py/obj: Add support for __int__ special method.

Based on the discussion, this special method is available unconditionally,
as converting to int is a common operation.

py/obj.c

@@ -235,12 +235,8 @@ mp_int_t mp_obj_get_int(mp_const_obj_t arg) {
     } else if (MP_OBJ_IS_TYPE(arg, &mp_type_int)) {
         return mp_obj_int_get_checked(arg);
     } else {
-        if (MICROPY_ERROR_REPORTING == MICROPY_ERROR_REPORTING_TERSE) {
-            mp_raise_TypeError("can't convert to int");
-        } else {
-            nlr_raise(mp_obj_new_exception_msg_varg(&mp_type_TypeError,
-                "can't convert %s to int", mp_obj_get_type_str(arg)));
-        }
+        mp_obj_t res = mp_unary_op(MP_UNARY_OP_INT, (mp_obj_t)arg);
+        return mp_obj_int_get_checked(res);
     }
 }
 

py/objint.c

@@ -62,8 +62,7 @@ STATIC mp_obj_t mp_obj_int_make_new(const mp_obj_type_t *type_in, size_t n_args,
                 return mp_obj_new_int_from_float(mp_obj_float_get(args[0]));
 #endif
             } else {
-                // try to convert to small int (eg from bool)
-                return MP_OBJ_NEW_SMALL_INT(mp_obj_get_int(args[0]));
+                return mp_unary_op(MP_UNARY_OP_INT, args[0]);
             }
 
         case 2:

py/objtype.c

@@ -376,6 +376,7 @@ const byte mp_unary_op_method_name[MP_UNARY_OP_NUM_RUNTIME] = {
     [MP_UNARY_OP_BOOL] = MP_QSTR___bool__,
     [MP_UNARY_OP_LEN] = MP_QSTR___len__,
     [MP_UNARY_OP_HASH] = MP_QSTR___hash__,
+    [MP_UNARY_OP_INT] = MP_QSTR___int__,
     #if MICROPY_PY_ALL_SPECIAL_METHODS
     [MP_UNARY_OP_POSITIVE] = MP_QSTR___pos__,
     [MP_UNARY_OP_NEGATIVE] = MP_QSTR___neg__,
@@ -421,9 +422,21 @@ STATIC mp_obj_t instance_unary_op(mp_unary_op_t op, mp_obj_t self_in) {
         return mp_unary_op(op, self->subobj[0]);
     } else if (member[0] != MP_OBJ_NULL) {
         mp_obj_t val = mp_call_function_1(member[0], self_in);
+
+        switch (op) {
+            case MP_UNARY_OP_HASH:
                 // __hash__ must return a small int
-        if (op == MP_UNARY_OP_HASH) {
                 val = MP_OBJ_NEW_SMALL_INT(mp_obj_get_int_truncated(val));
+                break;
+            case MP_UNARY_OP_INT:
+                // Must return int
+                if (!MP_OBJ_IS_INT(val)) {
+                    mp_raise_TypeError(NULL);
+                }
+                break;
+            default:
+                // No need to do anything
+                ;
         }
         return val;
     } else {

py/runtime.c

@@ -228,6 +228,7 @@ mp_obj_t mp_unary_op(mp_unary_op_t op, mp_obj_t arg) {
             case MP_UNARY_OP_HASH:
                 return arg;
             case MP_UNARY_OP_POSITIVE:
+            case MP_UNARY_OP_INT:
                 return arg;
             case MP_UNARY_OP_NEGATIVE:
                 // check for overflow
@@ -265,8 +266,18 @@ mp_obj_t mp_unary_op(mp_unary_op_t op, mp_obj_t arg) {
                 return result;
             }
         }
+        // With MP_UNARY_OP_INT, mp_unary_op() becomes a fallback for mp_obj_get_int().
+        // In this case provide a more focused error message to not confuse, e.g. chr(1.0)
         if (MICROPY_ERROR_REPORTING == MICROPY_ERROR_REPORTING_TERSE) {
+            if (op == MP_UNARY_OP_INT) {
+                mp_raise_TypeError("can't convert to int");
+            } else {
                 mp_raise_TypeError("unsupported type for operator");
+            }
+        } else {
+            if (op == MP_UNARY_OP_INT) {
+                nlr_raise(mp_obj_new_exception_msg_varg(&mp_type_TypeError,
+                    "can't convert %s to int", mp_obj_get_type_str(arg)));
             } else {
                 nlr_raise(mp_obj_new_exception_msg_varg(&mp_type_TypeError,
                     "unsupported type for %q: '%s'",
@@ -274,6 +285,7 @@ mp_obj_t mp_unary_op(mp_unary_op_t op, mp_obj_t arg) {
             }
         }
     }
+}
 
 mp_obj_t mp_binary_op(mp_binary_op_t op, mp_obj_t lhs, mp_obj_t rhs) {
     DEBUG_OP_printf("binary " UINT_FMT " %q %p %p\n", op, mp_binary_op_method_name[op], lhs, rhs);

py/runtime0.h

@@ -61,6 +61,7 @@ typedef enum {
     MP_UNARY_OP_LEN, // __len__
     MP_UNARY_OP_HASH, // __hash__; must return a small int
     MP_UNARY_OP_ABS, // __abs__
+    MP_UNARY_OP_INT, // __int__
     MP_UNARY_OP_SIZEOF, // for sys.getsizeof()
 
     MP_UNARY_OP_NUM_RUNTIME,