diff --git a/scrapy/utils/python.py b/scrapy/utils/python.py
index 20d4e6b5e..834773072 100644
--- a/scrapy/utils/python.py
+++ b/scrapy/utils/python.py
@@ -53,13 +53,13 @@ def flatten(x):
 
 def unique(list_, key=lambda x: x):
     """efficient function to uniquify a list preserving item order"""
-    seen = {}
+    seen = set()
     result = []
     for item in list_:
         seenkey = key(item)
-        if seenkey in seen: 
+        if seenkey in seen:
             continue
-        seen[seenkey] = 1
+        seen.add(seenkey)
         result.append(item)
     return result
 
@@ -70,7 +70,7 @@ def str_to_unicode(text, encoding=None, errors='strict'):
     object without the risk of double-decoding problems (which can happen if
     you don't use the default 'ascii' encoding)
     """
-    
+
     if encoding is None:
         encoding = 'utf-8'
     if isinstance(text, str):