Merge pull request #528 from pfalcon/native-subclass1

Initial support for subclassing native types

py/objtype.c

@@ -1,3 +1,5 @@
+#include <stdio.h>
+#include <stddef.h>
 #include <string.h>
 #include <assert.h>
 
@@ -9,6 +11,13 @@
 #include "runtime0.h"
 #include "runtime.h"
 
+#if 0 // print debugging info
+#define DEBUG_PRINT (1)
+#define DEBUG_printf DEBUG_printf
+#else // don't print debugging info
+#define DEBUG_printf(...) (void)0
+#endif
+
 /******************************************************************************/
 // class object
 // creating an instance of a class makes one of these objects
@@ -16,26 +25,88 @@
 typedef struct _mp_obj_class_t {
     mp_obj_base_t base;
     mp_map_t members;
+    mp_obj_t subobj[];
     // TODO maybe cache __getattr__ and __setattr__ for efficient lookup of them
 } mp_obj_class_t;
 
-STATIC mp_obj_t mp_obj_new_class(mp_obj_t class) {
-    mp_obj_class_t *o = m_new_obj(mp_obj_class_t);
+#define is_native_type(type) ((type)->make_new != class_make_new)
+STATIC mp_obj_t class_make_new(mp_obj_t self_in, uint n_args, uint n_kw, const mp_obj_t *args);
+
+STATIC mp_obj_t mp_obj_new_class(mp_obj_t class, uint subobjs) {
+    mp_obj_class_t *o = m_new_obj_var(mp_obj_class_t, mp_obj_t, subobjs);
     o->base.type = class;
     mp_map_init(&o->members, 0);
+    mp_seq_clear(o->subobj, 0, subobjs, sizeof(*o->subobj));
     return o;
 }
 
+STATIC int class_count_native_bases(const mp_obj_type_t *type, const mp_obj_type_t **last_native_base) {
+    uint len;
+    mp_obj_t *items;
+    mp_obj_tuple_get(type->bases_tuple, &len, &items);
+
+    int count = 0;
+    for (uint i = 0; i < len; i++) {
+        assert(MP_OBJ_IS_TYPE(items[i], &mp_type_type));
+        if (is_native_type((const mp_obj_type_t *)items[i])) {
+            *last_native_base = items[i];
+            count++;
+        } else {
+            count += class_count_native_bases(items[i], last_native_base);
+        }
+    }
+
+    return count;
+}
+
+// TODO
+// This implements depth-first left-to-right MRO, which is not compliant with Python3 MRO
+// http://python-history.blogspot.com/2010/06/method-resolution-order.html
+// https://www.python.org/download/releases/2.3/mro/
+//
 // will return MP_OBJ_NULL if not found
-STATIC mp_obj_t mp_obj_class_lookup(const mp_obj_type_t *type, qstr attr) {
+// will return MP_OBJ_SENTINEL if special method was found in a native type base
+// via slot id (meth_offset). As there can be only one native base, it's known that it
+// applies to instance->subobj[0]. In most cases, we also don't need to know which type
+// it was - because instance->subobj[0] is of that type. The only exception is when
+// object is not yet constructed, then we need to know base native type to construct
+// instance->subobj[0]. This case is handled via class_count_native_bases() though.
+STATIC void mp_obj_class_lookup(mp_obj_class_t *o, const mp_obj_type_t *type, qstr attr, machine_uint_t meth_offset, mp_obj_t *dest) {
+    assert(dest[0] == NULL);
+    assert(dest[1] == NULL);
     for (;;) {
+        // Optimize special method lookup for native types
+        // This avoids extra method_name => slot lookup. On the other hand,
+        // this should not be applied to class types, as will result in extra
+        // lookup either.
+        if (meth_offset != 0 && is_native_type(type)) {
+            if (*(void**)((char*)type + meth_offset) != NULL) {
+                DEBUG_printf("mp_obj_class_lookup: matched special meth slot for %s\n", qstr_str(attr));
+                dest[0] = MP_OBJ_SENTINEL;
+                return;
+            }
+        }
+
         if (type->locals_dict != NULL) {
             // search locals_dict (the set of methods/attributes)
             assert(MP_OBJ_IS_TYPE(type->locals_dict, &mp_type_dict)); // Micro Python restriction, for now
             mp_map_t *locals_map = mp_obj_dict_get_map(type->locals_dict);
             mp_map_elem_t *elem = mp_map_lookup(locals_map, MP_OBJ_NEW_QSTR(attr), MP_MAP_LOOKUP);
             if (elem != NULL) {
-                return elem->value;
+                dest[0] = elem->value;
+                if (o != MP_OBJ_NULL && is_native_type(type)) {
+                    dest[1] = o->subobj[0];
+                }
+                return;
+            }
+        }
+
+        // Try this for completeness, by all native methods should be statically defined
+        // in locals_dict, and would be handled by above.
+        if (o != MP_OBJ_NULL && is_native_type(type)) {
+            mp_load_method_maybe(o->subobj[0], attr, dest);
+            if (dest[0] != MP_OBJ_NULL) {
+                return;
             }
         }
 
@@ -43,20 +114,20 @@ STATIC mp_obj_t mp_obj_class_lookup(const mp_obj_type_t *type, qstr attr) {
 
         // for a const struct, this entry might be NULL
         if (type->bases_tuple == MP_OBJ_NULL) {
-            return MP_OBJ_NULL;
+            return;
         }
 
         uint len;
         mp_obj_t *items;
         mp_obj_tuple_get(type->bases_tuple, &len, &items);
         if (len == 0) {
-            return MP_OBJ_NULL;
+            return;
         }
         for (uint i = 0; i < len - 1; i++) {
             assert(MP_OBJ_IS_TYPE(items[i], &mp_type_type));
-            mp_obj_t obj = mp_obj_class_lookup((mp_obj_type_t*)items[i], attr);
-            if (obj != MP_OBJ_NULL) {
-                return obj;
+            mp_obj_class_lookup(o, (mp_obj_type_t*)items[i], attr, meth_offset, dest);
+            if (dest[0] != MP_OBJ_NULL) {
+                return;
             }
         }
 
@@ -69,14 +140,20 @@ STATIC mp_obj_t mp_obj_class_lookup(const mp_obj_type_t *type, qstr attr) {
 STATIC void class_print(void (*print)(void *env, const char *fmt, ...), void *env, mp_obj_t self_in, mp_print_kind_t kind) {
     mp_obj_class_t *self = self_in;
     qstr meth = (kind == PRINT_STR) ? MP_QSTR___str__ : MP_QSTR___repr__;
-    mp_obj_t member = mp_obj_class_lookup(self->base.type, meth);
-    if (member == MP_OBJ_NULL && kind == PRINT_STR) {
+    mp_obj_t member[2] = {MP_OBJ_NULL};
+    mp_obj_class_lookup(self, self->base.type, meth, offsetof(mp_obj_type_t, print), member);
+    if (member[0] == MP_OBJ_NULL && kind == PRINT_STR) {
         // If there's no __str__, fall back to __repr__
-        member = mp_obj_class_lookup(self->base.type, MP_QSTR___repr__);
+        mp_obj_class_lookup(self, self->base.type, MP_QSTR___repr__, 0, member);
     }
 
-    if (member != MP_OBJ_NULL) {
-        mp_obj_t r = mp_call_function_1(member, self_in);
+    if (member[0] == MP_OBJ_SENTINEL) {
+        mp_obj_print_helper(print, env, self->subobj[0], kind);
+        return;
+    }
+
+    if (member[0] != MP_OBJ_NULL) {
+        mp_obj_t r = mp_call_function_1(member[0], self_in);
         mp_obj_print_helper(print, env, r, PRINT_STR);
         return;
     }
@@ -89,21 +166,34 @@ STATIC mp_obj_t class_make_new(mp_obj_t self_in, uint n_args, uint n_kw, const m
     assert(MP_OBJ_IS_TYPE(self_in, &mp_type_type));
     mp_obj_type_t *self = self_in;
 
-    mp_obj_t o = mp_obj_new_class(self_in);
+    const mp_obj_type_t *native_base;
+    uint num_native_bases = class_count_native_bases(self, &native_base);
+    assert(num_native_bases < 2);
 
-    // look for __init__ function
-    mp_obj_t init_fn = mp_obj_class_lookup(self, MP_QSTR___init__);
+    mp_obj_class_t *o = mp_obj_new_class(self_in, num_native_bases);
 
-    if (init_fn != MP_OBJ_NULL) {
-        // call __init__ function
+    // look for __init__ function
+    mp_obj_t init_fn[2] = {MP_OBJ_NULL};
+    mp_obj_class_lookup(NULL, self, MP_QSTR___init__, offsetof(mp_obj_type_t, make_new), init_fn);
+
+    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.
+        o->subobj[0] = native_base->make_new((mp_obj_type_t*)native_base, n_args, n_kw, args);
+    } else if (init_fn[0] != MP_OBJ_NULL) {
+        // We need to default-initialize any native subobjs first
+        if (num_native_bases > 0) {
+            o->subobj[0] = native_base->make_new((mp_obj_type_t*)native_base, 0, 0, NULL);
+        }
+        // now call Python class __init__ function with all args
         mp_obj_t init_ret;
         if (n_args == 0 && n_kw == 0) {
-            init_ret = mp_call_function_n_kw(init_fn, 1, 0, (mp_obj_t*)&o);
+            init_ret = mp_call_function_n_kw(init_fn[0], 1, 0, (mp_obj_t*)&o);
         } else {
             mp_obj_t *args2 = m_new(mp_obj_t, 1 + n_args + 2 * n_kw);
             args2[0] = o;
             memcpy(args2 + 1, args, (n_args + 2 * n_kw) * sizeof(mp_obj_t));
-            init_ret = mp_call_function_n_kw(init_fn, n_args + 1, n_kw, args2);
+            init_ret = mp_call_function_n_kw(init_fn[0], n_args + 1, n_kw, args2);
             m_del(mp_obj_t, args2, 1 + n_args + 2 * n_kw);
         }
         if (init_ret != mp_const_none) {
@@ -111,9 +201,8 @@ STATIC mp_obj_t class_make_new(mp_obj_t self_in, uint n_args, uint n_kw, const m
         }
 
     } else {
-        // TODO
         if (n_args != 0) {
-            nlr_raise(mp_obj_new_exception_msg_varg(&mp_type_TypeError, "function takes 0 positional arguments but %d were given", n_args));
+            nlr_raise(mp_obj_new_exception_msg_varg(&mp_type_TypeError, "object() takes no parameters"));
         }
     }
 
@@ -132,12 +221,17 @@ STATIC const qstr unary_op_method_name[] = {
 STATIC mp_obj_t class_unary_op(int op, mp_obj_t self_in) {
     mp_obj_class_t *self = self_in;
     qstr op_name = unary_op_method_name[op];
+    /* Still try to lookup native slot
     if (op_name == 0) {
         return MP_OBJ_NOT_SUPPORTED;
     }
-    mp_obj_t member = mp_obj_class_lookup(self->base.type, op_name);
-    if (member != MP_OBJ_NULL) {
-        return mp_call_function_1(member, self_in);
+    */
+    mp_obj_t member[2] = {MP_OBJ_NULL};
+    mp_obj_class_lookup(self, self->base.type, op_name, offsetof(mp_obj_type_t, unary_op), member);
+    if (member[0] == MP_OBJ_SENTINEL) {
+        return mp_unary_op(op, self->subobj[0]);
+    } else if (member[0] != MP_OBJ_NULL) {
+        return mp_call_function_1(member[0], self_in);
     } else {
         return MP_OBJ_NOT_SUPPORTED;
     }
@@ -188,6 +282,7 @@ STATIC const qstr binary_op_method_name[] = {
 // Conversion means dealing with static/class methods, callables, and values.
 // see http://docs.python.org/3.3/howto/descriptor.html
 STATIC void class_convert_return_attr(mp_obj_t self, mp_obj_t member, mp_obj_t *dest) {
+    assert(dest[1] == NULL);
     if (MP_OBJ_IS_TYPE(member, &mp_type_staticmethod)) {
         // return just the function
         dest[0] = ((mp_obj_static_class_method_t*)member)->fun;
@@ -210,14 +305,19 @@ STATIC mp_obj_t class_binary_op(int op, mp_obj_t lhs_in, mp_obj_t rhs_in) {
     // __getattr__ or __getattribute__.  It only looks in the class dictionary.
     mp_obj_class_t *lhs = lhs_in;
     qstr op_name = binary_op_method_name[op];
+    /* Still try to lookup native slot
     if (op_name == 0) {
         return MP_OBJ_NOT_SUPPORTED;
     }
-    mp_obj_t member = mp_obj_class_lookup(lhs->base.type, op_name);
-    if (member != MP_OBJ_NULL) {
+    */
+    mp_obj_t member[2] = {MP_OBJ_NULL};
+    mp_obj_class_lookup(lhs, lhs->base.type, op_name, offsetof(mp_obj_type_t, binary_op), member);
+    if (member[0] == MP_OBJ_SENTINEL) {
+        return mp_binary_op(op, lhs->subobj[0], rhs_in);
+    } else if (member[0] != MP_OBJ_NULL) {
         mp_obj_t dest[3];
         dest[1] = MP_OBJ_NULL;
-        class_convert_return_attr(lhs_in, member, dest);
+        class_convert_return_attr(lhs_in, member[0], dest);
         dest[2] = rhs_in;
         return mp_call_method_n_kw(1, 0, dest);
     } else {
@@ -237,7 +337,8 @@ STATIC void class_load_attr(mp_obj_t self_in, qstr attr, mp_obj_t *dest) {
         return;
     }
 
-    mp_obj_t member = mp_obj_class_lookup(self->base.type, attr);
+    mp_obj_class_lookup(self, self->base.type, attr, 0, dest);
+    mp_obj_t member = dest[0];
     if (member != MP_OBJ_NULL) {
         if (0) {
 #if MICROPY_ENABLE_PROPERTY
@@ -255,7 +356,11 @@ STATIC void class_load_attr(mp_obj_t self_in, qstr attr, mp_obj_t *dest) {
 #endif
         } else {
             // not a property
-            class_convert_return_attr(self_in, member, dest);
+            // if we don't yet have bound method (supposedly from native base), go
+            // try to convert own attrs.
+            if (dest[1] == MP_OBJ_NULL) {
+                class_convert_return_attr(self_in, member, dest);
+            }
         }
         return;
     }
@@ -280,11 +385,12 @@ STATIC bool class_store_attr(mp_obj_t self_in, qstr attr, mp_obj_t value) {
 #if MICROPY_ENABLE_PROPERTY
     // for property, we need to do a lookup first in the class dict
     // this makes all stores slow... how to fix?
-    mp_obj_t member = mp_obj_class_lookup(self->base.type, attr);
-    if (member != MP_OBJ_NULL && MP_OBJ_IS_TYPE(member, &mp_type_property)) {
+    mp_obj_t member[2] = {MP_OBJ_NULL};
+    mp_obj_class_lookup(self, self->base.type, attr, 0, member);
+    if (member[0] != MP_OBJ_NULL && MP_OBJ_IS_TYPE(member[0], &mp_type_property)) {
         // attribute already exists and is a property
         // delegate the store to the property
-        const mp_obj_t *proxy = mp_obj_property_get(member);
+        const mp_obj_t *proxy = mp_obj_property_get(member[0]);
         if (proxy[1] == mp_const_none) {
             // TODO better error message
             return false;
@@ -309,25 +415,27 @@ STATIC bool class_store_attr(mp_obj_t self_in, qstr attr, mp_obj_t value) {
 
 STATIC mp_obj_t class_subscr(mp_obj_t self_in, mp_obj_t index, mp_obj_t value) {
     mp_obj_class_t *self = self_in;
-    mp_obj_t member;
+    mp_obj_t member[2] = {MP_OBJ_NULL};
     uint meth_args;
     if (value == MP_OBJ_NULL) {
         // delete item
-        member = mp_obj_class_lookup(self->base.type, MP_QSTR___delitem__);
+        mp_obj_class_lookup(self, self->base.type, MP_QSTR___delitem__, offsetof(mp_obj_type_t, subscr), member);
         meth_args = 2;
     } else if (value == MP_OBJ_SENTINEL) {
         // load item
-        member = mp_obj_class_lookup(self->base.type, MP_QSTR___getitem__);
+        mp_obj_class_lookup(self, self->base.type, MP_QSTR___getitem__, offsetof(mp_obj_type_t, subscr), member);
         meth_args = 2;
     } else {
         // store item
-        member = mp_obj_class_lookup(self->base.type, MP_QSTR___setitem__);
+        mp_obj_class_lookup(self, self->base.type, MP_QSTR___setitem__, offsetof(mp_obj_type_t, subscr), member);
         meth_args = 3;
     }
-    if (member != MP_OBJ_NULL) {
+    if (member[0] == MP_OBJ_SENTINEL) {
+        return mp_obj_subscr(self->subobj[0], index, value);
+    } else if (member[0] != MP_OBJ_NULL) {
         mp_obj_t args[3] = {self_in, index, value};
         // TODO probably need to call class_convert_return_attr, and use mp_call_method_n_kw
-        mp_obj_t ret = mp_call_function_n_kw(member, meth_args, 0, args);
+        mp_obj_t ret = mp_call_function_n_kw(member[0], meth_args, 0, args);
         if (value == MP_OBJ_SENTINEL) {
             return ret;
         } else {
@@ -340,11 +448,15 @@ STATIC mp_obj_t class_subscr(mp_obj_t self_in, mp_obj_t index, mp_obj_t value) {
 
 STATIC mp_obj_t class_call(mp_obj_t self_in, uint n_args, uint n_kw, const mp_obj_t *args) {
     mp_obj_class_t *self = self_in;
-    mp_obj_t member = mp_obj_class_lookup(self->base.type, MP_QSTR___call__);
-    if (member == MP_OBJ_NULL) {
-        return member;
+    mp_obj_t member[2] = {MP_OBJ_NULL};
+    mp_obj_class_lookup(self, self->base.type, MP_QSTR___call__, offsetof(mp_obj_type_t, call), member);
+    if (member[0] == MP_OBJ_NULL) {
+        return MP_OBJ_NULL;
+    }
+    if (member[0] == MP_OBJ_SENTINEL) {
+        return mp_call_function_n_kw(self->subobj[0], n_args, n_kw, args);
     }
-    mp_obj_t meth = mp_obj_new_bound_meth(member, self);
+    mp_obj_t meth = mp_obj_new_bound_meth(member[0], self);
     return mp_call_function_n_kw(meth, n_args, n_kw, args);
 }
 
@@ -403,21 +515,22 @@ STATIC void type_load_attr(mp_obj_t self_in, qstr attr, mp_obj_t *dest) {
         return;
     }
 #endif
-    mp_obj_t member = mp_obj_class_lookup(self, attr);
-    if (member != MP_OBJ_NULL) {
+    mp_obj_t member[2] = {MP_OBJ_NULL};
+    mp_obj_class_lookup(NULL, self, attr, 0, member);
+    if (member[0] != MP_OBJ_NULL) {
         // check if the methods are functions, static or class methods
         // see http://docs.python.org/3.3/howto/descriptor.html
-        if (MP_OBJ_IS_TYPE(member, &mp_type_staticmethod)) {
+        if (MP_OBJ_IS_TYPE(member[0], &mp_type_staticmethod)) {
             // return just the function
-            dest[0] = ((mp_obj_static_class_method_t*)member)->fun;
-        } else if (MP_OBJ_IS_TYPE(member, &mp_type_classmethod)) {
+            dest[0] = ((mp_obj_static_class_method_t*)member[0])->fun;
+        } else if (MP_OBJ_IS_TYPE(member[0], &mp_type_classmethod)) {
             // return a bound method, with self being this class
-            dest[0] = ((mp_obj_static_class_method_t*)member)->fun;
+            dest[0] = ((mp_obj_static_class_method_t*)member[0])->fun;
             dest[1] = self_in;
         } else {
             // return just the function
             // TODO need to wrap in a type check for the first argument; eg list.append(1,1) needs to throw an exception
-            dest[0] = (mp_obj_t)member;
+            dest[0] = member[0];
         }
     }
 }
@@ -474,6 +587,20 @@ const mp_obj_type_t mp_type_type = {
 mp_obj_t mp_obj_new_type(qstr name, mp_obj_t bases_tuple, mp_obj_t locals_dict) {
     assert(MP_OBJ_IS_TYPE(bases_tuple, &mp_type_tuple)); // Micro Python restriction, for now
     assert(MP_OBJ_IS_TYPE(locals_dict, &mp_type_dict)); // Micro Python restriction, for now
+
+    // Basic validation of base classes
+    uint len;
+    mp_obj_t *items;
+    mp_obj_tuple_get(bases_tuple, &len, &items);
+    for (uint i = 0; i < len; i++) {
+        assert(MP_OBJ_IS_TYPE(items[i], &mp_type_type));
+        mp_obj_type_t *t = items[i];
+        // TODO: Verify with CPy, tested on function type
+        if (t->make_new == NULL) {
+            nlr_raise(mp_obj_new_exception_msg_varg(&mp_type_TypeError, "type '%s' is not an acceptable base type", qstr_str(t->name)));
+        }
+    }
+
     mp_obj_type_t *o = m_new0(mp_obj_type_t, 1);
     o->base.type = &mp_type_type;
     o->name = name;
@@ -487,6 +614,13 @@ mp_obj_t mp_obj_new_type(qstr name, mp_obj_t bases_tuple, mp_obj_t locals_dict)
     o->call = class_call;
     o->bases_tuple = bases_tuple;
     o->locals_dict = locals_dict;
+
+    const mp_obj_type_t *native_base;
+    uint num_native_bases = class_count_native_bases(o, &native_base);
+    if (num_native_bases > 1) {
+        nlr_raise(mp_obj_new_exception_msg(&mp_type_TypeError, "multiple bases have instance lay-out conflict"));
+    }
+
     return o;
 }
 
@@ -536,9 +670,10 @@ STATIC void super_load_attr(mp_obj_t self_in, qstr attr, mp_obj_t *dest) {
     mp_obj_tuple_get(type->bases_tuple, &len, &items);
     for (uint i = 0; i < len; i++) {
         assert(MP_OBJ_IS_TYPE(items[i], &mp_type_type));
-        mp_obj_t member = mp_obj_class_lookup((mp_obj_type_t*)items[i], attr);
-        if (member != MP_OBJ_NULL) {
-            class_convert_return_attr(self->obj, member, dest);
+        mp_obj_t member[2] = {MP_OBJ_NULL};
+        mp_obj_class_lookup(self->obj, (mp_obj_type_t*)items[i], attr, 0, member);
+        if (member[0] != MP_OBJ_NULL) {
+            class_convert_return_attr(self->obj, member[0], dest);
             return;
         }
     }

tests/basics/subclass-native1.py

@@ -0,0 +1,31 @@
+class mylist(list):
+    pass
+
+a = mylist([1, 2, 5])
+# Test setting instance attr
+a.attr = "something"
+# Test base type __str__
+print(a)
+# Test getting instance attr
+print(a.attr)
+# Test base type ->subscr
+print(a[-1])
+a[0] = -1
+print(a)
+# Test another base type unary op
+print(len(a))
+
+# Test binary op of base type, with 2nd arg being raw base type
+print(a + [20, 30, 40])
+# Test binary op of base type, with 2nd arg being same class as 1st arg
+# TODO: Faults
+#print(a + a)
+
+def foo():
+    print("hello from foo")
+
+try:
+    class myfunc(type(foo)):
+        pass
+except TypeError:
+    print("TypeError")

tests/basics/subclass-native2.py

@@ -0,0 +1,37 @@
+class Base1:
+    def __init__(self, *args):
+        print("Base1.__init__",args)
+
+class Clist1(Base1, list):
+    pass
+
+class Ctuple1(Base1, tuple):
+    pass
+
+a = Clist1()
+print(len(a))
+a = Clist1([1, 2, 3])
+print(len(a))
+
+a = Ctuple1()
+print(len(a))
+a = Ctuple1([1, 2, 3])
+# TODO: Faults
+#print(len(a))
+
+print("---")
+
+class Clist2(list, Base1):
+    pass
+
+class Ctuple2(tuple, Base1):
+    pass
+a = Clist2()
+print(len(a))
+a = Clist2([1, 2, 3])
+print(len(a))
+
+#a = Ctuple2()
+#print(len(a))
+#a = Ctuple2([1, 2, 3])
+#print(len(a))

tests/basics/subclass-native3.py

@@ -0,0 +1,8 @@
+class MyExc(Exception):
+    pass
+
+e = MyExc(100, "Some error")
+print(e)
+# TODO: Prints native base class name
+#print(repr(e))
+print(e.args)

tests/basics/subclass-native4.py

@@ -0,0 +1,9 @@
+# Test calling non-special method inherited from native type
+
+class mylist(list):
+    pass
+
+l = mylist([1, 2, 3])
+print(l)
+l.append(10)
+print(l)