Use cached FX rate if fresh rate not available

agenda/fx.py

@@ -44,6 +44,17 @@ async def get_gbpusd(config: flask.config.Config) -> Decimal:
     return typing.cast(Decimal, 1 / data["quotes"]["USDGBP"])
 
 
+def read_cached_rates(filename: str, currencies: list[str]) -> dict[str, Decimal]:
+    """Read FX rates from cache."""
+    with open(filename) as file:
+        data = json.load(file, parse_float=Decimal)
+    return {
+        cur: Decimal(data["quotes"][f"GBP{cur}"])
+        for cur in currencies
+        if f"GBP{cur}" in data["quotes"]
+    }
+
+
 def get_rates(config: flask.config.Config) -> dict[str, Decimal]:
     """Get current values of exchange rates for a list of currencies against GBP."""
     currencies = config["CURRENCIES"]
@@ -65,22 +76,19 @@ def get_rates(config: flask.config.Config) -> dict[str, Decimal]:
         recent = datetime.strptime(recent_filename[:16], "%Y-%m-%d_%H:%M")
         delta = now - recent
 
+        full_path = os.path.join(fx_dir, recent_filename)
         if delta < timedelta(hours=12):
-            full_path = os.path.join(fx_dir, recent_filename)
-            with open(full_path) as file:
-                data = json.load(file, parse_float=Decimal)
-            return {
-                cur: Decimal(data["quotes"][f"GBP{cur}"])
-                for cur in currencies
-                if f"GBP{cur}" in data["quotes"]
-            }
+            return read_cached_rates(full_path, currencies)
 
     url = "http://api.exchangerate.host/live"
     params = {"currencies": currency_string, "source": "GBP", "access_key": access_key}
 
     filename = f"{now_str}_{file_suffix}"
-    with httpx.Client() as client:
-        response = client.get(url, params=params)
+    try:
+        with httpx.Client() as client:
+            response = client.get(url, params=params)
+    except httpx.ConnectError:
+        return read_cached_rates(full_path, currencies)
 
     with open(os.path.join(fx_dir, filename), "w") as file:
         file.write(response.text)