Python Currency Converter Project

"""Convert currencies with the public Frankfurter v2 exchange-rate API."""

from __future__ import annotations

import json
from decimal import Decimal, InvalidOperation, ROUND_HALF_UP
from urllib.error import HTTPError, URLError
from urllib.request import Request, urlopen


API_ROOT = "https://api.frankfurter.dev/v2"


def read_currency(prompt: str) -> str:
    while True:
        code = input(prompt).strip().upper()
        if len(code) == 3 and code.isascii() and code.isalpha():
            return code
        print("Enter a three-letter currency code such as INR, USD, or EUR.")


def read_amount() -> Decimal:
    while True:
        try:
            amount = Decimal(input("Amount: ").strip())
            if not amount.is_finite() or amount < 0:
                raise InvalidOperation
            return amount
        except InvalidOperation:
            print("Enter a valid zero or positive amount.")


def fetch_rate(base: str, quote: str) -> tuple[Decimal, str]:
    """Return the latest rate and provider date for one currency pair."""
    if base == quote:
        return Decimal("1"), "same currency"

    url = f"{API_ROOT}/rate/{base}/{quote}"
    request = Request(url, headers={"User-Agent": "SoftenantCurrencyProject/1.0"})
    try:
        with urlopen(request, timeout=10) as response:
            data = json.loads(response.read().decode("utf-8"), parse_float=Decimal)
        return Decimal(data["rate"]), str(data["date"])
    except HTTPError as error:
        if error.code == 422:
            raise ValueError("The API did not recognise one of those currency codes.") from error
        raise ConnectionError(f"The rate service returned HTTP {error.code}.") from error
    except (URLError, TimeoutError) as error:
        raise ConnectionError("Could not reach the exchange-rate service.") from error
    except (json.JSONDecodeError, KeyError, TypeError, InvalidOperation) as error:
        raise ConnectionError("The rate service returned an unexpected response.") from error


def convert(amount: Decimal, rate: Decimal) -> Decimal:
    return (amount * rate).quantize(Decimal("0.01"), rounding=ROUND_HALF_UP)


def main() -> None:
    print("Python Currency Converter")
    print("Rates are reference rates, not bank or card transaction quotes.\n")

    base = read_currency("From currency: ")
    quote = read_currency("To currency: ")
    amount = read_amount()

    try:
        rate, rate_date = fetch_rate(base, quote)
    except (ValueError, ConnectionError) as error:
        print(f"Conversion failed: {error}")
        return

    result = convert(amount, rate)
    print(f"\n{amount:.2f} {base} = {result:.2f} {quote}")
    print(f"Reference rate: 1 {base} = {rate} {quote} ({rate_date})")


if __name__ == "__main__":
    main()