<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def mark_dirty(self):
 """Invalidates memoized fingerprints for this payload. Exposed for testing. :API: public """

    self._fingerprint_memo_map = {}
    for field in self._fields.values():
      field.mark_dirty()