[web2py] How to rename column name of a projection on select

[Rafael Sales]

Hello,
How can I define the column name of a field which is a projection
(like sum()) on select?
Examplo:
        query = ((db.despesa.id_servico == db.servico.id))

        total_gasto = (db.despesa.valor_servico *
db.despesa.quantidade).sum()
        linhas = db(query).select(db.servico.descricao, total_gasto, groupby
= db.servico.id)

The return "linhas" have a column with name
'SUM((despesa.valor_servico * despesa.quantidade))'
When iteration by linhas array, how can I get the values of this
column?

Thanks