[Findit] Fix flaky tests due to importing pytz.

appengine/findit/common/test/time_util_test.py

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
+import mock
 import unittest
 
 from common import time_util
 from datetime import datetime
 from datetime import timedelta
 
-import pytz
-
 
 class DiffTest(unittest.TestCase):
   def testRemoveMicrosecondsFromDelta(self):
     date1 = datetime(2016, 5, 1, 1, 1, 1, 1)
     date2 = datetime(2016, 5, 1, 1, 1, 1, 2)
     delta = date2 - date1
 
     self.assertEqual(
         time_util.RemoveMicrosecondsFromDelta(delta).microseconds,
         0)
 
   def testFormatTimedelta(self):
     self.assertIsNone(time_util.FormatTimedelta(None))
     self.assertEqual(time_util.FormatTimedelta(timedelta(0, 1)),
                      '00:00:01')
     self.assertEqual(time_util.FormatTimedelta(timedelta(0, 60)),
                      '00:01:00')
     self.assertEqual(time_util.FormatTimedelta(timedelta(0, 3600)),
                      '01:00:00')
     self.assertEqual(time_util.FormatTimedelta(timedelta(0, 0, 1)),
                      '00:00:00')
 
   def testFormatDatetime(self):
     self.assertIsNone(time_util.FormatDatetime(None))
     self.assertEqual(
         time_util.FormatDatetime(datetime(2016, 1, 2, 1, 2, 3)),
         '2016-01-02 01:02:03 UTC')
 
-  def testGetDateTimeInTimezone(self):
-    utc_datetime = datetime(2016, 9, 27, 20, 46, 18, 1, pytz.utc)
-    result_pst_datetime = time_util.GetDatetimeInTimezone(
-        'US/Pacific', utc_datetime)
-    expected_pst_datetime = datetime(2016, 9, 27, 13, 46, 18, 1,
-                                     pytz.timezone('US/Pacific'))
-    self.assertEqual(result_pst_datetime.date(), expected_pst_datetime.date())
-    self.assertEqual(result_pst_datetime.time(), expected_pst_datetime.time())
+  @mock.patch('common.time_util.pytz')
+  def testGetDateTimeInTimezoneWithGivenDatetime(self, mocked_pytz_module):
+    mocked_datetime = mock.MagicMock()
+    mocked_datetime.astimezone.return_value = 'expected'
+
+    self.assertEqual('expected',
+                     time_util.GetDatetimeInTimezone('PST', mocked_datetime))
+    mocked_pytz_module.timezone.assert_called_with('PST')