class_docstr: Defined by John Bollinger in the 80s. It measures volatility by defining
  upper and lower bands at distance x standard deviations
  
  Formula:
    - midband = SimpleMovingAverage(close, period)
    - topband = midband + devfactor * StandardDeviation(data, period)
    - botband = midband - devfactor * StandardDeviation(data, period)
  
  See:
    - http://en.wikipedia.org/wiki/Bollinger_Bands
code: def __init__(self):
      self.lines.mid = ma = self.p.movav(
          self.data, period=self.p.period
      )
      stddev = self.p.devfactor * StdDev(
          self.data, ma, period=self.p.period, movav=self.p.movav
      )
      self.lines.top = ma + stddev
      self.lines.bot = ma - stddev
  
      super(BollingerBands, self).__init__()
  
file_path: backtrader/backtrader/indicators/bollinger.py
imports: from __future__ import absolute_import, division, print_function, unicode_literals
  from .None import Indicator, MovAv, StdDev
label_desc: Use image node_id 2 to create a new BollingerBands object from inherited base classes: Indicator with example: obj = BollingerBands()