Serializer field 'default' argument may be a callable

docs/api-guide/fields.md

@@ -41,7 +41,9 @@ Defaults to `True`.
 
 ### `default`
 
-If set, this gives the default value that will be used for the field if none is supplied.  If not set the default behavior is to not populate the attribute at all.
+If set, this gives the default value that will be used for the field if none is supplied.  If not set the default behavior is to not populate the attribute at all. 
+
+May be set to a function or other callable, in which case the value will be evaluated each time it is used.
 
 ### `validators`
 

docs/topics/release-notes.md

@@ -43,6 +43,7 @@ You can determine your currently installed version using `pip freeze`:
 ### Master
 
 * Added `get_url` hook to `HyperlinkedIdentityField`.
+* Serializer field `default` argument may be a callable.
 * Bugfix: The `lookup_field` option on `HyperlinkedIdentityField` should apply by default to the url field on the serializer.
 * Bugfix: `HyperlinkedIdentityField` should continue to support `pk_url_kwarg`, `slug_url_kwarg`, `slug_field`, in a pending deprecation state.
 * Bugfix: Ensure we always return 404 instead of 500 if a lookup field cannot be converted to the correct lookup type.  (Eg non-numeric `AutoInteger` pk lookup)

rest_framework/fields.py

@@ -250,9 +250,6 @@ class WritableField(Field):
         self.validators = self.default_validators + validators
         self.default = default if default is not None else self.default
 
-        if is_simple_callable(self.default):
-            self.default = self.default()
-
         # Widgets are ony used for HTML forms.
         widget = widget or self.widget
         if isinstance(widget, type):
@@ -298,7 +295,10 @@ class WritableField(Field):
         except KeyError:
             if self.default is not None and not self.partial:
                 # Note: partial updates shouldn't set defaults
-                native = self.default
+                if is_simple_callable(self.default):
+                    native = self.default()
+                else:
+                    native = self.default
             else:
                 if self.required:
                     raise ValidationError(self.error_messages['required'])

rest_framework/tests/test_fields.py

@@ -852,3 +852,17 @@ class FieldMetadata(TestCase):
     def test_label(self):
         for field in (self.required_field, self.optional_field):
             self.assertEqual(field.metadata()['label'], field.label)
+
+
+class FieldCallableDefault(TestCase):
+    def setUp(self):
+        self.simple_callable = lambda: 'foo bar'
+
+    def test_default_can_be_simple_callable(self):
+        """
+        Ensure that the 'default' argument can also be a simple callable.
+        """
+        field = serializers.WritableField(default=self.simple_callable)
+        into = {}
+        field.field_from_native({}, {}, 'field', into)
+        self.assertEquals(into, {'field': 'foo bar'})