Create New field equal to existing field - convert to integer - PyQGIS 3.4

Question

I'm using PyQGIS 3.4, I am trying to add a new field and set the values equal to an existing field. The existing field are population totals but are in string format, I want to convert these to integers or real numbers.

I can add the new field however the problem is adding values to the field.

I am getting the following error.

exec(code, self.locals)
  File "<input>", line 1, in <module>
  File "<string>", line 13, in <module>
TypeError: QgsExpression.prepare(): argument 1 has unexpected type 'int'

Script:

from qgis.core import *

from PyQt5.QtCore import *

output_layer = iface.activeLayer()

provider = output_layer.dataProvider()

provider.addAttributes([QgsField('new_pop', QVariant.Int)])

output_layer.updateFields()

output_layer.startEditing()

fieldIndex = output_layer.fields().indexFromName( 'new_pop' )

e = QgsExpression( 'toInt("exist_pop")' )

e.prepare(output_layer.fields().indexFromName( 'new_pop' ))

output_layer.commitChanges()

Answer 1

Thanks for your help, I decided to use a different strategy. Instead I created a .csvt file to accompany my csv file so that when I imported the csv the field was imported as a "integer".

opened notepad, saved the following as .csvt and saved in same location using same filename as my csv file.

"string","integer","string","string","integer"

PyQGIS code:

uri='file:///U:/filelocation/filename.csv?delimiter=,'
infoLyr=QgsVectorLayer(uri,'filename2','delimitedtext')
infoLyr.isValid()