Make _ext_getattr handle a default, and add _ext_replacement_getattr …

CHANGES.rst

@@ -11,6 +11,10 @@
   Document the intended API, ``_ext_replacement()``. See `issue 73
   <https://github.com/NextThought/nti.externalization/issues/73>`_.
 
+- Make ``AbstractDynamicObjectIO._ext_getattr`` handle a default
+  value, and add ``_ext_replacement_getattr``  for when it will only
+  be called once. See `issue 73
+  <https://github.com/NextThought/nti.externalization/issues/73>`_.
 
 1.0.0a3 (2018-07-28)
 ====================

src/nti/externalization/_datastructures.pxd

@@ -42,7 +42,8 @@ cdef class AbstractDynamicObjectIO(ExternalizableDictionaryMixin):
 
     cpdef _ext_all_possible_keys(self)
     cpdef _ext_setattr(self, ext_self, k, value)
-    cpdef _ext_getattr(self, ext_self, k)
+    cpdef _ext_getattr(self, ext_self, k, default=*)
+    cpdef _ext_replacement_getattr(self, k, default=*)
 
     @cython.locals(
         k=str # cython can optimize k.startswith('constantstring')

src/nti/externalization/datastructures.py

@@ -159,13 +159,28 @@ def _ext_all_possible_keys(self):
     def _ext_setattr(self, ext_self, k, value):
         raise NotImplementedError()
 
-    def _ext_getattr(self, ext_self, k):
+    def _ext_getattr(self, ext_self, k, default=NotGiven):
         """
-        Return the attribute of the `ext_self` object with the external name `k`.
-        If the attribute does not exist, should raise (typically :exc:`AttributeError`)
+        _ext_getattr(object, name[, default]) -> value
+
+        Return the attribute of the *ext_self* object with the internal name *name*.
+        If the attribute does not exist, should raise (typically :exc:`AttributeError`),
+        unless *default* is given, in which case it returns that.
+
+        .. versionchanged:: 1.0a4
+            Add the *default* argument.
         """
         raise NotImplementedError()
 
+    def _ext_replacement_getattr(self, name, default=NotGiven):
+        """
+        Like `_ext_getattr`, but automatically fills in `_ext_replacement`
+        for the *ext_self* argument.
+
+        .. versionadded:: 1.0a4
+        """
+        return self._ext_getattr(self._ext_replacement(), name, default)
+
     def _ext_keys(self):
         """
         Return only the names of attributes that should be externalized.
@@ -320,11 +335,13 @@ class ExternalizableInstanceDict(AbstractDynamicObjectIO):
     def _ext_all_possible_keys(self):
         return frozenset(self._ext_replacement().__dict__.keys())
 
-    def _ext_getattr(self, ext_self, k):
-        return getattr(ext_self, k)
+    def _ext_getattr(self, ext_self, k, default=NotGiven):
+        if default is NotGiven:
+            return getattr(ext_self, k)
+        return getattr(ext_self, k, default)
 
-    def _ext_setattr(self, ext_self, k, v):
-        setattr(ext_self, k, v)
+    def _ext_setattr(self, ext_self, k, value):
+        setattr(ext_self, k, value)
 
     def _ext_accept_update_key(self, k, ext_self, ext_keys):
         return (
@@ -450,9 +467,11 @@ def _ext_all_possible_keys(self):
             ])
         return cache.ext_all_possible_keys
 
-    def _ext_getattr(self, ext_self, k):
+    def _ext_getattr(self, ext_self, k, default=NotGiven):
         # TODO: Should this be directed through IField.get?
-        return getattr(ext_self, k)
+        if default is NotGiven:
+            return getattr(ext_self, k)
+        return getattr(ext_self, k, default)
 
     def _ext_setattr(self, ext_self, k, value):
         validate_named_field_value(ext_self, self._iface, k, value)()

src/nti/externalization/tests/test_datastructures.py

@@ -28,9 +28,31 @@
 # pylint: disable=inherit-non-class,attribute-defined-outside-init,abstract-method
 # pylint:disable=no-value-for-parameter,too-many-function-args
 
-class TestAbstractDynamicObjectIO(ExternalizationLayerTest):
+class CommonTestMixins(object):
 
-    def _makeOne(self):
+    def _makeOne(self, context=None):
+        raise NotImplementedError()
+
+    def test_ext_getattr_default(self):
+        io = self._makeOne()
+        assert_that(io._ext_getattr(self, 'no_such_attribute', None),
+                    is_(none()))
+
+    def test_ext_getattr_no_default(self):
+        io = self._makeOne()
+        get = io._ext_getattr
+        with self.assertRaises(AttributeError):
+            get(self, 'no_such_attribute')
+
+    def test_ext_replacement_getattr_default(self):
+        io = self._makeOne()
+        assert_that(io._ext_replacement_getattr('no_such_attribute', None),
+                    is_(none()))
+
+class TestAbstractDynamicObjectIO(CommonTestMixins,
+                                  ExternalizationLayerTest):
+
+    def _makeOne(self, context=None):
         from nti.externalization.datastructures import AbstractDynamicObjectIO
         class IO(AbstractDynamicObjectIO):
             _ext_setattr = staticmethod(setattr)
@@ -139,14 +161,15 @@ def test_update_takes_external_fields(self):
         assert_that(inst, has_property('id', 'id'))
 
 class TestInterfaceObjectIO(CleanUp,
+                            CommonTestMixins,
                             unittest.TestCase):
 
     def _getTargetClass(self):
         from nti.externalization.datastructures import InterfaceObjectIO
         return InterfaceObjectIO
 
-    def _makeOne(self, ext_self, *args, **kwargs):
-        return self._getTargetClass()(ext_self, *args, **kwargs)
+    def _makeOne(self, context=None, iface_upper_bound=interface.Interface):
+        return self._getTargetClass()(context, iface_upper_bound=iface_upper_bound)
 
     def test_find_primitive_keys_dne(self):
         ext_self = self
@@ -479,3 +502,15 @@ class Consistent(object):
 
         io = IO(Consistent())
         assert_that(io, has_property('_iface', IGrandChild))
+
+
+class TestExternalizableInstanceDict(CommonTestMixins,
+                                     unittest.TestCase):
+
+    def _makeOne(self, context=None):
+        from nti.externalization.datastructures import ExternalizableInstanceDict
+        class FUT(ExternalizableInstanceDict):
+            def _ext_replacement(self):
+                return context
+
+        return FUT()