improve missing start_time error message

Author: alessandrofelder

src/silverlabnwb/nwb_file.py

@@ -125,7 +125,11 @@ def create_from_metadata(self, metadata_file, user=None, session_id=None):
         try:
             start_time = sessions[user]['start_time']
         except KeyError:
-            raise ValueError("Start time for session not found!")
+            raise ValueError("Start time for session not found! "
+                             "Please add a `start_time` parameter to sessions for user {}, "
+                             "e.g. '10 May 2020 12:34:56.789103'"
+                             .format(user)
+                             )
         start_time = pd.to_datetime(
             start_time, infer_datetime_format=True).tz_localize(
             timezone('Europe/London'))

tests/test_metadata_import.py

@@ -32,7 +32,10 @@ def test_no_start_time_fails(tmpdir, ref_data_dir):
     with pytest.raises(ValueError) as exc_info:
         with NwbFile(nwb_path, 'w') as nwb:
             nwb.create_from_metadata(meta_path, user="A")
-    assert "Start time for session not found!" == str(exc_info.value)
+    expected_error_string = "Start time for session not found! " \
+                            "Please add a `start_time` parameter to sessions for user A, " \
+                            "e.g. '10 May 2020 12:34:56.789103'"
+    assert expected_error_string == str(exc_info.value)
 
 
 def test_metadata_import_correct(tmpdir, ref_data_dir):