code: def _extract_format_keys(self, fmt_string):
          
  
          class AccessSaver:
              def __init__(self):
                  self.keys = []
  
              def __getitem__(self, key):
                  self.keys.append(key)
  
          a = AccessSaver()
          fmt_string % a
  
          return a.keys
docs: Extract the keys of a format string
          (the 'stuff' in '%(stuff)s'