[Python] Small discrepancy in interest calculation: how to handle day-count / rounding to...

I’m implementing a simple annual compounding function with partial-year accrual using Actual/365. My test expects a final profit of 405.6, but my code consistently yields 405.74. The difference of 0.14 seems to come from either day-count details or a rounding/truncation rule. How can I adjust my calculation so it precisely matches the expected result?

The function:

def calculate_annual_capitalization(today: date, series: BondSeries, user_bond: Bond, nominal_value: float = 100.0,
cpi_data=None):

days_in_year = 365
first_year_rate = series.interest_rate / 100
issue_date = series.issue_date
maturity_date = series.maturity_date
purchase_date = user_bond.purchase_date
calculation_date = min(today, maturity_date)
value = user_bond.quantity * nominal_value
days_held = (calculation_date - purchase_date).days
profit = round(value * first_year_rate * (days_held / days_in_year), 2)
return profit


Entry data:

tos0327 = BondSeries(
series="TOS0327",
type="TOS",
issue_date=date(2024, 3, 1),
maturity_date=date(2027, 3, 31),
duration_years=3,
interest_rate=6.4,
margin_rate=None
)

user_bond = Bond(
account_uuid="1",
series="TOS0327",
purchase_date=date(2024, 3, 26),
quantity=65
)

profit = calculate_annual_capitalization(self.today, tos0327, user_bond)


Full code (you can run without dependencies by python name.py)

from datetime import date
from typing import Optional
import unittest
from dateutil.relativedelta import relativedelta
import math

class Bond:
def __init__(self, account_uuid: str, series: str, purchase_date: date, quantity: int):
self.account_uuid = account_uuid
self.series = series
self.purchase_date = purchase_date
self.quantity = quantity

class BondSeries:
def __init__(self, series: str, type: str, issue_date: date, maturity_date: date, duration_years: int, interest_rate: float, margin_rate: Optional[float] = None):
self.series = series
self.type = type
self.issue_date = issue_date
self.maturity_date = maturity_date
self.duration_years = duration_years
self.interest_rate = interest_rate
self.margin_rate = margin_rate



def calculate_annual_capitalization(today: date, series: BondSeries, user_bond: Bond, nominal_value: float = 100.0,
cpi_data=None):

days_in_year = 365
first_year_rate = series.interest_rate / 100
issue_date = series.issue_date
maturity_date = series.maturity_date
purchase_date = user_bond.purchase_date
calculation_date = min(today, maturity_date)
value = user_bond.quantity * nominal_value
days_held = (calculation_date - purchase_date).days
profit = round(value * first_year_rate * (days_held / days_in_year), 2)
return profit

class TestBondCalculations(unittest.TestCase):
def setUp(self):
self.today = date(2025, 3, 17)

def test_tos_ma1(self):
tos0128 = BondSeries(
series="TOS0128",
type="TOS",
issue_date=date(2025, 1, 1),
maturity_date=date(2028, 1, 31),
duration_years=3,
interest_rate=5.95,
margin_rate=None
)
user_bond = Bond(
account_uuid="1",
series="TOS0128",
purchase_date=date(2025, 1, 30),
quantity=2
)

profit = calculate_annual_capitalization(self.today, tos0128, user_bond)
self.assertEqual(profit, 1.5)

def test_tos_ma2(self):
tos0328 = BondSeries(
series="TOS0328",
type="TOS",
issue_date=date(2025, 3, 1),
maturity_date=date(2028, 3, 31),
duration_years=3,
interest_rate=5.95,
margin_rate=None
)

user_bond = Bond(
account_uuid="1",
series="TOS0328",
purchase_date=date(2025, 3, 14),
quantity=4
)

profit = calculate_annual_capitalization(self.today, tos0328, user_bond)
print(profit)
self.assertEqual(profit, 0.2)

def test_tos_mi1(self):
tos0327 = BondSeries(
series="TOS0327",
type="TOS",
issue_date=date(2024, 3, 1),
maturity_date=date(2027, 3, 31),
duration_years=3,
interest_rate=6.4,
margin_rate=None
)

user_bond = Bond(
account_uuid="1",
series="TOS0327",
purchase_date=date(2024, 3, 26),
quantity=65
)

profit = calculate_annual_capitalization(self.today, tos0327, user_bond)
print(profit)
self.assertEqual(profit, 405.6)

def test_tos_mi2(self):
tos0627 = BondSeries(
series="TOS0627",
type="TOS",
issue_date=date(2024, 6, 1),
maturity_date=date(2027, 6, 30),
duration_years=3,
interest_rate=6.2,
margin_rate=None
)

user_bond = Bond(
account_uuid="1",
series="TOS0627",
purchase_date=date(2024, 6, 10),
quantity=20
)

profit = calculate_annual_capitalization(self.today, tos0627, user_bond)
print(profit)
self.assertEqual(profit, 95.20)

if __name__ == "__main__":
unittest.main()

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