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I'm trying to write a script that will update a field with 2 values (Wrong/Ok) when specific conditions are met. This is what I've come up so far, but I can't get it to work.. 

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

layers = QgsMapLayerRegistry.instance().mapLayersByName('Mukono_Polygon') 
layer = layers[0]
it = layer.getFeatures()
idx = layer.fieldNameIndex('PlotArea')
idy = layer.fieldNameIndex('Pr')
idz = layer.fieldNameIndex('CTRL')

layer.startEditing()
for feat in it:
  if feat.attributes()[idx] <> '888.8888' and feat.attributes()[idx] <> '999.9999' and feat.attributes()[idy] > '120':
    layer.changeAttributeValue(feat.attributes()[idz], 'Wrong')
  elif feat.attributes()[idx] <> '888.8888' and feat.attributes()[idx] <> '999.9999' and feat.attributes()[idy] > '80':
    layer.changeAttributeValue(feat.attributes()[idz], 'Ok')
  else:
    print 'Cant find specific entries'

layer.commitChanges()


Traceback (most recent call last):
  File "", line 23, in 
IOError: [Errno 9] Bad file descriptor

If remove change the line 23 to print '', the script doesnt do anything.

I chose to get the fields by name instead of their number index because it's different from shapefile to shapefile.

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  • 4
    The QgsVectorLayer::changeAttributeValue() method requires a features id, field and the new value. Try using layer.changeAttributeValue(feat.id(), idz, 'Wrong')
    – Joseph
    Commented Mar 26, 2018 at 11:32
  • 1
    @kidd - What field types are idx and idy? If they are non-string, remove the string quotes around your values in your if statements. E.g. if feat.attributes()[idx] <> 888.8888 and feat.attributes()[idx] <> 999.9999 and feat.attributes()[idy] > 120:
    – Joseph
    Commented Mar 26, 2018 at 12:04
  • 2
    feat objects are dictionnary like, so you can change all feat.attributes()[id] to feat['field_name'].
    – YoLecomte
    Commented Mar 26, 2018 at 12:27
  • 1
    the first argument of changeAttributeValue must the id of the feature. so you need something like layer.changeAttributeValue(feat.id(), layer.fieldNameIndex('field_name'), new_value)
    – YoLecomte
    Commented Mar 26, 2018 at 12:58
  • 1
    That was great. Now it works perfectly. Thank you very much Joseph And YoLecomte. Dunno how to mark both your answers correct. I added + rep. I will post full working code for anyone else.
    – kidd
    Commented Mar 26, 2018 at 13:08

1 Answer 1

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4

Many thanks to YoLecomte and Joseph. Here is the final code:

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

layers = QgsMapLayerRegistry.instance().mapLayersByName('Mukono_Polygon') 
layer = layers[0]
it = layer.getFeatures()

layer.startEditing()
for feat in it:
  if feat['PlotArea'] <> 888.8888 and feat['PlotArea'] <> 999.9999 and feat['Pr'] > 120:
    layer.changeAttributeValue(feat.id(), layer.fieldNameIndex('CTRL'), 'Wrong')
  elif feat['PlotArea'] <> 888.8888 and feat['PlotArea'] <> 999.9999 and feat['Pr'] > 80:
    layer.changeAttributeValue(feat.id(), layer.fieldNameIndex('CTRL'), 'Ok')
  else:
    print 'Cant find specific entries'

layer.commitChanges()
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