write header line by default when using csv exporter

--HG--
extra : rebase_source : 2d2d7153dde5e3f77e682e16d2e4408f732f234e

docs/topics/exporters.rst

@@ -244,7 +244,7 @@ XmlItemExporter
 CsvItemExporter
 ---------------
 
-.. class:: CsvItemExporter(file, include_headers_line=False, \**kwargs)
+.. class:: CsvItemExporter(file, include_headers_line=True, \**kwargs)
 
    Exports Items in CSV format to the given file-like object. If the
    :attr:`fields_to_export` attribute is set, it will be used to define the
@@ -254,9 +254,8 @@ CsvItemExporter
    :param file: the file-like object to use for exporting the data.
 
    :param include_headers_line: If enabled, makes the exporter output a header
-       line with the field names taken from
-       :attr:`BaseItemExporter.fields_to_export` so that attribute must also be
-       set in order to work (otherwise it raises a :exc:`RuntimeError`)
+      line with the field names taken from 
+      :attr:`BaseItemExporter.fields_to_export` or the first exported item fields.
    :type include_headers_line: boolean
 
    The additional keyword arguments of this constructor are passed to the
@@ -266,6 +265,7 @@ CsvItemExporter
 
    A typical output of this exporter would be::
 
+      product,price
       Color TV,1200
       DVD player,200
       

scrapy/contrib/exporter/__init__.py

@@ -101,25 +101,29 @@ class XmlItemExporter(BaseItemExporter):
 
 class CsvItemExporter(BaseItemExporter):
 
-    def __init__(self, file, include_headers_line=False, **kwargs):
+    def __init__(self, file, include_headers_line=True, **kwargs):
         self._configure(kwargs, dont_fail=True)
         self.include_headers_line = include_headers_line
         self.csv_writer = csv.writer(file, **kwargs)
-
-    def start_exporting(self):
-        if self.include_headers_line:
-            if not self.fields_to_export:
-                raise RuntimeError("You must set fields_to_export in order" + \
-                                   " to use include_headers_line")
-            self.csv_writer.writerow(self.fields_to_export)
+        self._header_written = False
 
     def export_item(self, item):
+        if not self._header_written:
+            self._write_header(item)
+
         fields = self._get_serialized_fields(item, default_value='', \
             include_empty=True)
 
         values = [x[1] for x in fields]
         self.csv_writer.writerow(values)
 
+    def _write_header(self, item):
+        self._header_written = True
+        if self.include_headers_line:
+            if not self.fields_to_export:
+                self.fields_to_export = item.fields.keys()
+            self.csv_writer.writerow(self.fields_to_export)
+
 
 class PickleItemExporter(BaseItemExporter):
 

scrapy/tests/test_contrib_exporter.py

@@ -95,20 +95,38 @@ class CsvItemExporterTest(BaseItemExporterTest):
         return CsvItemExporter(self.output, **kwargs)
 
     def _check_output(self):
-        self.assertEqual(self.output.getvalue(), '22,John\xc2\xa3\r\n')
+        self.assertEqual(self.output.getvalue(), 'age,name\r\n22,John\xc2\xa3\r\n')
 
     def test_header(self):
         output = StringIO()
-        ie = CsvItemExporter(output, include_headers_line=True)
-        self.assertRaises(RuntimeError, ie.start_exporting)
-
-        ie = CsvItemExporter(output, include_headers_line=True, \
-            fields_to_export=self.i.fields.keys())
+        ie = CsvItemExporter(output, fields_to_export=self.i.fields.keys())
         ie.start_exporting()
         ie.export_item(self.i)
         ie.finish_exporting()
         self.assertEqual(output.getvalue(), 'age,name\r\n22,John\xc2\xa3\r\n')
 
+        output = StringIO()
+        ie = CsvItemExporter(output, fields_to_export=['age'])
+        ie.start_exporting()
+        ie.export_item(self.i)
+        ie.finish_exporting()
+        self.assertEqual(output.getvalue(), 'age\r\n22\r\n')
+
+        output = StringIO()
+        ie = CsvItemExporter(output)
+        ie.start_exporting()
+        ie.export_item(self.i)
+        ie.export_item(self.i)
+        ie.finish_exporting()
+        self.assertEqual(output.getvalue(), 'age,name\r\n22,John\xc2\xa3\r\n22,John\xc2\xa3\r\n')
+
+        output = StringIO()
+        ie = CsvItemExporter(output, include_headers_line=False)
+        ie.start_exporting()
+        ie.export_item(self.i)
+        ie.finish_exporting()
+        self.assertEqual(output.getvalue(), '22,John\xc2\xa3\r\n')
+
 
 class XmlItemExporterTest(BaseItemExporterTest):