63 lines
2.5 KiB
Python
63 lines
2.5 KiB
Python
|
"""Custom user-defined spending plan.
|
||
|
|
|
||
|
|
A flexible strategy where the user chooses every knob:
|
||
|
|
|
||
|
|
- `initial_spend` — year 0 withdrawal in real GBP (taken from
|
||
|
|
`state.initial_withdrawal` if not overridden).
|
||
|
|
- `annual_real_adjust_pct` — fraction by which last year's withdrawal
|
||
|
|
scales each subsequent year, on top of inflation. 0.0 = constant
|
||
|
|
real GBP (Trinity-shape). +0.02 = 2%/yr above-inflation creep
|
||
|
|
(e.g. healthcare). -0.005 = -0.5%/yr decreasing spend (slowing
|
||
|
|
down with age).
|
||
|
|
- `guardrail_threshold_pct` — if portfolio drops below this fraction
|
||
|
|
of the starting NW, apply a cut. None = no guardrail.
|
||
|
|
- `guardrail_cut_pct` — fraction by which to cut last year's
|
||
|
|
withdrawal when triggered. Applied multiplicatively each triggered
|
||
|
|
year — not "snap to threshold-implied rate", just a soft cut.
|
||
|
|
|
||
|
|
The cut is checked AFTER the annual adjustment, so a cut + an
|
||
|
|
increase don't double-apply: cut wins.
|
||
|
|
|
||
|
|
Compared to Guyton-Klinger this is simpler — one threshold, one
|
||
|
|
cut size, no prosperity rule. If the user wants the prosperity rule
|
||
|
|
behaviour they can pick the GK preset.
|
||
|
|
"""
|
||
|
|
from __future__ import annotations
|
||
|
|
|
||
|
|
from fire_planner.strategies.base import StrategyState, WithdrawalStrategy
|
||
|
|
|
||
|
|
|
||
|
|
class SpendingPlanStrategy(WithdrawalStrategy):
|
||
|
|
name = "custom"
|
||
|
|
|
||
|
|
def __init__(
|
||
|
|
self,
|
||
|
|
initial_spend: float | None = None,
|
||
|
|
annual_real_adjust_pct: float = 0.0,
|
||
|
|
guardrail_threshold_pct: float | None = None,
|
||
|
|
guardrail_cut_pct: float = 0.10,
|
||
|
|
) -> None:
|
||
|
|
self.initial_spend = initial_spend
|
||
|
|
self.annual_real_adjust_pct = annual_real_adjust_pct
|
||
|
|
self.guardrail_threshold_pct = guardrail_threshold_pct
|
||
|
|
self.guardrail_cut_pct = guardrail_cut_pct
|
||
|
|
|
||
|
|
def propose_withdrawal(self, state: StrategyState) -> float:
|
||
|
|
if state.year_idx == 0:
|
||
|
|
# Explicit override wins; otherwise take the user's target.
|
||
|
|
return (self.initial_spend
|
||
|
|
if self.initial_spend is not None and self.initial_spend > 0 else
|
||
|
|
state.initial_withdrawal)
|
||
|
|
if state.portfolio <= 0:
|
||
|
|
return 0.0
|
||
|
|
|
||
|
|
proposed = state.last_withdrawal * (1.0 + self.annual_real_adjust_pct)
|
||
|
|
|
||
|
|
if (self.guardrail_threshold_pct is not None
|
||
|
|
and state.initial_portfolio > 0):
|
||
|
|
trigger_at = state.initial_portfolio * self.guardrail_threshold_pct
|
||
|
|
if state.portfolio < trigger_at:
|
||
|
|
proposed = proposed * (1.0 - self.guardrail_cut_pct)
|
||
|
|
|
||
|
|
return max(0.0, proposed)
|