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I'm learning QuantLib-Python and trying to replicate an Interest Rate Swap valuation on a custom curve constructed by passing lists of dates and discounting factors. Please see my code below

import QuantLib as ql

today = ql.Date(31, ql.May, 2023)
ql.Settings.instance().setEvaluationDate(today)

dates = [ql.Date(31, ql.May, 2023), ql.Date(28, ql.August, 2023), ql.Date(27, ql.November, 2023), 
                 ql.Date(26, ql.February, 2024), ql.Date(27, ql.May, 2024), ql.Date(26, ql.May, 2025), ql.Date(26, ql.May, 2026)]

dfs = [1.00, 0.980626530240871, 0.961865563098355, 0.942995352345942, 0.923889577251464, 0.845638911735274, 0.770640534908645]

dayCounter = ql.ActualActual(ql.ActualActual.ISDA)
calendar = ql.UnitedStates()

curve = ql.DiscountCurve(dates, dfs, dayCounter, calendar)
curveHandle = ql.YieldTermStructureHandle(curve)

start = today
maturity = calendar.advance(start, ql.Period('1Y'))
fix_schedule = ql.MakeSchedule(start, maturity, ql.Period('1Y'))
float_schedule = ql.MakeSchedule(start, maturity, ql.Period('3M'))

customIndex = ql.IborIndex('index', ql.Period('3M'), 0, ql.USDCurrency(), ql.UnitedStates(), ql.ModifiedFollowing, True, dayCounter, curveHandle)
customIndex.addFixing(ql.Date(30, ql.May, 2023), 0.075)

notional = 100000000
swap = ql.VanillaSwap(ql.VanillaSwap.Payer, notional, fix_schedule, 0.0833, dayCounter, 
                     float_schedule, customIndex, 0, dayCounter)
swap_engine = ql.DiscountingSwapEngine(curveHandle)
swap.setPricingEngine(swap_engine)

print(swap.NPV())

There is a possibility to print out undiscounted fixed and floating leg cashflows by

print("Net Present Value: {0}".format(swap.NPV()))
print()
print("Fixed leg cashflows:")
for i, cf in enumerate(swap.leg(0)):
    print("%2d    %-18s  %10.2f"%(i+1, cf.date(), cf.amount()))
print()
print("Floating leg cashflows:")
for i, cf in enumerate(swap.leg(1)):
    print("%2d    %-18s  %10.2f"%(i+1, cf.date(), cf.amount()))

The Net Present Value of a swap should be given by the difference of discounted floating and fixed leg cashflows. I tried to replicate calculations manually via

fwd1 = curve.forwardRate(ql.Date(31, 5, 2023), ql.Date(31, 8, 2023), dayCounter, ql.Continuous).rate()
df1 = curve.discount(ql.Date(31, 8, 2023))
tau1 = dayCounter.yearFraction(ql.Date(31, 5, 2023), ql.Date(31, 8, 2023))
cashflow1 = df1 * notional * fwd1 * tau1
print(cashflow1)
print()

fwd2 = curve.forwardRate(ql.Date(1, 9, 2023), ql.Date(30, 11, 2023), dayCounter, ql.Simple).rate()
df2 = curve.discount(ql.Date(30, 11, 2023))
tau2 = dayCounter.yearFraction(ql.Date(1, 9, 2023), ql.Date(30, 11, 2023))
cashflow2 = df2 * notional * fwd2 * tau2
print(cashflow2)
print()

fwd3 = curve.forwardRate(ql.Date(1, 12, 2023), ql.Date(29, 2, 2024), dayCounter, ql.Simple).rate()
df3 = curve.discount(ql.Date(29, 2, 2024))
tau3 = dayCounter.yearFraction(ql.Date(1, 12, 2023), ql.Date(29, 2, 2024))
cashflow3 = df3 * notional * fwd3 * tau3
print(cashflow3)
print()

fwd4 = curve.forwardRate(ql.Date(1, 3, 2024), ql.Date(31, 5, 2024), dayCounter, ql.Simple).rate()
df4 = curve.discount(ql.Date(31, 5, 2024))
tau4 = dayCounter.yearFraction(ql.Date(1, 3, 2024), ql.Date(31, 5, 2024))
cashflow4 = df4 * notional * fwd4 * tau4
print(cashflow4)
print()

floatCashFlow = cashflow1 + cashflow2 + cashflow3 + cashflow4
print(floatCashFlow)
print()

df = df4
fixRate = 0.0833
tau = dayCounter.yearFraction(ql.Date(31, 5, 2023), ql.Date(31, 5, 2024))
fixCashFlow = df * notional * fixRate * tau
print(fixCashFlow)
print()

valueSwap = floatCashFlow - fixCashFlow
print(valueSwap)

and got a staggeringly different NPV. Can anyone show me how to precisely arrive at a built-in NPV valuation using built-in methods for calling discounting factors and forward rates? I suppose the difference is hidden somewhere in payment schedules but I failed to match the numbers by playing around with dates.

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2 Answers 2

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1/ the cf.amount() attribute returns undiscounted cashflows not PVd ones

2/ your curve.forwardRate() call is returning a continuously compounded zero rate, use customIndex(fixing date) to get the relevant ibor fwd

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3
  • $\begingroup$ 1. I'm well aware of it. It was mentioned in my question that these cashflows are undiscounted. 2. customIndex(fixing date) returns error IborIndex object is not callable. I tried customIndex.forecastFixing(fixing date) as well but it says that IborIndex object has no attribute forecastFixing. $\endgroup$
    – Hasek
    Jun 2 at 14:06
  • 2
    $\begingroup$ Try customIndex.fixing(fixing date) $\endgroup$
    – user35980
    Jun 2 at 14:12
  • $\begingroup$ customIndex.fixing(fixing date) works. I was able to replicate undiscounted floating and fixed cashflows and thus NPV. Thanks! $\endgroup$
    – Hasek
    Jun 2 at 14:32
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As a base for comparison you can also try rateslib

from rateslib import Curve, IRS
from pandas import Series

curve = Curve(
    nodes={
        dt(2023, 5, 31): 1.00,
        dt(2023, 8, 28): 0.980626530240871,
        dt(2023, 11, 27): 0.961865563098355,
        dt(2024, 2, 26): 0.942995352345942,
        dt(2024, 5, 27): 0.923889577251464,
        dt(2025, 5, 26): 0.845638911735274,
        dt(2026, 5, 26): 0.770640534908645,
    },
    interpolation = "log_linear",
    calendar="nyc",
    convention="act360",
)

irs = IRS(
    effective=dt(2023, 5, 31),
    termination="1Y",
    calendar="nyc",
    frequency="A",
    convention="act360",
    modifier="MF",
    notional=100e6,
    fixed_rate=8.33,
    leg2_frequency="Q",
    leg2_fixing_method="ibor",
    leg2_method_param=1,
    leg2_fixings=Series([7.5], index=[dt(2023, 5, 30)])
)

enter image description here

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