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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _update_length(self):
 """Update the length field of the struct."""

        action_length = 4 + len(self.field.pack())
        overflow = action_length % 8
        self.length = action_length
        if overflow:
            self.length = action_length + 8 - overflow