Download exchange rates

xrates.py

# RA, 2020-04-30

# Download exchange rates -- template

import json
import urllib.request
from retry import retry
from pathlib import Path


@retry(Exception, tries=2)
def get_exchange_rates(symbols: set, base="USD", path=Path("rates.txt")):
	"""
	Download exchange rates or load from file if existent
	"""

	try:
		# Use the old rates file, if available
		# Note: delete the file to update the exchange rates
		rates = json.load(path.open(mode='r'))
	except (json.decoder.JSONDecodeError, FileNotFoundError):
		# Re-download the exchange rates
		url = "https://api.exchangeratesapi.io/latest?symbols={symbols}&base={base}"
		url = url.format(symbols=(",".join(symbols)), base=base)
		data = urllib.request.urlopen(url).read().decode()
		# The data has the following format:
		# rates = {"rates": {"CHF": 0.9750046117, "EUR": 0.9223390518}, "base": "USD", "date": "2020-04-29"}
		rates = json.loads(data)
		json.dump(rates, path.open(mode='w'))

	return rates['rates']