Fix reading metadata from filenames when the author regexp does not match anything

src/calibre/ebooks/metadata/__init__.py

@@ -22,6 +22,8 @@ except:
     _author_pat = re.compile(r'(?i),?\s+(and|with)\s+')
 
 def string_to_authors(raw):
+    if not raw:
+        return []
     raw = raw.replace('&&', u'\uffff')
     raw = _author_pat.sub('&', raw)
     authors = [a.strip().replace(u'\uffff', '&') for a in raw.split('&')]

src/calibre/ebooks/metadata/meta.py

@@ -149,19 +149,20 @@ def metadata_from_filename(name, pat=None):
         try:
             au = match.group('author')
             aus = string_to_authors(au)
-            mi.authors = aus
+            if aus:
-            if prefs['swap_author_names'] and mi.authors:
+                mi.authors = aus
-                def swap(a):
+                if prefs['swap_author_names'] and mi.authors:
-                    if ',' in a:
+                    def swap(a):
-                        parts = a.split(',', 1)
+                        if ',' in a:
-                    else:
+                            parts = a.split(',', 1)
-                        parts = a.split(None, 1)
+                        else:
-                    if len(parts) > 1:
+                            parts = a.split(None, 1)
-                        t = parts[-1]
+                        if len(parts) > 1:
-                        parts = parts[:-1]
+                            t = parts[-1]
-                        parts.insert(0, t)
+                            parts = parts[:-1]
-                    return ' '.join(parts)
+                            parts.insert(0, t)
-                mi.authors = [swap(x) for x in mi.authors]
+                        return ' '.join(parts)
+                    mi.authors = [swap(x) for x in mi.authors]
         except (IndexError, ValueError):
             pass
         try: