py/objtype: Implement __delattr__ and __setattr__.

This patch implements support for class methods __delattr__ and __setattr__
for customising attribute access.  It is controlled by the config option
MICROPY_PY_DELATTR_SETATTR and is disabled by default.

py/mpconfig.h

@@ -635,6 +635,12 @@ typedef double mp_float_t;
 #define MICROPY_PY_DESCRIPTORS (0)
 #endif
 
+// Whether to support class __delattr__ and __setattr__ methods
+// This costs some code size and makes all del attrs and store attrs slow
+#ifndef MICROPY_PY_DELATTR_SETATTR
+#define MICROPY_PY_DELATTR_SETATTR (0)
+#endif
+
 // Support for async/await/async for/async with
 #ifndef MICROPY_PY_ASYNC_AWAIT
 #define MICROPY_PY_ASYNC_AWAIT (1)

py/objtype.c

@@ -533,6 +533,15 @@ STATIC void mp_obj_instance_load_attr(mp_obj_t self_in, qstr attr, mp_obj_t *des
 
     // try __getattr__
     if (attr != MP_QSTR___getattr__) {
+        #if MICROPY_PY_DELATTR_SETATTR
+        // If the requested attr is __setattr__/__delattr__ then don't delegate the lookup
+        // to __getattr__.  If we followed CPython's behaviour then __setattr__/__delattr__
+        // would have already been found in the "object" base class.
+        if (attr == MP_QSTR___setattr__ || attr == MP_QSTR___delattr__) {
+            return;
+        }
+        #endif
+
         mp_obj_t dest2[3];
         mp_load_method_maybe(self_in, MP_QSTR___getattr__, dest2);
         if (dest2[0] != MP_OBJ_NULL) {
@@ -626,10 +635,35 @@ STATIC bool mp_obj_instance_store_attr(mp_obj_t self_in, qstr attr, mp_obj_t val
 
     if (value == MP_OBJ_NULL) {
         // delete attribute
+        #if MICROPY_PY_DELATTR_SETATTR
+        // try __delattr__ first
+        mp_obj_t attr_delattr_method[3];
+        mp_load_method_maybe(self_in, MP_QSTR___delattr__, attr_delattr_method);
+        if (attr_delattr_method[0] != MP_OBJ_NULL) {
+            // __delattr__ exists, so call it
+            attr_delattr_method[2] = MP_OBJ_NEW_QSTR(attr);
+            mp_call_method_n_kw(1, 0, attr_delattr_method);
+            return true;
+        }
+        #endif
+
         mp_map_elem_t *elem = mp_map_lookup(&self->members, MP_OBJ_NEW_QSTR(attr), MP_MAP_LOOKUP_REMOVE_IF_FOUND);
         return elem != NULL;
     } else {
         // store attribute
+        #if MICROPY_PY_DELATTR_SETATTR
+        // try __setattr__ first
+        mp_obj_t attr_setattr_method[4];
+        mp_load_method_maybe(self_in, MP_QSTR___setattr__, attr_setattr_method);
+        if (attr_setattr_method[0] != MP_OBJ_NULL) {
+            // __setattr__ exists, so call it
+            attr_setattr_method[2] = MP_OBJ_NEW_QSTR(attr);
+            attr_setattr_method[3] = value;
+            mp_call_method_n_kw(2, 0, attr_setattr_method);
+            return true;
+        }
+        #endif
+
         mp_map_lookup(&self->members, MP_OBJ_NEW_QSTR(attr), MP_MAP_LOOKUP_ADD_IF_NOT_FOUND)->value = value;
         return true;
     }

tests/basics/class_delattr_setattr.py

@@ -0,0 +1,63 @@
+# test __delattr__ and __setattr__
+
+# feature test for __setattr__/__delattr__
+try:
+    class Test():
+        def __delattr__(self, attr): pass
+    del Test().noexist
+except AttributeError:
+    import sys
+    print('SKIP')
+    sys.exit()
+
+# this class just prints the calls to see if they were executed
+class A():
+    def __getattr__(self, attr):
+        print('get', attr)
+        return 1
+    def __setattr__(self, attr, val):
+        print('set', attr, val)
+    def __delattr__(self, attr):
+        print('del', attr)
+a = A()
+
+# check basic behaviour
+print(getattr(a, 'foo'))
+setattr(a, 'bar', 2)
+delattr(a, 'baz')
+
+# check meta behaviour
+getattr(a, '__getattr__') # should not call A.__getattr__
+getattr(a, '__setattr__') # should not call A.__getattr__
+getattr(a, '__delattr__') # should not call A.__getattr__
+setattr(a, '__setattr__', 1) # should call A.__setattr__
+delattr(a, '__delattr__') # should call A.__delattr__
+
+# this class acts like a dictionary
+class B:
+    def __init__(self, d):
+        # store the dict in the class, not instance, so
+        # we don't get infinite recursion in __getattr_
+        B.d = d
+
+    def __getattr__(self, attr):
+        if attr in B.d:
+            return B.d[attr]
+        else:
+            raise AttributeError(attr)
+
+    def __setattr__(self, attr, value):
+        B.d[attr] = value
+
+    def __delattr__(self, attr):
+        del B.d[attr]
+
+a = B({"a":1, "b":2})
+print(a.a, a.b)
+a.a = 3
+print(a.a, a.b)
+del a.a
+try:
+    print(a.a)
+except AttributeError:
+    print("AttributeError")

unix/mpconfigport_coverage.h

@@ -32,6 +32,7 @@
 
 #include <mpconfigport.h>
 
+#define MICROPY_PY_DELATTR_SETATTR     (1)
 #define MICROPY_PY_BUILTINS_HELP       (1)
 #define MICROPY_PY_BUILTINS_HELP_MODULES (1)
 #define MICROPY_PY_URANDOM_EXTRA_FUNCS (1)