Thank you very much for considering my pull request.

I have added a test: tests/test_note_append.py
And have made the isuggested code changes.
Please let me know if anything else is needed.
https://github.com/pubs/pubs/pull/201
https://github.com/pubs/pubs/issues/200

pubs/commands/note_cmd.py

@@ -3,6 +3,7 @@ from ..uis import get_ui
 from ..utils import resolve_citekey
 from ..completion import CiteKeyCompletion
 from ..events import NoteEvent
+from ..content import write_file
 
 
 def parser(subparsers, conf):
@@ -10,7 +11,8 @@ def parser(subparsers, conf):
                                    help='edit the note attached to a paper')
     parser.add_argument('citekey', help='citekey of the paper',
                         ).completer = CiteKeyCompletion(conf)
-    parser.add_argument('-a', '--append', help='append a line of text to the notes file', default=None)
+    parser.add_argument('-a', '--append',
+                        help='append a line of text to the notes file', default=None)
     return parser
 
 
@@ -19,12 +21,11 @@ def command(conf, args):
     ui = get_ui()
     rp = repo.Repository(conf)
     citekey = resolve_citekey(rp, args.citekey, ui=ui, exit_on_fail=True)
-    append = args.append
+    latestnote = '{TXT}\n'.format(TXT=args.append)
     notepath = rp.databroker.real_notepath(citekey, rp.conf['main']['note_extension'])
-    if append is None:
+    if latestnote is None:
         ui.edit_file(notepath, temporary=False)
     else:
-        with open(notepath, 'a') as fd:
+        write_file(notepath, latestnote, 'a')
-            fd.write(append)
     NoteEvent(citekey).send()
     rp.close()

tests/test_note_append.py

@@ -0,0 +1,50 @@
+# -*- coding: utf-8 -*-
+"""Test appending a note file from the command-line using the '-a' arg"""
+
+from __future__ import print_function, unicode_literals
+
+import unittest
+import os
+
+from tests.test_usecase import DataCommandTestCase
+
+
+class TestNoteAppend(DataCommandTestCase):
+    """Test appending a note file from the command-line using the '-a' arg"""
+
+    def setUp(self, nsec_stat=True):
+        """Initialize a bib entry containing citation key, Page99, for testing"""
+        super(TestNoteAppend, self).setUp()
+        init = ['pubs init',
+                'pubs add data/pagerank.bib',
+               ]
+        self.execute_cmds(init)
+        self.note_dir = os.path.join(self.default_pubs_dir, 'notes')
+
+    def test_note_edit(self):
+        """Test appending the note file using the command-line argument, -a"""
+        fin_notes = os.path.join(self.note_dir, 'Page99.txt')
+        # Test adding first line
+        cmds = [('pubs note Page99 -a aaa')]
+        self.execute_cmds(cmds)
+        note_lines = ['aaa']
+        self.assertFileContentEqual(fin_notes, self._get_note_content(note_lines))
+        # Test adding additional line
+        cmds = [('pubs note Page99 -a bbb')]
+        self.execute_cmds(cmds)
+        note_lines.append('bbb')
+        self.assertFileContentEqual(fin_notes, self._get_note_content(note_lines))
+        # Test adding Japanese character
+        cmds = [('pubs note Page99 -a ソ')]
+        self.execute_cmds(cmds)
+        note_lines.append('ソ')
+        self.assertFileContentEqual(fin_notes, self._get_note_content(note_lines))
+
+    @staticmethod
+    def _get_note_content(note_lines):
+        """Given a list of note lines, return full note file content"""
+        return '{LINES}\n'.format(LINES='\n'.join(note_lines))
+
+
+if __name__ == '__main__':
+    unittest.main(verbosity=2)