Generating attributes in an automatic way with QGIS?

If you right-click your layer and go to Properties > Fields then click the Text Edit, you can set Default values (either by typing in a value or using an expression) which will appear in the attribute table automatically.

Unfortunately, using an expression like $rownum (which should get you unique ids for your features) does not work (my guess is because the feature has to be committed first?). But we can create a function which counts the number of features made and adds 1. You can access the Function Editor as shown in the image:

Then in the Function Editor, create a new file or edit an existing one and use the following code:

from qgis.core import *
from qgis.gui import *

@qgsfunction(args='auto', group='Custom')
def update(feature, parent):
    layer = qgis.utils.iface.activeLayer()
    x = layer.featureCount()
    return x + 1

Click Load then go to the Expression tab next to it and insert the expression:

update()

Then click OK and you should see a preview value:

Click OK, Apply etc and now hopefully when you create a feature, it will automatically create a unique id in your chosen field.

You can use the plugin "AutoFields" for Automatic attribute updates when creating or modifying vector features

I think you can not set default values when creating a new feature but you could try the following work around:

• when done creating your features go to the attribute table
• Go to the field calculator and create a new column called id with type Int
• As value insert @row_number which will yield unique vales according to the row number