predict.py

#!/usr/bin/env python3
"""Prediction program: estimate a car's price from its mileage.

Prompts for a mileage (or takes it as an argument) and prints
``estimatePrice(mileage) = theta0 + theta1 * mileage``. Before training,
theta0 and theta1 are 0, so the estimate is 0.

Usage:
    python3 predict.py            # interactive prompt
    python3 predict.py 50000      # one-shot
"""

from __future__ import annotations

import math
import sys

from linear_regression import THETA_FILE, estimate_price, load_model


def read_mileage(argv: list[str]) -> float | None:
    """Return a valid non-negative mileage, or None on bad/absent input."""
    if len(argv) > 1:
        raw = argv[1]
    else:
        try:
            raw = input("Enter the mileage (km): ").strip()
        except EOFError:
            print()
            return None
    try:
        km = float(raw)
    except ValueError:
        print(f"predict: '{raw}' is not a valid number", file=sys.stderr)
        return None
    if not math.isfinite(km):
        print("predict: mileage must be a finite number", file=sys.stderr)
        return None
    if km < 0:
        print("predict: mileage cannot be negative", file=sys.stderr)
        return None
    return km


def main(argv: list[str] | None = None) -> int:
    argv = sys.argv if argv is None else argv
    model = load_model(THETA_FILE)
    km = read_mileage(argv)
    if km is None:
        return 1
    price = estimate_price(km, model.theta0, model.theta1)
    print(f"Estimated price for {km:.0f} km: {price:.2f}")
    return 0


if __name__ == "__main__":
    raise SystemExit(main())