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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 _calculate(self, field):
        '''
        If the offset is unknown, return 0
        '''
        base_offset = 0
        if self.base_field is not None:
            base_offset = self.base_field.offset
        target_offset = self._field.offset
        if (target_offset is None) or (base_offset is None):
            return 0
        return target_offset - base_offset