Make deep copy of fileds from class attr to instance attr (#550)

* Make deep copy of fileds from class attr to instance attr

The fields resource class attribute is the *class-wide* definition of
fields. Because a particular *instance* of the class might want to
alter self.fields, we create self.fields here by copying cls.fields.
Instances should always modify self.fields; they should not modify
cls.fields.

* Update test for resource fields

Test checks that resource instance *fields* attr doesn't refer to attr *fields* of resource class

import_export/resources.py

@@ -192,6 +192,14 @@ class Resource(six.with_metaclass(DeclarativeMetaclass)):
     representations and handle importing and exporting data.
     """
 
+    def __init__(self):
+        # The fields class attribute is the *class-wide* definition of
+        # fields. Because a particular *instance* of the class might want to
+        # alter self.fields, we create self.fields here by copying cls.fields.
+        # Instances should always modify self.fields; they should not modify
+        # cls.fields.
+        self.fields = deepcopy(self.fields)
+
     @classmethod
     def get_result_class(self):
         """
@@ -233,16 +241,15 @@ class Resource(six.with_metaclass(DeclarativeMetaclass)):
         """
         return [self.fields[f] for f in self.get_export_order()]
 
-    @classmethod
-    def get_field_name(cls, field):
+    def get_field_name(self, field):
         """
         Returns the field name for a given field.
         """
-        for field_name, f in cls.fields.items():
+        for field_name, f in self.fields.items():
             if f == field:
                 return field_name
         raise AttributeError("Field %s does not exists in %s resource" % (
-            field, cls))
+            field, self.__class__))
 
     def init_instance(self, row=None):
         raise NotImplementedError()

tests/core/tests/resources_tests.py

@@ -24,6 +24,11 @@ from ..models import (
     WithFloatField,
 )
 
+try:
+    from collections import OrderedDict
+except ImportError:
+    from django.utils.datastructures import SortedDict as OrderedDict
+
 try:
     from django.utils.encoding import force_text
 except ImportError:
@@ -45,8 +50,35 @@ class ResourceTestCase(TestCase):
         self.my_resource = MyResource()
 
     def test_fields(self):
-        fields = self.my_resource.fields
-        self.assertIn('name', fields)
+        """Check that fields were determined correctly """
+
+        # check that our fields were determined
+        self.assertIn('name', self.my_resource.fields)
+
+        # check that resource instance fields attr isn't link to resource cls
+        # fields
+        self.assertFalse(
+            MyResource.fields is self.my_resource.fields
+        )
+
+        # dynamically add new resource field into resource instance
+        self.my_resource.fields.update(
+            OrderedDict([
+                ('new_field', fields.Field()),
+            ])
+        )
+
+        # check that new field in resource instance fields
+        self.assertIn(
+            'new_field',
+            self.my_resource.fields
+        )
+
+        # check that new field not in resource cls fields
+        self.assertNotIn(
+            'new_field',
+            MyResource.fields
+        )
 
     def test_field_column_name(self):
         field = self.my_resource.fields['name']