Add a management command to dump xml courses

This command loads the courses with the XMLModuleStore, and then exports
the fields contents as json in a diff-friendly way. This can then be
used to validate against regressions in the xml parsing code.

lms/djangoapps/debug/management/commands/dump_xml_courses.py

+"""
+Export all xml courses in a diffable format.
+
+This command loads all of the xml courses in the configured DATA_DIR.
+For each of the courses, it loops through all of the modules, and dumps
+each as a separate output file containing the json representation
+of each of its fields (including those fields that are set as default values).
+"""
+
+from __future__ import print_function
+
+import json
+from path import path
+
+from django.core.management.base import BaseCommand, CommandError
+from django.conf import settings
+
+from xmodule.modulestore.xml import XMLModuleStore
+
+
+class Command(BaseCommand):
+    """
+    Django management command to export diffable representations of all xml courses
+    """
+    help = '''Dump the in-memory representation of all xml courses in a diff-able format'''
+    args = '<export path>'
+
+    def handle(self, *args, **options):
+        if len(args) != 1:
+            raise CommandError('Must called with arguments: {}'.format(self.args))
+
+        xml_module_store = XMLModuleStore(
+            data_dir=settings.DATA_DIR,
+            default_class='xmodule.hidden_module.HiddenDescriptor',
+            load_error_modules=True,
+            xblock_mixins=settings.XBLOCK_MIXINS,
+        )
+
+        export_dir = path(args[0])
+
+        for course_id, course_modules in xml_module_store.modules.iteritems():
+            course_path = course_id.replace('/', '_')
+            for location, descriptor in course_modules.iteritems():
+                location_path = location.url().replace('/', '_')
+                data = {}
+                for field_name, field in descriptor.fields.iteritems():
+                    try:
+                        data[field_name] = field.read_json(descriptor)
+                    except Exception as exc:  # pylint: disable=broad-except
+                        data[field_name] = {
+                            '$type': str(type(exc)),
+                            '$value': descriptor._field_data.get(descriptor, field_name)  # pylint: disable=protected-access
+                        }
+
+                outdir = export_dir / course_path
+                outdir.makedirs_p()
+                with open(outdir / location_path + '.json', 'w') as outfile:
+                    json.dump(data, outfile, sort_keys=True, indent=4)
+                    print('', file=outfile)