I'm not quite as good as calculating income tax as I thought I was...

gb_income_tax.py

from decimal import Decimal


DEFAULT_PERSONAL_ALLOWANCE = Decimal(11_850)
PERSONAL_ALLOWANCE_ADJUSTMENT_BEGIN = Decimal(100_000)

BASIC_RATE_BEGIN = DEFAULT_PERSONAL_ALLOWANCE
HIGHER_RATE_BEGIN = Decimal(46_350)
ADDITIONAL_RATE_BEGIN = Decimal(150_000)

BASIC_RATE = Decimal(0.20)
HIGHER_RATE = Decimal(0.40)
ADDITIONAL_RATE = Decimal(0.45)


def income_tax(s):
    # make sure salary is stored as Decimal type
    salary = Decimal(s)
    # everyone gets the default personal allowance to start with
    personal_allowance = DEFAULT_PERSONAL_ALLOWANCE
    # calculate any personal allowance adjustments
    if salary > PERSONAL_ALLOWANCE_ADJUSTMENT_BEGIN:
        # personal allowance goes down by GBP1 for every GBP2 over 100k
        personal_allowance -= (
            (salary - PERSONAL_ALLOWANCE_ADJUSTMENT_BEGIN) / 2
        )
    # this guards against ending up with a negative personal allowance
    personal_allowance = max(personal_allowance, Decimal(0))
    # take_home tracks take-home pay, taxable tracks tax to pay
    take_home = min(personal_allowance, salary)
    taxable = Decimal(0)
    salary -= take_home
    # now that the personal allowance is out of the way, calculate up tax bands
    if salary > 0:
        # basic rate
        basic_rate_taxable = min(HIGHER_RATE_BEGIN - BASIC_RATE_BEGIN, salary)
        take_home += basic_rate_taxable * (1 - BASIC_RATE)
        taxable += basic_rate_taxable * BASIC_RATE
        salary -= basic_rate_taxable
    if salary > 0:
        # higher rate
        higher_rate_taxable = min(ADDITIONAL_RATE_BEGIN - HIGHER_RATE_BEGIN, salary)
        take_home += higher_rate_taxable * (1 - HIGHER_RATE)
        taxable += higher_rate_taxable * HIGHER_RATE
        salary -= higher_rate_taxable
    if salary > 0:
        # additional rate -this is calculated on any remaining income
        additional_rate_taxable = salary
        take_home += additional_rate_taxable * (1 - ADDITIONAL_RATE)
        taxable += additional_rate_taxable * ADDITIONAL_RATE
        salary -= additional_rate_taxable
    # these post-conditional assertions perform basic sanity checks
    assert salary == 0. # we should have accounted for all income
    # take-home + taxable = original salary
    # FIXME: below assertion doesn't always work. maybe being too picky
    # assert (take_home + taxable) == Decimal(s)
    return (take_home, taxable)


for i in range(0, 201_000, 1_000):
    take_home, taxable = income_tax(i)
    print(i, take_home, taxable)