refactor: convert from_xml to parse_xml in CourseBlock

xmodule/course_module.py

@@ -6,7 +6,6 @@ Django module container for classes and operations related to the "Course Module
 import json
 import logging
 from datetime import datetime, timedelta
-from io import BytesIO
 
 import dateutil.parser
 import requests
@@ -1128,18 +1127,11 @@ class CourseBlock(
         return policy_str
 
     @classmethod
-    def from_xml(cls, xml_data, system, id_generator):
-        instance = super().from_xml(xml_data, system, id_generator)
-
-        # bleh, have to parse the XML here to just pull out the url_name attribute
-        # I don't think it's stored anywhere in the instance.
-        if isinstance(xml_data, str):
-            xml_data = xml_data.encode('ascii', 'ignore')
-        course_file = BytesIO(xml_data)
-        xml_obj = etree.parse(course_file, parser=edx_xml_parser).getroot()
+    def parse_xml(cls, node, runtime, keys, id_generator):
+        instance = super().parse_xml(node, runtime, keys, id_generator)
 
         policy_dir = None
-        url_name = xml_obj.get('url_name', xml_obj.get('slug'))
+        url_name = node.get('url_name')
         if url_name:
             policy_dir = 'policies/' + url_name
 
@@ -1149,9 +1141,9 @@ class CourseBlock(
             paths = [policy_dir + '/grading_policy.json'] + paths
 
         try:
-            policy = json.loads(cls.read_grading_policy(paths, system))
+            policy = json.loads(cls.read_grading_policy(paths, runtime))
         except ValueError:
-            system.error_tracker("Unable to decode grading policy as json")
+            runtime.error_tracker("Unable to decode grading policy as json")
             policy = {}
 
         # now set the current instance. set_grading_policy() will apply some inheritance rules