I am working With Odoo ERP and I want to restrict for return None Value from the List of Dictionary from the generate SQL Query.

form_data = self.read(cr, uid, ids, ['start_date','end_date'], context=context)[0]
sql = "select i.date_invoice, i.number, (select name from res_partner where id=i.partner_id) as partner,i.currency_id, (select name from res_currency where id=i.currency_id) as currency, (select description from account_tax where name=t.name), t.amount, t.base, (t.amount+t.base) as total from account_invoice i, account_invoice_tax t where t.invoice_id = i.id and i.state = 'open' and i.type = 'out_invoice' and i.date_invoice >= '%s' and i.date_invoice <= '%s' order by t.name"%(form_data['start_date'],form_data['end_date']) 

cr.execute(sql)
data = cr.dictfetchall()

Generated Result :

data=[

{'currency_id': 38, 'description': u'GST 7%', 'number': u'SAJ/2015/0021', 'date_invoice': '2015-05-05', 'currency': u'SGD', 'amount': 283.08, 'base': None, 'partner': u'partner1', 'total': None},

{'currency_id': 38, 'description': None, 'number': u'SAJ/2015/0021', 'date_invoice': '2015-05-05', 'currency': u'SGD', 'amount': 283.08, 'base': 4044.0, 'partner': u'partner1', 'total': 4327.08},

{'currency_id': 38, 'description': None, 'number': u'SAJ/2015/0020', 'date_invoice': '2015-05-04', 'currency': u'SGD', 'amount': 0.0, 'base': 3550.0, 'partner': u'EAST MARINE PTE LTD', 'total': 3550.0},

{'currency_id': 38, 'description': None, 'number': u'SAJ/2015/0021', 'date_invoice': '2015-05-05', 'currency': u'SGD', 'amount': 0.0, 'base': 500.0, 'partner': u'partner1', 'total': 500.0},

{'currency_id': 38, 'description': None, 'number': u'SAJ/2015/0023', 'date_invoice': '2015-05-05', 'currency': u'SGD', 'amount': 0.0, 'base': 11100.0, 'partner': u'partner2', 'total': 11100.0}

]

Above Generated List of Dictionary Values found the description and total keys of values got the None result using above SQL Query.

I do not want None Value but instead of None i want to blank string or other blank space(empty string).

What should i do for make restriction on None Value. ?