core: improvements for warnings, twitter/rss: try using @warn_if_empty

my/twitter/all.py

@@ -3,11 +3,9 @@ Unified Twitter data (merged from the archive and periodic updates)
 """
 
 # NOTE: you can comment out the sources you don't need
-
-
 from . import twint, archive
-from .common import merge_tweets
 
+from .common import merge_tweets
 
 def tweets():
     yield from merge_tweets(
@@ -15,6 +13,7 @@ def tweets():
         archive.tweets(),
     )
 
+from .common import merge_tweets
 
 def likes():
     yield from merge_tweets(

my/twitter/common.py

@@ -2,7 +2,9 @@ from itertools import chain
 
 from more_itertools import unique_everseen
 
+from ..core import warn_if_empty
 
+@warn_if_empty
 def merge_tweets(*sources):
     yield from unique_everseen(
         chain(*sources),