Lazy loading of fields and validators. Closes #1963.

rest_framework/fields.py

@@ -143,7 +143,7 @@ class Field(object):
     def __init__(self, read_only=False, write_only=False,
                  required=None, default=empty, initial=empty, source=None,
                  label=None, help_text=None, style=None,
-                 error_messages=None, validators=[], allow_null=False):
+                 error_messages=None, validators=None, allow_null=False):
         self._creation_counter = Field._creation_counter
         Field._creation_counter += 1
 
@@ -166,9 +166,11 @@ class Field(object):
         self.label = label
         self.help_text = help_text
         self.style = {} if style is None else style
-        self.validators = validators[:] or self.default_validators[:]
         self.allow_null = allow_null
 
+        if validators is not None:
+            self.validators = validators[:]
+
         # These are set up by `.bind()` when the field is added to a serializer.
         self.field_name = None
         self.parent = None
@@ -214,6 +216,21 @@ class Field(object):
         else:
             self.source_attrs = self.source.split('.')
 
+    # .validators is a lazily loaded property, that gets its default
+    # value from `get_validators`.
+    @property
+    def validators(self):
+        if not hasattr(self, '_validators'):
+            self._validators = self.get_validators()
+        return self._validators
+
+    @validators.setter
+    def validators(self, validators):
+        self._validators = validators
+
+    def get_validators(self):
+        return self.default_validators[:]
+
     def get_initial(self):
         """
         Return a value to use when the field is being returned as a primitive

rest_framework/serializers.py

@@ -282,21 +282,23 @@ class SerializerMetaclass(type):
 
 @six.add_metaclass(SerializerMetaclass)
 class Serializer(BaseSerializer):
-    def __init__(self, *args, **kwargs):
-        super(Serializer, self).__init__(*args, **kwargs)
-
+    @property
+    def fields(self):
+        if not hasattr(self, '_fields'):
+            self._fields = BindingDict(self)
+            for key, value in self.get_fields().items():
+                self._fields[key] = value
+        return self._fields
+
+    def get_fields(self):
         # Every new serializer is created with a clone of the field instances.
         # This allows users to dynamically modify the fields on a serializer
         # instance without affecting every other serializer class.
-        self.fields = BindingDict(self)
-        for key, value in self._get_base_fields().items():
-            self.fields[key] = value
-
-        self.validators = getattr(getattr(self, 'Meta', None), 'validators', [])
-
-    def _get_base_fields(self):
         return copy.deepcopy(self._declared_fields)
 
+    def get_validators(self):
+        return getattr(getattr(self, 'Meta', None), 'validators', [])
+
     def get_initial(self):
         if self._initial_data is not None:
             return ReturnDict([
@@ -520,14 +522,6 @@ class ModelSerializer(Serializer):
     })
     _related_class = PrimaryKeyRelatedField
 
-    def __init__(self, *args, **kwargs):
-        super(ModelSerializer, self).__init__(*args, **kwargs)
-        if 'validators' not in kwargs:
-            validators = self.get_default_validators()
-            if validators:
-                self.validators.extend(validators)
-                self._kwargs['validators'] = validators
-
     def create(self, validated_attrs):
         # Check that the user isn't trying to handle a writable nested field.
         # If we don't do this explicitly they'd likely get a confusing
@@ -578,13 +572,13 @@ class ModelSerializer(Serializer):
         instance.save()
         return instance
 
-    def get_default_validators(self):
+    def get_validators(self):
         field_names = set([
             field.source for field in self.fields.values()
             if (field.source != '*') and ('.' not in field.source)
         ])
 
-        validators = []
+        validators = getattr(getattr(self, 'Meta', None), 'validators', [])
         model_class = self.Meta.model
 
         # Note that we make sure to check `unique_together` both on the
@@ -627,7 +621,7 @@ class ModelSerializer(Serializer):
 
         return validators
 
-    def _get_base_fields(self):
+    def get_fields(self):
         declared_fields = copy.deepcopy(self._declared_fields)
 
         ret = SortedDict()

rest_framework/utils/representation.py

@@ -82,6 +82,11 @@ def serializer_repr(serializer, indent, force_many=None):
             ret += field_repr(field.child_relation, force_many=field.child_relation)
         else:
             ret += field_repr(field)
+
+    if serializer.validators:
+        ret += '\n' + indent_str + 'class Meta:'
+        ret += '\n' + indent_str + '    validators = ' + smart_repr(serializer.validators)
+
     return ret
 
 

tests/test_fields.py

@@ -85,7 +85,7 @@ class TestSource:
         class ExampleSerializer(serializers.Serializer):
             example_field = serializers.CharField(source='example_field')
         with pytest.raises(AssertionError) as exc_info:
-            ExampleSerializer()
+            ExampleSerializer().fields
         assert str(exc_info.value) == (
             "It is redundant to specify `source='example_field'` on field "
             "'CharField' in serializer 'ExampleSerializer', because it is the "
@@ -1018,7 +1018,7 @@ class TestSerializerMethodField:
             example_field = serializers.SerializerMethodField('get_example_field')
 
         with pytest.raises(AssertionError) as exc_info:
-            ExampleSerializer()
+            ExampleSerializer().fields
         assert str(exc_info.value) == (
             "It is redundant to specify `get_example_field` on "
             "SerializerMethodField 'example_field' in serializer "

tests/test_model_serializer.py

@@ -181,7 +181,7 @@ class TestRegularFieldMappings(TestCase):
                 fields = ('auto_field', 'invalid')
 
         with self.assertRaises(ImproperlyConfigured) as excinfo:
-            TestSerializer()
+            TestSerializer().fields
         expected = 'Field name `invalid` is not valid for model `ModelBase`.'
         assert str(excinfo.exception) == expected
 
@@ -198,7 +198,7 @@ class TestRegularFieldMappings(TestCase):
                 fields = ('auto_field',)
 
         with self.assertRaises(ImproperlyConfigured) as excinfo:
-            TestSerializer()
+            TestSerializer().fields
         expected = (
             'Field `missing` has been declared on serializer '
             '`TestSerializer`, but is missing from `Meta.fields`.'

tests/test_validators.py

@@ -86,10 +86,12 @@ class TestUniquenessTogetherValidation(TestCase):
     def test_repr(self):
         serializer = UniquenessTogetherSerializer()
         expected = dedent("""
-            UniquenessTogetherSerializer(validators=[<UniqueTogetherValidator(queryset=UniquenessTogetherModel.objects.all(), fields=('race_name', 'position'))>]):
+            UniquenessTogetherSerializer():
                 id = IntegerField(label='ID', read_only=True)
                 race_name = CharField(max_length=100)
                 position = IntegerField()
+                class Meta:
+                    validators = [<UniqueTogetherValidator(queryset=UniquenessTogetherModel.objects.all(), fields=('race_name', 'position'))>]
         """)
         assert repr(serializer) == expected
 
@@ -173,10 +175,12 @@ class TestUniquenessForDateValidation(TestCase):
     def test_repr(self):
         serializer = UniqueForDateSerializer()
         expected = dedent("""
-            UniqueForDateSerializer(validators=[<UniqueForDateValidator(queryset=UniqueForDateModel.objects.all(), field='slug', date_field='published')>]):
+            UniqueForDateSerializer():
                 id = IntegerField(label='ID', read_only=True)
                 slug = CharField(max_length=100)
                 published = DateField(required=True)
+                class Meta:
+                    validators = [<UniqueForDateValidator(queryset=UniqueForDateModel.objects.all(), field='slug', date_field='published')>]
         """)
         assert repr(serializer) == expected
 
@@ -231,10 +235,12 @@ class TestHiddenFieldUniquenessForDateValidation(TestCase):
 
         serializer = TestSerializer()
         expected = dedent("""
-            TestSerializer(validators=[<UniqueForDateValidator(queryset=HiddenFieldUniqueForDateModel.objects.all(), field='slug', date_field='published')>]):
+            TestSerializer():
                 id = IntegerField(label='ID', read_only=True)
                 slug = CharField(max_length=100)
                 published = HiddenField(default=CreateOnlyDefault(<function now>))
+                class Meta:
+                    validators = [<UniqueForDateValidator(queryset=HiddenFieldUniqueForDateModel.objects.all(), field='slug', date_field='published')>]
         """)
         assert repr(serializer) == expected
 
@@ -246,9 +252,11 @@ class TestHiddenFieldUniquenessForDateValidation(TestCase):
 
         serializer = TestSerializer()
         expected = dedent("""
-            TestSerializer(validators=[<UniqueForDateValidator(queryset=HiddenFieldUniqueForDateModel.objects.all(), field='slug', date_field='published')>]):
+            TestSerializer():
                 id = IntegerField(label='ID', read_only=True)
                 slug = CharField(max_length=100)
                 published = DateTimeField(default=CreateOnlyDefault(<function now>), read_only=True)
+                class Meta:
+                    validators = [<UniqueForDateValidator(queryset=HiddenFieldUniqueForDateModel.objects.all(), field='slug', date_field='published')>]
         """)
         assert repr(serializer) == expected