fewer loops are good

src/sourmash/commands.py

@@ -59,16 +59,16 @@ def compare(args):
                                                        yield_all_files=args.force,
                                                        progress=progress,
                                                        pattern=pattern_search)
-        loaded = list(loaded)
-        if not loaded:
-            notify(f'\nwarning: no signatures loaded at given ksize/molecule type/picklist from {filename}')
-
         # add to siglist; track ksizes/moltypes
+        s = None
         for s in loaded:
             siglist.append((s, filename))
             ksizes.add(s.minhash.ksize)
             moltypes.add(sourmash_args.get_moltype(s))
 
+        if s is None:
+            notify(f'\nwarning: no signatures loaded at given ksize/molecule type/picklist from {filename}')
+
         # error out while loading if we have more than one ksize/moltype
         if len(ksizes) > 1 or len(moltypes) > 1:
             break
@@ -134,6 +134,10 @@ def compare(args):
         if track_abundances:
             notify('NOTE: --containment, --max-containment, --avg-containment, and --estimate-ani ignore signature abundances.')
 
+    # CTB: note, up to this point, we could do everything with manifests
+    # w/o actually loading any signatures. I'm not sure the manifest
+    # API allows it tho.
+
     # if using scaled sketches or --scaled, downsample to common max scaled.
     printed_scaled_msg = False
     if is_scaled: