support FEED_EXPORT_FIELDS=[]

docs/topics/feed-exports.rst

@@ -236,17 +236,20 @@ The serialization format to be used for the feed. See
 FEED_EXPORT_FIELDS
 ------------------
 
+Default: ``None``
+
 A list of fields to export, optional.
 Example: ``FEED_EXPORT_FIELDS = ["foo", "bar", "baz"]``.
 
 Use FEED_EXPORT_FIELDS option to define fields to export and their order.
 
-When omitted or empty, Scrapy uses fields defined in :class:`~.Item` subclasses
-a spider is yielding. If raw dicts are used as items, FEED_EXPORT_FIELDS
-is omitted and an exporter requires a fixed set of fields (this is the case
-for :ref:`CSV <topics-feed-format-csv>` export format) then Scrapy tries
-to infer field names from the exported data - currently it uses field names
-from the first item.
+When FEED_EXPORT_FIELDS is None (default), Scrapy uses fields
+defined in :class:`~.Item` subclasses a spider is yielding.
+If FEED_EXPORT_FIELDS is None, raw dicts are used as items and
+an exporter requires a fixed set of fields (this is the case for
+:ref:`CSV <topics-feed-format-csv>` export format), then
+Scrapy tries to infer field names from the exported data - currently it
+uses field names from the first item.
 
 .. setting:: FEED_STORE_EMPTY
 

scrapy/extensions/feedexport.py

@@ -151,7 +151,12 @@ class FeedExporter(object):
         if not self._exporter_supported(self.format):
             raise NotConfigured
         self.store_empty = settings.getbool('FEED_STORE_EMPTY')
-        self.export_fields = settings.getlist('FEED_EXPORT_FIELDS') or None
+
+        if settings.get('FEED_EXPORT_FIELDS') is None:
+            self.export_fields = None  # don't promote None to []
+        else:
+            self.export_fields = settings.getlist('FEED_EXPORT_FIELDS')
+
         uripar = settings['FEED_URI_PARAMS']
         self._uripar = load_object(uripar) if uripar else lambda x, y: None
 

tests/test_feedexport.py

@@ -246,6 +246,10 @@ class FeedExportTest(unittest.TestCase):
         yield self.assertExported(items, header, rows,
                                   settings=settings, ordered=True)
 
+        # edge case: FEED_EXPORT_FIELDS==[] means nothing is exported
+        settings = {'FEED_EXPORT_FIELDS': []}
+        yield self.assertExportedJsonLines(items, [{},{},{},{}], settings)
+
     @defer.inlineCallbacks
     def test_export_dicts(self):
         # When dicts are used, only keys from the first row are used as