gistfile1.py

computeFieldIndex = layer.fieldNameIndex('population')
allFieldsMap = layer.pendingFields()
expression = QgsExpression('"affected" = 1')
expression.prepare(allFieldsMap)

if expression.hasParserError():
  raise Exception('Parser error !!')

def getFeatures():
  if self.selectedOnly:
    # Just return the selected features
    return layer.selectedFeatures()
  else:
    # Select all the features
    layer.select(allFieldsMap)
    # The layer is an iterator that returns a QgsFeature on next()
    # so just return the layer
    return layer

# Loop the features.
for feature in getFeatures():
    result = expression.evaluate(feature)
    if expression.hasEvalError():
      raise Exception('Eval error !!')
      continue

    if result.toBool():
        total += feature.attributeMap()[computeFieldIndex].toDouble()[0]