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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 data(self):
        '''Returns data formatted for a POST request to the Postmark send API.

        :rtype: `dict`
        '''
        d = {}
        for val, key in self._fields.items():
            val = getattr(self, val)
            if val is not None:
                d[key] = val
        return d