How to use the agate.warns.warn_null_calculation function in agate

def validate(self, table):
        column = table.columns[self._column_name]

        if not isinstance(column.data_type, Number):
            raise DataTypeError('Percentiles can only be applied to columns containing Number data.')

        has_nulls = HasNulls(self._column_name).run(table)

        if has_nulls:
            warn_null_calculation(self, column)

def validate(self, table):
        before_column = table.columns[self._before_column_name]
        after_column = table.columns[self._after_column_name]

        for data_type in (Number, Date, DateTime, TimeDelta):
            if isinstance(before_column.data_type, data_type):
                if not isinstance(after_column.data_type, data_type):
                    raise DataTypeError('Specified columns must be of the same type')

                if HasNulls(self._before_column_name).run(table):
                    warn_null_calculation(self, before_column)

                if HasNulls(self._after_column_name).run(table):
                    warn_null_calculation(self, after_column)

                return

        raise DataTypeError('Change before and after columns must both contain data that is one of: Number, Date, DateTime or TimeDelta.')

def validate(self, table):
        column = table.columns[self._column_name]
        if not isinstance(column.data_type, Number):
            raise DataTypeError('Percent column must contain Number data.')
        if self._total is not None and self._total <= 0:
            raise DataTypeError('The total must be a positive number')
        # Throw a warning if there are nulls in there
        if HasNulls(self._column_name).run(table):
            warn_null_calculation(self, column)

def validate(self, table):
        column = table.columns[self._column_name]

        if not isinstance(column.data_type, Number):
            raise DataTypeError('Percent column must contain Number data.')
        if self._total is not None and self._total <= 0:
            raise DataTypeError('The total must be a positive number')

        # Throw a warning if there are nulls in there
        if HasNulls(self._column_name).run(table):
            warn_null_calculation(self, column)

def validate(self, table):
        before_column = table.columns[self._before_column_name]
        after_column = table.columns[self._after_column_name]

        if not isinstance(before_column.data_type, Number):
            raise DataTypeError('PercentChange before column must contain Number data.')

        if not isinstance(after_column.data_type, Number):
            raise DataTypeError('PercentChange after column must contain Number data.')

        if HasNulls(self._before_column_name).run(table):
            warn_null_calculation(self, before_column)

        if HasNulls(self._after_column_name).run(table):
            warn_null_calculation(self, after_column)

def validate(self, table):
        column = table.columns[self._column_name]

        if not isinstance(column.data_type, Number):
            raise DataTypeError('StDev can only be applied to columns containing Number data.')

        has_nulls = HasNulls(self._column_name).run(table)

        if has_nulls:
            warn_null_calculation(self, column)

def validate(self, table):
        column = table.columns[self._column_name]

        if not isinstance(column.data_type, Number):
            raise DataTypeError('PopulationVariance can only be applied to columns containing Number data.')

        has_nulls = HasNulls(self._column_name).run(table)

        if has_nulls:
            warn_null_calculation(self, column)

def validate(self, table):
        before_column = table.columns[self._before_column_name]
        after_column = table.columns[self._after_column_name]

        for data_type in (Number, Date, DateTime, TimeDelta):
            if isinstance(before_column.data_type, data_type):
                if not isinstance(after_column.data_type, data_type):
                    raise DataTypeError('Specified columns must be of the same type')

                if HasNulls(self._before_column_name).run(table):
                    warn_null_calculation(self, before_column)

                if HasNulls(self._after_column_name).run(table):
                    warn_null_calculation(self, after_column)

                return

        raise DataTypeError('Change before and after columns must both contain data that is one of: Number, Date, DateTime or TimeDelta.')

def validate(self, table):
        column = table.columns[self._column_name]

        if not isinstance(column.data_type, Number):
            raise DataTypeError('MAD can only be applied to columns containing Number data.')

        has_nulls = HasNulls(self._column_name).run(table)

        if has_nulls:
            warn_null_calculation(self, column)

def validate(self, table):
        column = table.columns[self._column_name]

        if not isinstance(column.data_type, Number):
            raise DataTypeError('Deciles can only be applied to columns containing Number data.')

        has_nulls = HasNulls(self._column_name).run(table)

        if has_nulls:
            warn_null_calculation(self, column)