diff -r a9a136c9d857 Doc/library/csv.rst
--- a/Doc/library/csv.rst	Fri Oct 21 12:32:46 2016 +0900
+++ b/Doc/library/csv.rst	Thu Oct 20 21:27:41 2016 -0700
@@ -195,7 +195,10 @@
    written if the dictionary is missing a key in *fieldnames*.  If the
    dictionary passed to the :meth:`writerow` method contains a key not found in
    *fieldnames*, the optional *extrasaction* parameter indicates what action to
-   take.  If it is set to ``'raise'`` a :exc:`ValueError` is raised.  If it is
+   take.
+   If it is set to ``'raise'``, the default value, a :exc:`ValueError`
+   is raised.
+   If it is
    set to ``'ignore'``, extra values in the dictionary are ignored.  Any other
    optional or keyword arguments are passed to the underlying :class:`writer`
    instance.
diff -r a9a136c9d857 Lib/csv.py
--- a/Lib/csv.py	Fri Oct 21 12:32:46 2016 +0900
+++ b/Lib/csv.py	Thu Oct 20 21:27:41 2016 -0700
@@ -131,7 +131,8 @@
 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 = fieldnames    # list of keys for the dict
+        self._fieldnames_set = set(self._fieldnames)
         self.restval = restval          # for writing short dicts
         if extrasaction.lower() not in ("raise", "ignore"):
             raise ValueError("extrasaction (%s) must be 'raise' or 'ignore'"
@@ -139,13 +140,22 @@
         self.extrasaction = extrasaction
         self.writer = writer(f, dialect, *args, **kwds)
 
+    @property
+    def fieldnames(self):
+        return self._fieldnames
+
+    @fieldnames.setter
+    def fieldnames(self, value):
+        self._fieldnames = value
+        self._fieldnames_set = set(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 = rowdict.keys() - self._fieldnames_set
             if wrong_fields:
                 raise ValueError("dict contains fields not in fieldnames: "
                                  + ", ".join([repr(x) for x in wrong_fields]))
diff -r a9a136c9d857 Lib/test/test_csv.py
--- a/Lib/test/test_csv.py	Fri Oct 21 12:32:46 2016 +0900
+++ b/Lib/test/test_csv.py	Thu Oct 20 21:27:41 2016 -0700
@@ -626,6 +626,31 @@
             self.assertNotIn("'f2'", exception)
             self.assertIn("1", exception)
 
+    def test_typo_in_extrasaction_raises_error(self):
+        fileobj = StringIO()
+        self.assertRaises(ValueError, csv.DictWriter, fileobj, ['f1', 'f2'], extrasaction="raised")
+
+    def test_uppercase_extrasaction_raises_error(self):
+        fileobj = StringIO()
+        csv
+        self.assertRaises(ValueError, csv.DictWriter, fileobj, ['f1', 'f2'], extrasaction="RAise")
+
+    def test_extrasaction_ignore(self):
+        fileobj = StringIO()
+        csv.DictWriter(fileobj, ['f1', 'f2'], extrasaction="ignore")
+
+    def test_write_field_not_in_field_names_raise(self):
+        fileobj = StringIO()
+        writer = csv.DictWriter(fileobj, ['f1','f2'], extrasaction="raise")
+        dictrow = {'f0':0, 'f1':1, 'f2':2, 'f3':3}
+        self.assertRaises(ValueError, csv.DictWriter.writerow, writer, dictrow)
+
+    def test_write_field_not_in_field_names_ignore(self):
+        fileobj = StringIO()
+        writer = csv.DictWriter(fileobj, ['f1','f2'], extrasaction="ignore")
+        dictrow = {'f0':0, 'f1':1, 'f2':2, 'f3':3}
+        csv.DictWriter.writerow(writer, dictrow)
+
     def test_read_dict_fields(self):
         with TemporaryFile("w+") as fileobj:
             fileobj.write("1,2,abc\r\n")