explain try-catch

src/werkzeug/datastructures/structures.py

@@ -129,6 +129,8 @@ def pop(self, key, default=_missing, type=None):
                     return None
                 return default
         try:
+            # This method is not meant to be thread-safe, but at least lets not
+            # fall over if the dict was mutated between the get and the delete.
             del self[key]
         except KeyError:
             pass