<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def unpatch(self):
 """un-applies this patch."""

        if not self._patched:
            return
        for func in self._read_compilers + self._write_compilers:
            func.execute_sql = self._original[func]
        self.cache_backend.unpatch()
        self._patched = False