refactor currency converter

to allow multiple exchange rate getters defaulting to user defined csv file or hard coded exchange rates
spiral-project · Oct 28, 2024 · 23d5b73 · 23d5b73
1 parent 6582e2c
commit 23d5b73
Showing 1 changed file with 85 additions and 188 deletions.
diff --git a/ihatemoney/currency_convertor.py b/ihatemoney/currency_convertor.py
@@ -1,10 +1,17 @@
+import csv
 import traceback
 import warnings
+from abc import ABC, abstractmethod
+from decimal import Decimal
+from typing import Dict, Optional
 
 from cachetools import TTLCache, cached
 import requests
 
 
+NO_CURRENCY = "XXX"
+ExchangeRates = Dict[str, float]
+
 class Singleton(type):
     _instances = {}
 
@@ -14,205 +21,93 @@ def __call__(cls, *args, **kwargs):
         return cls._instances[cls]
 
 
-class CurrencyConverter(object, metaclass=Singleton):
-    # Get exchange rates
-    no_currency = "XXX"
+class ExchangeRateGetter(ABC):
+
+    def get_rates(self) -> Optional[ExchangeRates]:
+        """Method to retrieve a list of currency conversion rates.
+
+        Returns:
+            currencies: dict - key is a three-letter currency, value is float of conversion to base currency
+        """
+        try:
+            return self._get_rates()
+        except Exception:
+            warnings.warn(
+                f"Exchange rate getter failed - {traceback.format_exc(limit=0).strip()}"
+            )
+
+    @abstractmethod
+    def _get_rates(self) -> Optional[ExchangeRates]:
+        """Actual implementation of the exchange rate getter."""
+        raise NotImplementedError
+
+
+class ApiExchangeRate(ExchangeRateGetter):
     api_url = "https://api.exchangerate.host/latest?base=USD"
 
+    def _get_rates(self) -> Optional[ExchangeRates]:
+        return requests.get(self.api_url).json()["rates"]  # TODO not working currently probably
+
+
+class UserExchangeRate(ExchangeRateGetter):
+    user_csv_file = "path/to/file.csv"
+
+    def _get_rates(self) -> Optional[ExchangeRates]:
+        """Get rates from user defined csv.
+
+        The user_csv_file should contain the currency conversions to "USD" without 1 header row
+        Example:
+        ```
+        currency_code,fx_rate_to_USD
+        CZK,25.0
+        ...
+        ```
+
+        TODO: make it work bi-directionally
+        TODO: document for the user where to place the file
+        """
+        reader = csv.reader(self.user_csv_file)
+
+        rates = {}
+        for row in reader:
+            from_currency = row[0]
+            rate = float(row[1])
+            # TODO add validation and exception handling for typos
+            rates[from_currency] = rate
+        return rates
+
+
+class HardCodedExchangeRate(ExchangeRateGetter):
+
+    def _get_rates(self) -> Optional[dict]:
+        return {"USD": 1.0}  # TODO fill in more
+
+
+class CurrencyConverter(object, metaclass=Singleton):
+    no_currency = NO_CURRENCY
+
     def __init__(self):
         pass
 
     @cached(cache=TTLCache(maxsize=1, ttl=86400))
     def get_rates(self):
-        try:
-            rates = requests.get(self.api_url).json()["rates"]
-        except Exception:
-            warnings.warn(
-                f"Call to {self.api_url} failed: {traceback.format_exc(limit=0).strip()}"
-            )
-            # In case of any exception, let's have an empty value
+        """Try to retrieve the exchange rate from various sources, defaulting to hard coded values."""
+        for provider in [ApiExchangeRate, UserExchangeRate, HardCodedExchangeRate]:
+            if rates:= provider.get_rates():
+                break
+        else:
             rates = {}
-        rates[self.no_currency] = 1.0
+        rates[NO_CURRENCY] = 1.0
         return rates
 
-    def get_currencies(self, with_no_currency=True):
-        currencies = [
-            "AED",
-            "AFN",
-            "ALL",
-            "AMD",
-            "ANG",
-            "AOA",
-            "ARS",
-            "AUD",
-            "AWG",
-            "AZN",
-            "BAM",
-            "BBD",
-            "BDT",
-            "BGN",
-            "BHD",
-            "BIF",
-            "BMD",
-            "BND",
-            "BOB",
-            "BRL",
-            "BSD",
-            "BTC",
-            "BTN",
-            "BWP",
-            "BYN",
-            "BZD",
-            "CAD",
-            "CDF",
-            "CHF",
-            "CLF",
-            "CLP",
-            "CNH",
-            "CNY",
-            "COP",
-            "CRC",
-            "CUC",
-            "CUP",
-            "CVE",
-            "CZK",
-            "DJF",
-            "DKK",
-            "DOP",
-            "DZD",
-            "EGP",
-            "ERN",
-            "ETB",
-            "EUR",
-            "FJD",
-            "FKP",
-            "GBP",
-            "GEL",
-            "GGP",
-            "GHS",
-            "GIP",
-            "GMD",
-            "GNF",
-            "GTQ",
-            "GYD",
-            "HKD",
-            "HNL",
-            "HRK",
-            "HTG",
-            "HUF",
-            "IDR",
-            "ILS",
-            "IMP",
-            "INR",
-            "IQD",
-            "IRR",
-            "ISK",
-            "JEP",
-            "JMD",
-            "JOD",
-            "JPY",
-            "KES",
-            "KGS",
-            "KHR",
-            "KMF",
-            "KPW",
-            "KRW",
-            "KWD",
-            "KYD",
-            "KZT",
-            "LAK",
-            "LBP",
-            "LKR",
-            "LRD",
-            "LSL",
-            "LYD",
-            "MAD",
-            "MDL",
-            "MGA",
-            "MKD",
-            "MMK",
-            "MNT",
-            "MOP",
-            "MRU",
-            "MUR",
-            "MVR",
-            "MWK",
-            "MXN",
-            "MYR",
-            "MZN",
-            "NAD",
-            "NGN",
-            "NIO",
-            "NOK",
-            "NPR",
-            "NZD",
-            "OMR",
-            "PAB",
-            "PEN",
-            "PGK",
-            "PHP",
-            "PKR",
-            "PLN",
-            "PYG",
-            "QAR",
-            "RON",
-            "RSD",
-            "RUB",
-            "RWF",
-            "SAR",
-            "SBD",
-            "SCR",
-            "SDG",
-            "SEK",
-            "SGD",
-            "SHP",
-            "SLL",
-            "SOS",
-            "SRD",
-            "SSP",
-            "STD",
-            "STN",
-            "SVC",
-            "SYP",
-            "SZL",
-            "THB",
-            "TJS",
-            "TMT",
-            "TND",
-            "TOP",
-            "TRY",
-            "TTD",
-            "TWD",
-            "TZS",
-            "UAH",
-            "UGX",
-            "USD",
-            "UYU",
-            "UZS",
-            "VEF",
-            "VES",
-            "VND",
-            "VUV",
-            "WST",
-            "XAF",
-            "XAG",
-            "XAU",
-            "XCD",
-            "XDR",
-            "XOF",
-            "XPD",
-            "XPF",
-            "XPT",
-            "YER",
-            "ZAR",
-            "ZMW",
-            "ZWL",
-        ]
+    def get_currencies(self, with_no_currency: bool=True) -> list:
+        currencies = list(self.get_rates.keys())
         if with_no_currency:
             currencies.append(self.no_currency)
         return currencies
 
-    def exchange_currency(self, amount, source_currency, dest_currency):
+    def exchange_currency(self, amount: float, source_currency: str, dest_currency: str) -> float:
+        """Return the money amount converted from source_currency to dest_currency."""
         if (
             source_currency == dest_currency
             or source_currency == self.no_currency
@@ -223,6 +118,8 @@ def exchange_currency(self, amount, source_currency, dest_currency):
         rates = self.get_rates()
         source_rate = rates[source_currency]
         dest_rate = rates[dest_currency]
-        new_amount = (float(amount) / source_rate) * dest_rate
-        # round to two digits because we are dealing with money
-        return round(new_amount, 2)
+        # Using Decimal to not introduce floating-point operation absolute errors
+        new_amount = (Decimal(amount) / Decimal(source_rate)) * Decimal(dest_rate)
+        # dealing with money - only round the shown amount before showing it to user
+        # - think about 10 * 0.0003 == 0?
+        return float(new_amount)