Adding calculated fields with PyQGIS

Question

In the Memory provider described in PyQGIS Cookbook and in the question Creating new empty vector layer with PyQGIS I can see how to create a Vector Layer programmatically using standard fields (String, Int, Double).

With PyQGIS/QGIS 1.8.0, is it possible to programmatically add calculated fields like the ones defined using the Field Calculator?

After reading about QgsExpression I'm thinking in something like this:

exp = QgsExpression('Column * 3')
pr.addAttributes( [ QgsField( "fieldName", QVariant.Expression, exp ) ] )

• Matthias Kuhn: Is you question about adding the attribute, modifying the value of an attribute or about the expression syntax/parser present in the field calculator? Could you outline a short example of what you want to accomplish?
• rpet: Is about adding a new attribute defined with the parser present in the field calculator. I've edited the question to add an example that tries to clarify what I want to achieve.
• Taras: Please, do not forget about "What should I do when someone answers my question?"

Answer 1

To accomplish this, you have to take a two-step approach. The first step is to create the field, the second one is to loop over all your features, evaluate the expression and update the attribute for each feature.

vl is your QgsVectorLayer

QGIS 2.0

from PyQt4.QtCore import QVariant
from qgis.core import QgsField, QgsExpression, QgsFeature
vl.startEditing()
#step 1
myField = QgsField( 'myattr', QVariant.Int )
vl.addAttribute( myField )
idx = vl.fieldNameIndex( 'myattr' )
#step 2
e = QgsExpression( 'Column * 3' )
e.prepare( vl.pendingFields() )
for f in vl.getFeatures():
 f[idx] = e.evaluate( f )
 vl.updateFeature( f )
vl.commitChanges()

QGIS 1.8

from PyQt4.QtCore import QVariant
from qgis.core import QgsField, QgsExpression, QgsFeature
vl.startEditing()
#step 1
myField = QgsField( 'myattr', QVariant.Int )
vl.addAttribute( myField )
idx = vl.fieldNameIndex( 'myattr' )
#step 2
e = QgsExpression( 'Column * 3' )
e.prepare( vl.pendingFields() )
f = QgsFeature()
vl.select( vl.pendingAllAttributesList() )
while vl.nextFeature( f ):
 vl.changeAttributeValue( f.id(), idx, e.evaluate( f ) )
vl.commitChanges()