how to update a column

Hello,

I want to update only one colomn ('cout') of a PostgreSQL table (table name:
aretes; colomns: gid, long, vit, acces, cout). For each line of the table
I calculate the 'cout' attribut with Python, then I want to update the
'cout'attribut of each line with the new value. I tried with the update
function
of the pg module, but it wasn't sucessfull.

Can anyone give me some help or an example of an update command?

Thanks
Daniela