[fetch] add tests for fetch subcommand parsing.

recipe_engine/unittests/fetch_test.py

 #!/usr/bin/env python
 # Copyright 2016 The LUCI Authors. All rights reserved.
 # Use of this source code is governed under the Apache License, Version 2.0
 # that can be found in the LICENSE file.
 
 import base64
 import itertools
 import json
 import unittest
 
+from cStringIO import StringIO
+
 import test_env
 
+import argparse  # this is vendored
+
 import mock
 import subprocess42
 
+from recipe_engine import common_args
 from recipe_engine import fetch
 from recipe_engine import package_pb2
 from recipe_engine import requests_ssl
 
 
 CPE = subprocess42.CalledProcessError
 
 
 class NoMoreExpectatedCalls(ValueError):
   pass
@@ skipping 418 matching lines @@
       self.j('repo/+/%s?name-status=1&format=JSON' % self.a, self.a_dat),
       self.d('repo/+/%s/infra/config/recipes.cfg?format=TEXT' % self.a,
              self.proto_text),
     )
 
     result = fetch.GitilesBackend('dir', 'repo', True).commit_metadata(
         'revision')
     self.assertEqual(result, self.a_meta)
     self.assertMultiDone(requests_get)
 
+
+class TestArgs(unittest.TestCase):
+  def setUp(self):
+    self.p = argparse.ArgumentParser()
+    self.followup = common_args.add_common_args(self.p)
+    subp = self.p.add_subparsers()
+    fetch.add_subparser(subp)
+
+  @mock.patch('argparse._sys.stderr', new_callable=StringIO)
+  def test_no_fetch(self, stderr):
+    with self.assertRaises(SystemExit):
+      args = self.p.parse_args(['--no-fetch', 'fetch'])
+      args.postprocess_func(self.p, args)
+    self.assertIn('--no-fetch does not make sense', stderr.getvalue())
+
+
 if __name__ == '__main__':
   unittest.main()