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'''

A single function that calculates IRR using Newton's Method

'''

from datetime import date

def xirr(transactions):

'''

Calculates the Internal Rate of Return (IRR) for an irregular series of cash flows (XIRR)

Takes a list of tuples [(date,cash-flow),(date,cash-flow),...]

Returns a rate of return as a percentage

'''

years = [(ta[0] - transactions[0][0]).days / 365. for ta in transactions]

residual = 1.0

step = 0.05

guess = 0.05

epsilon = 0.0001

limit = 10000

while abs(residual) > epsilon and limit > 0:

limit -= 1

residual = 0.0

for i, trans in enumerate(transactions):

residual += trans[1] / pow(guess, years[i])

if abs(residual) > epsilon:

if residual > 0:

guess += step

else:

guess -= step

step /= 2.0

return guess - 1

tas = [(date(2006, 1, 24), -39967),

(date(2008, 2, 6), -19866),

(date(2010, 10, 18), 245706),

(date(2013, 9, 14), 52142)]

if __name__ == '__main__':

print(tas,)

print("IRR for test data = {:.2%}".format(xirr(tas)))