do not overwrite outputfiles

tests/basic_stream/test_basic_stream.py

@@ -115,6 +115,26 @@ def test_construct_listener_with_file(self, PrintingListener):
         PrintingListener.assert_called_once_with(out=mock_open.return_value)
         self.assertEquals(result, PrintingListener.return_value)
 
+    @mock.patch('os.path.exists')
+    @mock.patch('twitter_monitor.basic_stream.PrintingListener')
+    def test_construct_listener_with_existing_file(self, PrintingListener, os_path_exists):
+        PrintingListener.return_value = 53234
+
+        outfile = "some_file.txt"
+
+        mock_open = mock.mock_open()
+        mock_open.return_value = 348374387
+
+        os_path_exists.return_value = True
+
+        with mock.patch('twitter_monitor.basic_stream.open', mock_open, create=True):
+            # An error should be raised
+            self.assertRaises(IOError, basic_stream.construct_listener, outfile)
+
+        # The file should not have been opened
+        self.assertEquals(mock_open.call_count, 0)
+
+
     def test_get_tweepy_auth(self):
         twitter_api_key = 'ak'
         twitter_api_secret = 'as'

twitter_monitor/basic_stream.py

@@ -137,6 +137,9 @@ def get_tweepy_auth(twitter_api_key,
 def construct_listener(outfile=None):
     """Create the listener that prints tweets"""
     if outfile is not None:
+        if os.path.exists(outfile):
+            raise IOError("File %s already exists" % outfile)
+
         outfile = open(outfile, 'wb')
 
     return PrintingListener(out=outfile)