, mp_obj_t rhs_in) {
     mp_obj_type_t *type = mp_obj_get_type(lhs_in);
-    return type->binary_op(MP_BINARY_OP_IN, lhs_in, rhs_in);
+    return type->binary_op(MP_BINARY_OP_CONTAINS, lhs_in, rhs_in);
 }
 MP_DEFINE_CONST_FUN_OBJ_2(mp_op_contains_obj, op_contains);
diff --git a/py/runtime.c b/py/runtime.c
index 08a35c2e60..c7fe393675 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");
diff --git a/py/runtime0.h b/py/runtime0.h
index a72b7feb7a..960532d176 100644
--- a/py/runtime0.h
+++ b/py/runtime0.h
@@ -131,6 +131,10 @@ typedef enum {
         #endif
     ,
 
+    // The runtime will convert MP_BINARY_OP_IN to this operator with swapped args.
+    // A type should implement this containment operator instead of MP_BINARY_OP_IN.
+    MP_BINARY_OP_CONTAINS,
+
     MP_BINARY_OP_NUM_RUNTIME,
 
     // These 2 are not supported by the runtime and must be synthesised by the emitter
diff --git a/tests/micropython/viper_error.py.exp b/tests/micropython/viper_error.py.exp
index 96be5a5902..a44fb3ff0a 100644
--- a/tests/micropython/viper_error.py.exp
+++ b/tests/micropython/viper_error.py.exp
@@ -18,7 +18,7 @@ ViperTypeError('must raise an object',)
 ViperTypeError('unary op __pos__ not implemented',)
 ViperTypeError('unary op __neg__ not implemented',)
 ViperTypeError('unary op __invert__ not implemented',)
-ViperTypeError('binary op __contains__ not implemented',)
+ViperTypeError('binary op  not implemented',)
 NotImplementedError('native yield',)
 NotImplementedError('native yield from',)
 NotImplementedError('conversion to object',)