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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 _set_size_code(self):
 """Set the code for a size operation. """

        if not self._op.startswith(self.SIZE):
            self._size_code = None
            return

        if len(self._op) == len(self.SIZE):
            self._size_code = self.SZ_EQ
        else:
            suffix = self._op[len(self.SIZE):]
            self._size_code = self.SZ_MAPPING.get(suffix, None)
            if self._size_code is None:
                raise ValueError('invalid "{}" suffix "{}"'.format(self.SIZE, suffix))