diff --git a/setup/__init__.py b/setup/__init__.py
index 922e8b7197..21b2b28468 100644
--- a/setup/__init__.py
+++ b/setup/__init__.py
@@ -32,9 +32,9 @@ _cache_dir_built = False
 
 
 def newer(targets, sources):
-    if isinstance(targets, basestring):
+    if hasattr(targets, 'rjust'):
         targets = [targets]
-    if isinstance(sources, basestring):
+    if hasattr(sources, 'rjust'):
         sources = [sources]
     for f in targets:
         if not os.path.exists(f):
diff --git a/setup/build.py b/setup/build.py
index 18bf304d21..6b5ee5216c 100644
--- a/setup/build.py
+++ b/setup/build.py
@@ -73,7 +73,7 @@ def expand_file_list(items, is_paths=True):
     for item in items:
         if item.startswith('!'):
             item = lazy_load(item)
-            if isinstance(item, basestring):
+            if hasattr(item, 'rjust'):
                 item = [item]
             ans.extend(expand_file_list(item, is_paths=is_paths))
         else: