Lazily load nerc-rates to avoid Python 3.9 incompat

Author: knikolla

src/coldfront_plugin_cloud/management/commands/calculate_storage_gb_hours.py

@@ -14,11 +14,25 @@
 from coldfront.core.allocation.models import Allocation, AllocationStatusChoice
 import pytz
 
-from nerc_rates import load_from_url
 
 logging.basicConfig(level=logging.INFO)
 logger = logging.getLogger(__name__)
 
+_RATES = None
+
+
+def get_rates():
+    # nerc-rates doesn't work with Python 3.9, which is what ColdFront is currently
+    # using in Production. Lazily load the rates only when either of the storage rates
+    # is not set via CLI arguments, so we can keep providing them via CLI until we upgrade
+    # Python version.
+    global _RATES
+
+    if _RATES is None:
+        from nerc_rates import load_from_url
+        _RATES = load_from_url()
+    return _RATES
+
 
 @dataclasses.dataclass
 class InvoiceRow:
@@ -200,20 +214,18 @@ def process_invoice_row(allocation, attrs, su_name, rate):
             resources__in=openshift_resources
         )
 
-        rates = load_from_url()
-        
         if options['openstack_gb_rate']:
             openstack_storage_rate = options['openstack_gb_rate']
         else:
             openstack_storage_rate = Decimal(
-                rates.get_value_at('Storage GB Rate', options["invoice_month"])
+                get_rates().get_value_at('Storage GB Rate', options["invoice_month"])
             )
 
         if options['openshift_gb_rate']:
             openshift_storage_rate = options['openshift_gb_rate']
         else:
             openshift_storage_rate = Decimal(
-                rates.get_value_at('Storage GB Rate', options["invoice_month"])
+                get_rates().get_value_at('Storage GB Rate', options["invoice_month"])
             )
 
         logger.info(f'Using storage rate {openstack_storage_rate} (Openstack) and '