exchangerates.py

from bisect import bisect_left
import csv
import datetime
import pkg_resources

class UnknownCurrencyException(Exception):
    pass

def make_date_from_iso(iso_str):
    return datetime.date(int(iso_str[:4]), int(iso_str[5:7]),
                            int(iso_str[8:10]))

def take_closest(myList, myNumber):
    # Source: http://stackoverflow.com/a/12141511
    """
    Assumes myList is sorted. Returns closest value to myNumber.
    If two numbers are equally close, return the smallest number.
    """
    pos = bisect_left(myList, myNumber)
    if pos == 0:
        return myList[0]
    if pos == len(myList):
        return myList[-1]
    before = myList[pos - 1]
    after = myList[pos]
    if after - myNumber < myNumber - before:
       return after
    else:
       return before

class CurrencyConverter(object):
    def __init__(self, source=None):
        def load_rates():
            """
            Read CSV file as generator function
            """
            if not source:
                resource_package = __name__ 
                resource_path = '/'.join(('data', 'consolidated_rates.csv'))
                data = pkg_resources.resource_stream(resource_package, resource_path)
            else:
                data = open(source, "rU")
            csv_reader = csv.reader(data)
            csv_reader.next()
            for row in csv_reader:
                yield row

        def make_rates(rates_list):
            """
            Sort rates into nice dictionary of currency: dates
            """
            def append_path(root, paths):
                child = root.setdefault(paths[0], {})
                root[paths[0]].update(paths[1])
            rates_dates = {}
            for row in rates_list:
                append_path(rates_dates, (row[2],
                    { make_date_from_iso(row[0]): row[1] } ))
            currencies_dates = dict(map(lambda currency: 
                            (currency, sorted(list(rates_dates[currency]))), 
                            rates_dates.keys()))
            return currencies_dates, rates_dates

        self.currencies_dates, self.dates_rates = make_rates(load_rates())

    def known_currencies(self):
        return ",".join(sorted(self.currencies_dates.keys()))

    def closest_rate(self, currency, date):
        """
        Accepts a list with (currency, date)
        returns currency, date, conversion date, exchange rate
        """
        if currency == u"USD": 
            return { "closest_date": date, "conversion_rate": 1.0 }
        try:
            the_date = take_closest(self.currencies_dates[currency], date)
            return {
                "closest_date": the_date, 
                "conversion_rate": self.dates_rates[currency][the_date]
            }
        except KeyError:
            raise UnknownCurrencyException("Unknown currency: {}".format(
                currency))
        
if __name__ == "__main__":
    """
    Example output
    """
    converter = CurrencyConverter()
    print "Available currencies: {}".format(converter.known_currencies())
    print converter.closest_rate("USD", datetime.date(2012,7,20))
    print converter.closest_rate("EUR", datetime.date(2014,7,20))
    print converter.closest_rate("EUR", datetime.date(2014,7,20))