diff -r cc27d50bd91a Lib/csv.py
--- a/Lib/csv.py	Fri Jun 14 23:04:02 2013 -0400
+++ b/Lib/csv.py	Sat Jun 15 11:19:03 2013 -0700
@@ -129,7 +129,8 @@
 class DictWriter:
     def __init__(self, f, fieldnames, restval="", extrasaction="raise",
                  dialect="excel", *args, **kwds):
-        self.fieldnames = fieldnames    # list of keys for the dict
+        self._fieldnames = tuple(fieldnames)    # list of keys for the dict
+        self._fieldset = frozenset(fieldnames)  # set for key lookup
         self.restval = restval          # for writing short dicts
         if extrasaction.lower() not in ("raise", "ignore"):
             raise ValueError("extrasaction (%s) must be 'raise' or 'ignore'"
@@ -137,13 +138,17 @@
         self.extrasaction = extrasaction
         self.writer = writer(f, dialect, *args, **kwds)
 
+    @property
+    def fieldnames(self):
+        return self._fieldnames
+        
     def writeheader(self):
         header = dict(zip(self.fieldnames, self.fieldnames))
         self.writerow(header)
 
     def _dict_to_list(self, rowdict):
         if self.extrasaction == "raise":
-            wrong_fields = [k for k in rowdict if k not in self.fieldnames]
+            wrong_fields = [k for k in rowdict if k not in self._fieldset]
             if wrong_fields:
                 raise ValueError("dict contains fields not in fieldnames: "
                                  + ", ".join(wrong_fields))