drop support for FEED_EXPORT_FIELD=[] meaning "no fields"

docs/topics/feed-exports.rst

@@ -243,13 +243,13 @@ Example: ``FEED_EXPORT_FIELDS = ["foo", "bar", "baz"]``.
 
 Use FEED_EXPORT_FIELDS option to define fields to export and their order.
 
-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.
+When FEED_EXPORT_FIELDS is empty or None (default), Scrapy uses fields
+defined in dicts or :class:`~.Item` subclasses a spider is yielding.
+
+If an exporter requires a fixed set of fields (this is the case for
+:ref:`CSV <topics-feed-format-csv>` export format) and FEED_EXPORT_FIELDS
+is empty or None, 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

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

tests/test_feedexport.py

@@ -234,7 +234,12 @@ class FeedExportTest(unittest.TestCase):
         yield self.assertExportedCsv(items, header, rows_csv, ordered=False)
         yield self.assertExportedJsonLines(items, rows_jl)
 
-        # but it is possible to override fields using FEED_EXPORT_FIELDS
+        # edge case: FEED_EXPORT_FIELDS==[] means the same as default None
+        settings = {'FEED_EXPORT_FIELDS': []}
+        yield self.assertExportedCsv(items, header, rows_csv, ordered=False)
+        yield self.assertExportedJsonLines(items, rows_jl, settings)
+
+        # it is possible to override fields using FEED_EXPORT_FIELDS
         header = ["foo", "baz", "hello"]
         settings = {'FEED_EXPORT_FIELDS': header}
         rows = [
@@ -246,10 +251,6 @@ 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