92 lines
3.2 KiB
Python
92 lines
3.2 KiB
Python
|
"""Tax-regime abstract base — every jurisdiction implements this.
|
||
|
|
|
||
|
|
Inputs are split by income source because each source carries different
|
||
|
|
tax treatment (e.g. ISA withdrawals are always 0%, capital gains may be
|
||
|
|
exempt in some jurisdictions, pension withdrawals are partially tax-free
|
||
|
|
in the UK). The regime decides how to combine them.
|
||
|
|
|
||
|
|
Outputs are split per tax type so we can attribute lifetime tax — the
|
||
|
|
Grafana panel shows e.g. "lifetime CGT paid" separately from "lifetime
|
||
|
|
income tax".
|
||
|
|
"""
|
||
|
|
from __future__ import annotations
|
||
|
|
|
||
|
|
from abc import ABC, abstractmethod
|
||
|
|
from dataclasses import dataclass, field
|
||
|
|
from decimal import Decimal
|
||
|
|
|
||
|
|
|
||
|
|
@dataclass(frozen=True)
|
||
|
|
class TaxInputs:
|
||
|
|
"""Annual gross flows for a single tax year. All amounts in GBP, all
|
||
|
|
non-negative — withdrawals are absolute values.
|
||
|
|
|
||
|
|
`years_since_uk_departure` lets the UK regime apply the 5-year
|
||
|
|
Temporary Non-Residence claw-back: gains realised abroad get clawed
|
||
|
|
back if you return within 5y. Non-UK regimes ignore it.
|
||
|
|
"""
|
||
|
|
earned_income: Decimal = Decimal("0")
|
||
|
|
pension_withdrawal: Decimal = Decimal("0")
|
||
|
|
capital_gains: Decimal = Decimal("0")
|
||
|
|
dividends: Decimal = Decimal("0")
|
||
|
|
isa_withdrawals: Decimal = Decimal("0")
|
||
|
|
interest: Decimal = Decimal("0")
|
||
|
|
years_since_uk_departure: int = 0
|
||
|
|
|
||
|
|
|
||
|
|
@dataclass(frozen=True)
|
||
|
|
class TaxBreakdown:
|
||
|
|
"""Tax due, split by category. `total` is the sum — every regime
|
||
|
|
must keep `total == sum of categories` for the integrity check.
|
||
|
|
"""
|
||
|
|
income_tax: Decimal = Decimal("0")
|
||
|
|
national_insurance: Decimal = Decimal("0")
|
||
|
|
capital_gains_tax: Decimal = Decimal("0")
|
||
|
|
dividend_tax: Decimal = Decimal("0")
|
||
|
|
healthcare_levy: Decimal = Decimal("0")
|
||
|
|
other: Decimal = Decimal("0")
|
||
|
|
notes: tuple[str, ...] = field(default_factory=tuple)
|
||
|
|
|
||
|
|
@property
|
||
|
|
def total(self) -> Decimal:
|
||
|
|
return (self.income_tax + self.national_insurance + self.capital_gains_tax +
|
||
|
|
self.dividend_tax + self.healthcare_levy + self.other)
|
||
|
|
|
||
|
|
|
||
|
|
class TaxRegime(ABC):
|
||
|
|
"""Per-jurisdiction tax engine. Stateless — every call gets fresh
|
||
|
|
inputs. Sub-classes set `name` for the scenario key.
|
||
|
|
"""
|
||
|
|
name: str
|
||
|
|
|
||
|
|
@abstractmethod
|
||
|
|
def compute_tax(self, inputs: TaxInputs) -> TaxBreakdown:
|
||
|
|
"""Return the year's tax due given gross income/gains/dividends."""
|
||
|
|
raise NotImplementedError
|
||
|
|
|
||
|
|
|
||
|
|
def apply_brackets(amount: Decimal, brackets: list[tuple[Decimal, Decimal]]) -> Decimal:
|
||
|
|
"""Apply a progressive bracket schedule to `amount`.
|
||
|
|
|
||
|
|
`brackets` is a list of (band_top, marginal_rate) — band_top is the
|
||
|
|
upper bound of the band (use Decimal('Infinity') for the last band).
|
||
|
|
Bands are evaluated in order from lowest to highest.
|
||
|
|
|
||
|
|
Example UK PAYE 2026/27 above the personal allowance:
|
||
|
|
[(50_270 - 12_570, Decimal("0.20")),
|
||
|
|
(125_140 - 12_570, Decimal("0.40")),
|
||
|
|
(Decimal("Infinity"), Decimal("0.45"))]
|
||
|
|
where `amount` is taxable income net of the allowance.
|
||
|
|
"""
|
||
|
|
if amount <= 0:
|
||
|
|
return Decimal("0")
|
||
|
|
tax = Decimal("0")
|
||
|
|
prev_top = Decimal("0")
|
||
|
|
for band_top, rate in brackets:
|
||
|
|
if amount <= prev_top:
|
||
|
|
break
|
||
|
|
slice_top = min(amount, band_top)
|
||
|
|
tax += (slice_top - prev_top) * rate
|
||
|
|
prev_top = band_top
|
||
|
|
return tax
|