Backtest opening vs closing odds

import os
from fivedollarfootball import Client

client = Client(os.environ["FIVEDOLLARFOOTBALL_API_KEY"])
SEASON = "25/26"

leagues = client.leagues(search="Premier League", country="GB-ENG")
league = next(l for l in leagues if l["name"] == "England Premier League")

# 1. A season of results, each row with its opening and closing 1X2 odds.
matches = client.iter_all(
    client.league_fixtures, league_id=league["id"], season=SEASON,
    status="finished", include="odds", per_page=50,
)
rows = [m for m in matches if m["odds"]["1x2"]["opening"] and m["odds"]["1x2"]["closing"]]


def result(m):
    home, away = m["goals"]["home"], m["goals"]["away"]
    return "home" if home > away else "away" if away > home else "draw"


# 2. Four simple bets, one unit on every match.
bets = {
    "home team": lambda price: "home",
    "draw": lambda price: "draw",
    "away team": lambda price: "away",
    "favourite": lambda price: min(price, key=price.get),
}

# 3. Settle each bet twice: at the opening price and at the closing price.
print(f'{league["name"]} {SEASON}: {len(rows)} matches, one unit on each')
print(f'{"Back every":<12}{"At opening":>12}{"At closing":>12}')
for name, pick in bets.items():
    returns = []
    for stage in ("opening", "closing"):
        profit = 0
        for m in rows:
            price = m["odds"]["1x2"][stage]
            side = pick(price)
            profit += price[side] - 1 if side == result(m) else -1
        returns.append(profit / len(rows))
    print(f'{name:<12}{returns[0]:>+12.1%}{returns[1]:>+12.1%}')

England Premier League 25/26: 380 matches, one unit on each
Back every    At opening  At closing
home team          -9.7%       -8.5%
draw               +3.0%       +6.4%
away team          -9.0%      -11.4%
favourite         -10.9%       -8.6%