Script for tracking fuel usage and efficiency. I use this on my iPhone within Pythonista so it's convenient to use right at after filling up.

fuel_tracker.py

import csv
from datetime import datetime
from typing import Optional

STARTING_ODOMETER_READING = 188590
LOG_FILE = "fuel_log.csv"


def log_fuel_efficiency(
    current_miles: int,
    gallons: float,
    total_paid: float,
    filename: str = LOG_FILE,
) -> tuple[Optional[float], Optional[float]]:
    """
    Logs a fuel efficiency entry to a CSV file.

    Parameters
    ----------
    current_miles : int
        Current odometer reading.
    gallons : float
        Number of gallons of fuel purchased.
    total_paid : float
        Total amount paid in USD.
    filename : str, optional
        Name of the file to write to.

    Returns
    -------
    tuple
        A tuple of two values: the calculated MPG and price per gallon.
    """
    try:
        with open(filename, "r") as f:
            reader = csv.DictReader(f)
            rows = list(reader)
            start_miles = (
                int(rows[-1]["End Miles"]) if rows else STARTING_ODOMETER_READING
            )
    except (FileNotFoundError, KeyError):
        start_miles = STARTING_ODOMETER_READING

    miles_driven = current_miles - start_miles
    mpg = round(miles_driven / gallons, 1) if gallons else 0
    price_per_gallon = round(total_paid / gallons, 3) if gallons else 0

    # Get current month for cost tracking
    now = datetime.now()
    current_month = now.strftime("%Y-%m")

    new_row = [
        now.strftime("%Y-%m-%d %H:%M:%S"),
        start_miles,
        current_miles,
        gallons,
        mpg,
        price_per_gallon,
        total_paid,
        current_month,
    ]

    try:
        with open(filename, "a", newline="") as f:
            writer = csv.writer(f)
            if f.tell() == 0:
                writer.writerow(
                    [
                        "Timestamp",
                        "Start Miles",
                        "End Miles",
                        "Gallons",
                        "MPG",
                        "Price per Gallon",
                        "Total Paid",
                        "Month",
                    ]
                )
            writer.writerow(new_row)
        return mpg, price_per_gallon
    except Exception as e:
        print(f"Error saving data: {e}")
        return None, None


def calculate_cost_stats(filename: str = LOG_FILE) -> tuple[float, float]:
    """
    Calculates cost statistics for the current month.

    Parameters
    ----------
    filename : str, optional
        Name of the file to read from.

    Returns
    -------
    tuple
        A tuple of two values: the total cost for the month and the average cost per week.
    """
    now = datetime.now()
    current_month = now.strftime("%Y-%m")
    day_of_month = now.day
    weeks_in_month_so_far = max(1, (day_of_month - 1) // 7 + 1)

    try:
        with open(filename, "r") as f:
            reader = csv.DictReader(f)
            monthly_data = [row for row in reader if row.get("Month") == current_month]
            total_cost = sum(float(row["Total Paid"]) for row in monthly_data)
            avg_cost_per_week = total_cost / weeks_in_month_so_far
            return round(total_cost, 2), round(avg_cost_per_week, 2)
    except (FileNotFoundError, KeyError):
        print("No historical data found")
        return 0, 0


def main():
    """
    Prompts user for odometer reading, gallons consumed, and cost.

    Logs the fuel efficiency to a CSV file.

    Calculates and prints the current MPG and price per gallon.

    Calculates and prints the total cost for the month and average cost per week.

    """
    odometer = int(input("Odometer: "))
    gallons = float(input(" Gallons: "))
    cost = float(input("    Cost: "))

    mpg, price_per_gallon = log_fuel_efficiency(odometer, gallons, cost)
    print()
    print(f"     Current MPG: {mpg}")
    print(f"Price per gallon: ${price_per_gallon:.2f}")

    monthly_total, avg_weekly = calculate_cost_stats()
    print(f"Total this month: ${monthly_total:.2f}")
    print(f"  Weekly average: ${avg_weekly}")


if __name__ == "__main__":
    main()