code: def test_ensure_timezone(self):
      @preprocess(tz=ensure_timezone)
      def f(tz):
          return tz
  
      valid = {
          "utc",
          "EST",
          "US/Eastern",
      }
      invalid = {
          # unfortunately, these are not actually timezones (yet)
          "ayy",
          "lmao",
      }
  
      # test coercing from string
      for tz in valid:
          self.assertEqual(f(tz), pytz.timezone(tz))
  
      # test pass through of tzinfo objects
      for tz in map(pytz.timezone, valid):
          self.assertEqual(f(tz), tz)
  
      # test invalid timezone strings
      for tz in invalid:
          self.assertRaises(pytz.UnknownTimeZoneError, f, tz)
  
imports: from operator import attrgetter
  from types import FunctionType
  from unittest import TestCase
  from nose_parameterized import parameterized
  from numpy import arange, array, dtype
  import pytz
  from six import PY3
  from zipline.utils.preprocess import call, preprocess
  from zipline.utils.input_validation import expect_dimensions, ensure_timezone, expect_element, expect_dtypes, expect_types, optional, optionally
label_desc: Use image node_id 18 for calling the PreprocessTestCase obj's underlying member method code with example usage: obj.test_ensure_timezone() and returns: tz