py/objtype: Refactor object's handling of __new__ to not create 2 objs.

Before this patch, if a user defined the __new__() function for a class
then two instances of that class would be created: once before __new__ is
called and once during the __new__ call (assuming the user creates some
instance, eg using super().__new__, which is most of the time).  The first
one was then discarded.  This refactor makes it so that a new instance is
only created if the user __new__ function doesn't exist.

py/objobject.c

@@ -52,9 +52,12 @@ STATIC mp_obj_t object___new__(mp_obj_t cls) {
     if (!MP_OBJ_IS_TYPE(cls, &mp_type_type) || !mp_obj_is_instance_type((mp_obj_type_t*)MP_OBJ_TO_PTR(cls))) {
         mp_raise_TypeError("__new__ arg must be a user-type");
     }
-    mp_obj_t o = MP_OBJ_SENTINEL;
-    mp_obj_t res = mp_obj_instance_make_new(MP_OBJ_TO_PTR(cls), 1, 0, &o);
-    return res;
+    // This executes only "__new__" part of instance creation.
+    // TODO: This won't work well for classes with native bases.
+    // TODO: This is a hack, should be resolved along the lines of
+    // https://github.com/micropython/micropython/issues/606#issuecomment-43685883
+    const mp_obj_type_t *native_base;
+    return MP_OBJ_FROM_PTR(mp_obj_new_instance(MP_OBJ_TO_PTR(cls), &native_base));
 }
 STATIC MP_DEFINE_CONST_FUN_OBJ_1(object___new___fun_obj, object___new__);
 STATIC MP_DEFINE_CONST_STATICMETHOD_OBJ(object___new___obj, MP_ROM_PTR(&object___new___fun_obj));

py/objtype.c

@@ -90,17 +90,22 @@ STATIC mp_obj_t native_base_init_wrapper(size_t n_args, const mp_obj_t *args) {
 }
 STATIC MP_DEFINE_CONST_FUN_OBJ_VAR_BETWEEN(native_base_init_wrapper_obj, 1, MP_OBJ_FUN_ARGS_MAX, native_base_init_wrapper);
 
-STATIC mp_obj_t mp_obj_new_instance(const mp_obj_type_t *class, size_t subobjs) {
-    mp_obj_instance_t *o = m_new_obj_var(mp_obj_instance_t, mp_obj_t, subobjs);
+#if !MICROPY_CPYTHON_COMPAT
+STATIC
+#endif
+mp_obj_instance_t *mp_obj_new_instance(const mp_obj_type_t *class, const mp_obj_type_t **native_base) {
+    size_t num_native_bases = instance_count_native_bases(class, native_base);
+    assert(num_native_bases < 2);
+    mp_obj_instance_t *o = m_new_obj_var(mp_obj_instance_t, mp_obj_t, num_native_bases);
     o->base.type = class;
     mp_map_init(&o->members, 0);
     // Initialise the native base-class slot (should be 1 at most) with a valid
     // object.  It doesn't matter which object, so long as it can be uniquely
     // distinguished from a native class that is initialised.
-    if (subobjs != 0) {
+    if (num_native_bases != 0) {
         o->subobj[0] = MP_OBJ_FROM_PTR(&native_base_init_wrapper_obj);
     }
-    return MP_OBJ_FROM_PTR(o);
+    return o;
 }
 
 // TODO
@@ -267,20 +272,6 @@ STATIC void instance_print(const mp_print_t *print, mp_obj_t self_in, mp_print_k
 mp_obj_t mp_obj_instance_make_new(const mp_obj_type_t *self, size_t n_args, size_t n_kw, const mp_obj_t *args) {
     assert(mp_obj_is_instance_type(self));
 
-    const mp_obj_type_t *native_base = NULL;
-    size_t num_native_bases = instance_count_native_bases(self, &native_base);
-    assert(num_native_bases < 2);
-
-    mp_obj_instance_t *o = MP_OBJ_TO_PTR(mp_obj_new_instance(self, num_native_bases));
-
-    // This executes only "__new__" part of instance creation.
-    // TODO: This won't work well for classes with native bases.
-    // TODO: This is a hack, should be resolved along the lines of
-    // https://github.com/micropython/micropython/issues/606#issuecomment-43685883
-    if (n_args == 1 && *args == MP_OBJ_SENTINEL) {
-        return MP_OBJ_FROM_PTR(o);
-    }
-
     // look for __new__ function
     mp_obj_t init_fn[2] = {MP_OBJ_NULL};
     struct class_lookup_data lookup = {
@@ -292,10 +283,12 @@ mp_obj_t mp_obj_instance_make_new(const mp_obj_type_t *self, size_t n_args, size
     };
     mp_obj_class_lookup(&lookup, self);
 
-    mp_obj_t new_ret = MP_OBJ_FROM_PTR(o);
-    if (init_fn[0] == MP_OBJ_SENTINEL) {
-        // Native type's constructor is what wins - it gets all our arguments,
-        // and none Python classes are initialized at all.
+    const mp_obj_type_t *native_base = NULL;
+    mp_obj_instance_t *o;
+    if (init_fn[0] == MP_OBJ_NULL || init_fn[0] == MP_OBJ_SENTINEL) {
+        // Either there is no __new__() method defined or there is a native
+        // constructor.  In both cases create a blank instance.
+        o = mp_obj_new_instance(self, &native_base);
 
         // Since type->make_new() implements both __new__() and __init__() in
         // one go, of which the latter may be overridden by the Python subclass,
@@ -303,8 +296,9 @@ mp_obj_t mp_obj_instance_make_new(const mp_obj_type_t *self, size_t n_args, size
         // constructor to give a chance for the Python __init__() method to call
         // said native constructor.
 
-    } else if (init_fn[0] != MP_OBJ_NULL) {
-        // now call Python class __new__ function with all args
+    } else {
+        // Call Python class __new__ function with all args to create an instance
+        mp_obj_t new_ret;
         if (n_args == 0 && n_kw == 0) {
             mp_obj_t args2[1] = {MP_OBJ_FROM_PTR(self)};
             new_ret = mp_call_function_n_kw(init_fn[0], 1, 0, args2);
@@ -316,16 +310,17 @@ mp_obj_t mp_obj_instance_make_new(const mp_obj_type_t *self, size_t n_args, size
             m_del(mp_obj_t, args2, 1 + n_args + 2 * n_kw);
         }
 
-    }
+        // https://docs.python.org/3.4/reference/datamodel.html#object.__new__
+        // "If __new__() does not return an instance of cls, then the new
+        // instance's __init__() method will not be invoked."
+        if (mp_obj_get_type(new_ret) != self) {
+            return new_ret;
+        }
 
-    // https://docs.python.org/3.4/reference/datamodel.html#object.__new__
-    // "If __new__() does not return an instance of cls, then the new instance's __init__() method will not be invoked."
-    if (mp_obj_get_type(new_ret) != self) {
-        return new_ret;
+        // The instance returned by __new__() becomes the new object
+        o = MP_OBJ_TO_PTR(new_ret);
     }
 
-    o = MP_OBJ_TO_PTR(new_ret);
-
     // now call Python class __init__ function with all args
     // This method has a chance to call super().__init__() to construct a
     // possible native base class.

py/objtype.h

@@ -37,6 +37,11 @@ typedef struct _mp_obj_instance_t {
     // TODO maybe cache __getattr__ and __setattr__ for efficient lookup of them
 } mp_obj_instance_t;
 
+#if MICROPY_CPYTHON_COMPAT
+// this is needed for object.__new__
+mp_obj_instance_t *mp_obj_new_instance(const mp_obj_type_t *cls, const mp_obj_type_t **native_base);
+#endif
+
 // this needs to be exposed for MICROPY_OPT_CACHE_MAP_LOOKUP_IN_BYTECODE to work
 void mp_obj_instance_attr(mp_obj_t self_in, qstr attr, mp_obj_t *dest);