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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 _serialize(self, value, attr, obj, **kwargs):
 """Return a string if `self.as_string=True`, otherwise return this field's `num_type`."""

        ret = self._validated(value)
        return (
            self._to_string(ret)
            if (self.as_string and ret not in (None, missing_))
            else ret
        )