[formatter] do NOT apply :J to strings (#2833)

gallery_dl/formatter.py

@@ -296,12 +296,14 @@ def _parse_maxlen(format_spec, default):
 
 def _parse_join(format_spec, default):
     separator, _, format_spec = format_spec.partition(_SEPARATOR)
-    separator = separator[1:]
+    join = separator[1:].join
     fmt = _build_format_func(format_spec, default)
 
-    def join(obj):
-        return fmt(separator.join(obj))
-    return join
+    def apply_join(obj):
+        if isinstance(obj, str):
+            return fmt(obj)
+        return fmt(join(obj))
+    return apply_join
 
 
 def _parse_replace(format_spec, default):

test/test_formatter.py

@@ -177,7 +177,7 @@ class TestFormatter(unittest.TestCase):
         self._run_test("{l:J - />20}", "           a - b - c")
 
         self._run_test("{a:J/}"      , self.kwdict["a"])
-        self._run_test("{a:J, /}"    , ", ".join(self.kwdict["a"]))
+        self._run_test("{a:J, /}"    , self.kwdict["a"])
 
     def test_replace(self):
         self._run_test("{a:Rh/C/}"  , "CElLo wOrLd")