code: def web(ticker, field=None, start=None, end=None,
          mrefresh=False, source='yahoo'):
      
      if source == 'yahoo' and field is None:
          field = 'Adj Close'
  
      tmp = _download_web(ticker, data_source=source,
                          start=start, end=end)
  
      if tmp is None:
          raise ValueError('failed to retrieve data for %s:%s' % (ticker, field))
  
      if field:
          return tmp[field]
      else:
          return tmp
docs: Data provider wrapper around pandas.io.data provider. Provides
      memoization.