Added an explanatory comment. Refs #22017

django/utils/autoreload.py

@@ -81,6 +81,9 @@ def gen_filenames():
     Yields a generator over filenames referenced in sys.modules and translation
     files.
     """
+    # N.B. ``list(...)`` is needed, because this runs in parallel with
+    # application code which might be mutating ``sys.modules``, and this will
+    # fail with RuntimeError: cannot mutate dictionary while iterating
     filenames = [filename.__file__ for filename in list(sys.modules.values())
                 if hasattr(filename, '__file__')]