You probably need to import the datetime class.

http://docs.python.org/2/library/datetime.html

import datetime  

Ahhh.... well that was easy. (Sorry bout that). Side question - anyone know if vwap() computes the volume weighted average price for a security for the trailing number of trading days or is it actual calendar days? I'd think the former but I could be wrong.

I'm fairly certain it's trading days.

Which could make things a lot easier for you.

# initialize  
context.daycounter = 0

# handle_data  
context.daycounter += 1  
if context.daycounter >= 126:  
    # do stuff

Correct me if I'm wrong Dennis but #handle_data is called more than daily, no? Maybe there is a way to switch to daily data in the backtest? Right now I am using minute data and counting 390 calls per day (60 per hour from 930am-4pm) so maybe just

if context.daycounter >= 126*390  

thanks for the tip

Sorry, you didn't share a backtest so I didn't know you were using minute data.

You will get events outside of trading hours (especially if you use Fetcher). And some trading days close early at 1pm. So I don't think it's wise to count on 390 events per day.

In that case I would check for a new day using get_datetime() and update the counter once per day.

# initialize  
context.date = None  
context.daycounter = 0

#handle_data  
if get_datetime().date <> context.date:  
    context.date = get_datetime().date  
    context.daycounter += 1  

Yeah thats good. Just curious, how do I switch to daily data from minute data?

EDIT: Also, are there pre-defined variables within context (e.g. context.date, context.daycounter) or can you just append anything after context?

You should be able to switch between daily and minute with the dropdown right next to where you change your starting cash and beginning/end dates.

And you can just append anything after context. FYI, the reason context.blah should be used instead of just blah is so the variable can be passed between days/minutes easily. If you use the latter, your variable will just be erased at the end of the day since the function ends.

Sorry for my ignorance but I see only one dropdown menu on the backtest screen, to the right of "Share Results" and there are two options in that dropdown: View Code and Backtest Details. Neither seem to offer the ability to edit the backtest settings.

As far as context.blah I've been using

#initialize  
Global myvar  
myvar=0

#handle_data  
myvar=something_new  

which seems to work but is less elegant. Thanks alot for the help Gus.

@Daniel, look on the Algorithm page to set the mode (daily/minute) before you run the backtest.

Yup, found it.

@Dennis - I tried to implement your algorithm (see below) to count the days but the program seems to count each minute as a new day

#Buy and sell SPY when 15 day MA is above 126 day MA



import datetime  
import math

def initialize(context):  
    #  
    context.date=None  
    context.timestep=0  
    #  
    #Add SPDR SPY  
    context.SPY = sid(8554)

    #  
    #Set min/max notional  
    context.max_notional = 1000000.0  
    context.min_notional = -1000000.0

def handle_data(context, data):

    #Handle assigning the timestep number  
    if get_datetime().date <> context.date: #look for a new day  
        context.date=get_datetime().date  
        context.timestep += 1  
        #Handle the first timestep (buy and hold SPY until we start using the algorithm  
        if context.timestep==1:  
            nshares = math.floor ( context.max_notional/(data[context.SPY].price) )  
            order(context.SPY,+nshares)  
        #Exit the loop if timestep is less than the longer MA  
        if context.timestep < 126:  
            print("Exiting because context.timestep="+str(context.timestep))  
            return()  
    print("Did not exit loop because context.timestep = "+str(context.timestep))   

    #compute a notional value where short positions require 2x cash  
#    notional = abs(context.portfolio.positions[context.SPY].amount * data[context.SPY].price)  
    #test the market MA  
#    MAR_SPY = data[context.SPY].vwap(15) / data[context.SPY].vwap(126)  

Log:

2012-01-03PRINTExiting because context.timestep=1
2012-01-03PRINTExiting because context.timestep=2
2012-01-03PRINTExiting because context.timestep=3
2012-01-03PRINTExiting because context.timestep=4
2012-01-03PRINTExiting because context.timestep=5
2012-01-03PRINTExiting because context.timestep=6
2012-01-03PRINTExiting because context.timestep=7
2012-01-03PRINTExiting because context.timestep=8
2012-01-03PRINTExiting because context.timestep=9
2012-01-03PRINTExiting because context.timestep=10
... ...

NOTE: if get_datetime().date is changed to get_datetime().day the program works correctly.

@Daniel, it seems I left the parentheses off .date()

if get_datetime().date() <> context.date: #look for a new day  
        context.date=get_datetime().date()