code: def testPeriod50_ClosingPrice(self):
      barFeed = self.__getFeed()
      bars = barFeed[VWAPTestCase.Instrument]
      vwap_ = vwap.VWAP(bars, 50)
      barFeed.loadAll()
      for i in xrange(49):
          self.assertEqual(vwap_[i], None)
      for i in xrange(49, len(vwap_)):
          self.assertNotEqual(vwap_[i], None)
  
file_path: pyalgotrade/testcases/technical_vwap_test.py
imports: from six.moves import xrange
  from .None import common
  from pyalgotrade.technical import vwap
  from pyalgotrade.barfeed import yahoofeed
label_desc: Use image node_id 6 for calling the VWAPTestCase obj's underlying member method code with example usage: obj.testPeriod50_ClosingPrice() without return types