Fix #1537721: add writeheader() method to csv.DictWriter.

Reviewed by skip.montanaro and thomas.wouters.

2 files changed, 8 insertions, 0 deletions

diff --git a/Lib/csv.py b/Lib/csv.py
index 3db5dac..1df5062 100644
--- a/Lib/csv.py
+++ b/Lib/csv.py
@@ -132,6 +132,10 @@ class DictWriter:
         self.extrasaction = extrasaction
         self.writer = writer(f, dialect, *args, **kwds)
 
+    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]
diff --git a/Lib/test/test_csv.py b/Lib/test/test_csv.py
index 196d18c..7eef4c0 100644
--- a/Lib/test/test_csv.py
+++ b/Lib/test/test_csv.py
@@ -598,8 +598,12 @@ class TestDictFields(unittest.TestCase):
         fileobj = os.fdopen(fd, "w+b")
         try:
             writer = csv.DictWriter(fileobj, fieldnames = ["f1", "f2", "f3"])
+            writer.writeheader()
+            fileobj.seek(0)
+            self.assertEqual(fileobj.readline(), "f1,f2,f3\r\n")
             writer.writerow({"f1": 10, "f3": "abc"})
             fileobj.seek(0)
+            fileobj.readline() # header
             self.assertEqual(fileobj.read(), "10,,abc\r\n")
         finally:
             fileobj.close()