Things to consider before swapping XIN for VEA. I’m still on the fence whether it makes sense for me.

from __future__ import division
import unittest, csv, string, datetime
from numpy import floor, reshape, size, array, arange
from dateutil.relativedelta import relativedelta
import sys

def xirr(cash_flows, dates, yld=0.1, maxiter=50):
    """
    @param cash_flows list of cashflows
    @param dates list of datetime.date objects
    @param yld guess for rate of return
    @param maxiter maximum iterations
    """
    assert len(dates) == len(cash_flows)
    for date in dates:
        assert type(date) == datetime.date

    date_ordinals = array([x.toordinal() for x in dates])
    cash_flows = array(cash_flows)

    #number of years in cash flow FIXME: use max and min date, not first and last in list
    func = int(floor(yearfrac(dates[0], dates[-1])))
    if func == 0:
        func = 1
    #matlab: tf = func*(dates(:,loop)-dates(1,loop))/(datemnth(dates(1,loop),12*func,0,0)-dates(1,loop));
    tf = func*(date_ordinals - date_ordinals[0]) / (dates[0] + relativedelta(months = +12*func) - dates[0]).days

    # Determine the best guess for yld
    min_result = sys.maxint
    best_yld = None
    for yld in arange(-0.9, 1, 0.1):
        if yld == 0:
            continue
        result_f = abs(sum(cash_flows / ((1 + yld)**tf)))
        if result_f < min_result:
            min_result = result_f
            best_yld = yld

    yld = best_yld
    print "Using guess yld=", yld

    func = 2
    k = 1
    #Newton's Method
    while abs(func) > 1.e-6:
        #cash flow polynomial
        func = sum(cash_flows / ((1 + yld)**tf))
        #%(CF poly)’
        f_prime = -sum((cash_flows/((1 + yld)**tf)) * (tf/(1 +yld)))
        if f_prime == 0:
            yld = None
            break

        delta = -func/f_prime
        yld = yld + delta
        k += 1
        if k == maxiter + 1:
            print ‘Number of maximum iterations reached.’
            print ‘Please increase MAXITER or use different GUESS.’
            yld = None
            break
    return yld

def yearfrac(date1, date2):
    wYears = int(floor(abs((date1 - date2).days) / 365.))
    date1W = datetime.date(date1.year + wYears, date1.month, date1.day)
    numerator = (date1W - date2).days
    try:
        denominator = (date1W - datetime.date(date1W.year + 1, date1W.month, date1W.day)).days
    #—–handle weird leap year case
    except ValueError:
        date1W = date1W + datetime.timedelta(1)
        denominator = (date1W - datetime.date(date1W.year + 1, date1W.month, date1W.day)).days
    yearFraction = numerator / float(denominator)
    return yearFraction + wYears