curpy.py

#!/usr/bin/env python3
from decimal import Decimal
import re
from urllib.request import urlopen
from xml.etree import ElementTree as etree

EUROFXREF_URL = 'https://www.ecb.europa.eu/stats/eurofxref/eurofxref-daily.xml'
XPATH_PATTERN = './/*[@currency]'
RATES = {}

_CONVERT_RE = r'\W*'.join([
    r'(?P<amount>\d+(\.\d+)?)',
    r'(?P<from_currency>[A-Z]{3})',
    r'->',
    r'(?P<to_currency>[A-Z]{3})',
])

def update_rates(url=EUROFXREF_URL, pattern=XPATH_PATTERN):
    with urlopen(url) as stream:
        tree = etree.parse(stream)
    for elem in tree.iterfind(pattern):
        RATES[elem.get('currency')] = Decimal(elem.get('rate'))
    RATES['EUR'] = Decimal(1)

def get_rate(currency):
    if not RATES:
        update_rates()
    if currency not in RATES:
        raise ValueError(f'Unknown currency: {currency}')
    return RATES[currency]

def get_currencies():
    if not RATES:
        update_rates()
    return sorted(RATES.keys())

def convert(amount, from_currency, to_currency):
    if from_currency != 'EUR':
        # All rates refer to Euro
        amount /= get_rate(from_currency)
    return amount * get_rate(to_currency)

def convert_string(string, format_spec='.2f', show_currency=False):
    match = re.match(_CONVERT_RE, string.strip().upper())
    if not match:
        raise ValueError('Invalid string format')
    params = match.groupdict()
    params['amount'] = Decimal(params['amount'])
    result = format(convert(**params), format_spec)
    if not show_currency:
        return result
    return f'{result} {params["to_currency"]}'


def main():
    all_currencies = ', '.join(get_currencies())
    print(f'Available currencies:\n{all_currencies}\n')
    print(f'1 EUR equals {get_rate("USD")} US Dollar or {get_rate("JPY")} Yen')
    print(f'42 US Dollar = {convert(42, "USD", "JPY"):.2f} Yen')
    print(f'50 Euro = {convert_string("50 EUR -> CNY")} Chinese Yuan')

if __name__ == '__main__':
    main()