Warn if user checks is_stored for plugin not always saved

strax/context.py

@@ -1981,7 +1981,7 @@ def run_defaults(self, run_id):
         self._run_defaults_cache[run_id] = defs
         return defs
 
-    def is_stored(self, run_id, target, **kwargs):
+    def is_stored(self, run_id, target, detailed=True, **kwargs):
         """Return whether data type target has been saved for run_id through any of the registered
         storage frontends.
 
@@ -2003,6 +2003,31 @@ def is_stored(self, run_id, target, **kwargs):
             if self._is_stored_in_sf(run_id, target, sf):
                 return True
         # None of the frontends has the data
+
+        # Before returning False, check if the data can be made trivially
+        plugin = self._plugin_class_registry[target]
+        save_when = plugin.save_when
+
+        # Mutli-target plugins provide a save_when per target
+        if isinstance(save_when, immutabledict):
+            save_when = save_when[target]
+
+        if save_when < strax.SaveWhen.ALWAYS and detailed:
+            msg = (
+                f"{target} is not set to always be saved. "
+                "This is probably because it can be trivially made from other data. "
+            )
+
+            try:
+                components = self.get_components(run_id, target)
+                targets_plugin_made_from = tuple(components.loaders.keys())
+                msg += f"{target} can be made from: {targets_plugin_made_from}."
+            except strax.DataNotAvailable as e:
+                # Warn that data cannot be made
+                msg += str(e)
+
+            warnings.warn(msg)
+
         return False
 
     def _check_forbidden(self):

tests/test_superruns.py

@@ -251,17 +251,17 @@ def test_superrun_triggers_subrun_processing(self):
 
     def test_superruns_and_save_when(self):
         """Tests if only the highest level for save_when.EXPLICIT plugins is stored."""
-        assert not self.context.is_stored(self.superrun_name, "peaks")
-        assert not self.context.is_stored(self.subrun_ids[0], "peaks")
-        assert not self.context.is_stored(self.subrun_ids[0], "peaks_extension")
-        assert not self.context.is_stored(self.superrun_name, "peaks_extension")
+        assert not self.context.is_stored(self.superrun_name, "peaks", detailed=False)
+        assert not self.context.is_stored(self.subrun_ids[0], "peaks", detailed=False)
+        assert not self.context.is_stored(self.subrun_ids[0], "peaks_extension", detailed=False)
+        assert not self.context.is_stored(self.superrun_name, "peaks_extension", detailed=False)
         self.context._plugin_class_registry["peaks"].save_when = strax.SaveWhen.EXPLICIT
 
         self.context.make(self.superrun_name, "peaks_extension", save=("peaks_extension",))
-        assert not self.context.is_stored(self.superrun_name, "peaks")
-        assert not self.context.is_stored(self.subrun_ids[0], "peaks")
-        assert self.context.is_stored(self.superrun_name, "peaks_extension")
-        assert self.context.is_stored(self.subrun_ids[0], "peaks_extension")
+        assert not self.context.is_stored(self.superrun_name, "peaks", detailed=False)
+        assert not self.context.is_stored(self.subrun_ids[0], "peaks", detailed=False)
+        assert self.context.is_stored(self.superrun_name, "peaks_extension", detailed=False)
+        assert self.context.is_stored(self.subrun_ids[0], "peaks_extension", detailed=False)
 
     def test_storing_with_second_sf(self):
         """Tests if only superrun is written to new sf if subruns already exist in different sf."""