How to multiply two field values in pyqgis?

Question

In a vector layer table there are two fields F1 and F2 with float type. I want to multiply these fields and insert results to new field F3. In below code new field created but without any value. What is problem?

layer = iface.activelayer()
F1 = self.ui.cBox_Width.currentText()
F2 = self.ui.cBox_Length.currentText()

new_Field = "F3"
provider = layer.dataProvider()
caps = provider.capabilities()

idx = provider.fieldNameIndex(new_Field)
try:
    if idx == -1:
        if caps & QgsVectorDataProvider.AddAttributes:
            res = provider.addAttributes([QgsField(new_Field, QVariant.Double)])
except:
    return False

layer.startEditing()
bufferfield = layer.fieldNameIndex('buffer_km')
formula = QgsExpression("\"F1\" * \"F2\"")
formula.prepare(layer.pendingFields())
for f in layer.getFeatures():
    f[idx] = formula.evaluate(f)
    layer.updateFeature(f)

layer.updateFields()
layer.commitChanges()

Answer 1

Try moving:

layer.updateFields()

before you start editing to ensure all newly created fields are recognised. You also need to replace:

f[idx] = formula.evaluate(f) 

with something like:

new_Field_idx = layer.fieldNameIndex(new_Field)
f[new_Field_idx] = formula.evaluate(f) 

as the old index would be -1 if it wasn't already created. And if you're calling your fields in your expression, you could use:

formula = QgsExpression(str(F1) + '*' + str(F2))

---

So you could try using something like the following:

layer = iface.activeLayer()
F1 = self.ui.cBox_Width.currentText()
F2 = self.ui.cBox_Length.currentText()

new_Field = "F3"
provider = layer.dataProvider()

idx = layer.fieldNameIndex(new_Field)
if idx == -1:
    layer.dataProvider().addAttributes([QgsField(new_Field, QVariant.Double)])

layer.updateFields()
layer.startEditing()
#bufferfield = layer.fieldNameIndex('buffer_km')
new_Field_idx = layer.fieldNameIndex(new_Field)
formula = QgsExpression(str(F1) + '*' + str(F2))
formula.prepare(layer.pendingFields())
for f in layer.getFeatures():
    f[new_Field_idx] = formula.evaluate(f)
    layer.updateFeature(f)

layer.commitChanges()