Mean reversion with Linear Regression and Bollinger Band for Spread Trading within Python

Following code demonstrates how to utilize to linear regression to estimate hedge ratio and Bollinger band for spread trading. The code can be back tested at Quantopian.com

#   Mean reversion Spread Trading  with Linear Regression
#
#   Deniz Turan, (denizstij AT gmail DOT com), 19-Jan-2014
import numpy as np
from scipy.stats import linregress

R_P = 1 # refresh period in days
W_L = 30 # window length in days
def initialize(context):
    context.y=sid(14517) # EWC
    context.x=sid(14516) # EWA
    
    
    # for long and shorting 
    context.max_notional = 1000000
    context.min_notional = -1000000.0
    # set a fixed slippage
    set_slippage(slippage.FixedSlippage(spread=0.01))
        
    context.long=False;
    context.short=False;
    
    
def handle_data(context, data):
    xpx=data[context.x].price
    ypx=data[context.y].price
    
    retVal=linearRegression(data,context)    
    # lets dont do anything if we dont have enough data yet    
    if retVal is None:
        return  None 
    
    hedgeRatio,intercept=retVal;
    spread=ypx-hedgeRatio*xpx      
    data[context.y]['spread'] = spread

    record(ypx=ypx,spread=spread,xpx=xpx)

    # find moving average 
    rVal=getMeanStd(data, context)       
    # lets dont do anything if we dont have enough data yet    
    if rVal is None:
        return   
    
    meanSpread,stdSpread = rVal
    # zScore is the number of unit
    zScore=(spread-meanSpread)/stdSpread;
    QTY=1000
    qtyX=-hedgeRatio*QTY*xpx;        
    qtyY=QTY*ypx;        

    entryZscore=1;
    exitZscore=0;

    if zScore < -entryZscore and canEnterLong(context):
        # enter long the spread
        order(context.y, qtyY)
        order(context.x, qtyX)
        context.long=True
        context.short=False    
 
    if zScore > entryZscore and canEnterShort(context):
        #  enter short the spread
        order(context.y, -qtyY)
        order(context.x, -qtyX)
        context.short=True
        context.long=False
        
    record(cash=context.portfolio.cash, stock=context.portfolio.positions_value)
    
@batch_transform(window_length=W_L, refresh_period=R_P) 
def linearRegression(datapanel, context):
    xpx = datapanel['price'][context.x]
    ypx = datapanel['price'][context.y]

    beta, intercept, r, p, stderr = linregress(ypx, xpx)
#    record(beta=beta, intercept=intercept)
    return (beta, intercept)
        
@batch_transform(window_length=W_L, refresh_period=R_P) 
def getMeanStd(datapanel, context):    

    spread = datapanel['spread'][context.y]
    meanSpread=spread.mean()
    stdSpread=spread.std()
    if meanSpread is not None and stdSpread is not None :
        return (meanSpread, stdSpread)
    else:
        return None

def canEnterLong(context):
    notional=context.portfolio.positions_value

    if notional < context.max_notional and not context.long: # and not context.short:
        return True
    else:
        return False

def canEnterShort(context):
    notional=context.portfolio.positions_value

    if notional > context.max_notional and not context.short:  #and not context.short:
        return True
    else:
        return False