Fast quit IROC reader on empty tag-list

Earlier the IROC reader started crawling the whole file-system
looking for no tags.

This closes #149.

gordo_components/data_provider/iroc_reader.py

@@ -40,6 +40,14 @@ def load_dataframes(
         See GordoBaseDataProvider for documentation
         """
 
+        if not tag_list:
+            logger.warning("Iroc reader called with empty tag_list, returning none")
+            return
+        if to_ts < from_ts:
+            raise ValueError(
+                f"Iroc reader called with to_ts: {to_ts} before from_ts: {from_ts}"
+            )
+
         base_path = base_path.strip("/")
 
         # We query with an extra day on both sides since the way the files are

gordo_components/data_provider/ncs_reader.py

@@ -55,6 +55,10 @@ def load_dataframes(
         """
         See GordoBaseDataProvider for documentation
         """
+        if to_ts < from_ts:
+            raise ValueError(
+                f"NCS reader called with to_ts: {to_ts} before from_ts: {from_ts}"
+            )
         adls_file_system_client = self.client
 
         years = range(from_ts.year, to_ts.year + 1)

gordo_components/data_provider/providers.py

@@ -45,17 +45,20 @@ def load_dataframes_from_multiple_providers(
         for tag_reader in data_providers:
             if tag_reader.can_handle_tag(tag):
                 readers_to_tags[tag_reader].append(tag)
+                logger.info(f"Assigning tag: {tag} to reader {tag_reader}")
                 # In case of a tag matching two readers, we let the "first"
                 # one handle it
                 break
         # The else branch is executed if the break is not called
         else:
             raise ValueError(f"Found no data providers able to download the tag {tag}")
     for tag_reader, readers_tags in readers_to_tags.items():
-        for df in tag_reader.load_dataframes(
-            from_ts=from_ts, to_ts=to_ts, tag_list=readers_tags
-        ):
-            yield df
+        if readers_tags:
+            logger.info(f"Using tag reader {tag_reader} to fetch tags {readers_tags}")
+            for df in tag_reader.load_dataframes(
+                from_ts=from_ts, to_ts=to_ts, tag_list=readers_tags
+            ):
+                yield df
 
 
 class DataLakeProvider(GordoBaseDataProvider):
@@ -113,6 +116,10 @@ def load_dataframes(
         """
         # We create them here so we only try to get a auth-token once we actually need
         # it, otherwise we would have constructed them in the constructor.
+        if to_ts < from_ts:
+            raise ValueError(
+                f"DataLakeReader called with to_ts: {to_ts} before from_ts: {from_ts}"
+            )
         data_providers = self._get_sub_dataproviders()
 
         yield from load_dataframes_from_multiple_providers(

tests/test_data_provider_iroc.py

@@ -92,6 +92,30 @@ def test_load_dataframes_no_data(self, _mocked_method):
                 )
             )
 
+    def test_load_dataframes_no_tag_list(self):
+        """load_dataframe will return an empty generator when called with no tags"""
+        iroc_reader = IrocReader(client=None, threads=1)
+        res = list(
+            iroc_reader.load_dataframes(
+                from_ts=isoparse("2018-05-02T01:56:00+00:00"),
+                to_ts=isoparse("2018-05-03T01:56:00+00:00"),
+                tag_list=[],
+            )
+        )
+        self.assertEqual([], res)
+
+    def test_load_dataframes_checks_date(self):
+        """load_dataframe will raise ValueError if to_ts<from_ts"""
+        iroc_reader = IrocReader(client=None, threads=1)
+        with self.assertRaises(ValueError):
+            list(
+                iroc_reader.load_dataframes(
+                    from_ts=isoparse("2018-05-03T01:56:00+00:00"),
+                    to_ts=isoparse("2018-05-02T01:56:00+00:00"),
+                    tag_list=["jalla"],  # Not a tag in the input
+                )
+            )
+
     @mock.patch.object(
         IrocReader,
         "_fetch_all_iroc_files_from_paths",