An example on how one can use the typings module to make currency conversion work like a charm. The code uses only python 3.7 features.

typed_currencies_37.py

import urllib.request
import json
import typing
import functools


SEK = typing.NewType('SEK',float)
EUR = typing.NewType('EUR', float)
ConversionRate = typing.NewType('ConversionRate',float)
ThreeLetterCurrencyCode = str # ISO4217
ApiResponse = typing.Dict

@functools.lru_cache(maxsize=10)
def get_api_response() -> ApiResponse:
    url = "https://api.exchangerate-api.com/v4/latest/" + "EUR"
    contents = urllib.request.urlopen(url).read()
    return json.loads(contents)

def extract_conversion_to(api_response: ApiResponse, to: ThreeLetterCurrencyCode) -> ConversionRate:
    return api_response['rates'][to]

def eur_to_sek(rate:ConversionRate,eur:EUR) -> SEK:
    return SEK(rate*eur)

full_response = get_api_response()
current_rate = extract_conversion_to(full_response,'SEK')

a = EUR(2.3)
b = eur_to_sek(current_rate,a)