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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 self_build(self, field_pos_list=None):
 # type: (Any) -> str """self_build is overridden because type and len are determined at build time, based on the "data" field internal type """

        if self.getfieldval('type') is None:
            self.type = 1 if isinstance(self.getfieldval('data'), HPackZString) else 0  # noqa: E501
        return super(HPackHdrString, self).self_build(field_pos_list)