Programmatically populating shapefile field with polygon areas in OGR?

Question

I'm trying to programmatically populate a shp field with polygon areas:

import sys
import ogr

ds = ogr.Open( 'tttttttttt.shp', update = 1 )
if ds is None:
    print "Open failed./n"
    sys.exit( 1 )

lyr = ds.GetLayerByName( "tttttttttt" )
lyr.ResetReading()

field_defn = ogr.FieldDefn( "Area", ogr.OFTReal )
lyr.CreateField(field_defn)

for i in lyr:
    # feat = lyr.GetFeature(i) 
    geom = i.GetGeometryRef()
    area = geom.GetArea()
    print 'Area =', area
    lyr.SetFeature(i)
    i.SetField( "Area", area )

ds = None

But I get an empty values in the field...

Answer 1

You have to use lyr.SetFeature(i) to trigger the update in your shape file. You'll have to close the data sources in the end so things get written.

import sys
import ogr

ds = ogr.Open( 'tttttttttt.shp', update = 1 )
if ds is None:
    print "Open failed./n"
    sys.exit( 1 )

lyr = ds.GetLayerByName( "tttttttttt" )
lyr.ResetReading()

field_defn = ogr.FieldDefn( "Area", ogr.OFTReal )
lyr.CreateField(field_defn)

for i in lyr:
    # feat = lyr.GetFeature(i) 
    geom = i.GetGeometryRef()
    area = geom.GetArea()
    print 'Area =', area
    lyr.SetFeature(i)
    i.SetField( "Area", area )
    lyr.SetFeature(i)
ds = None

*Low-rep comment and concern: If the 'Area' field already exists, this code creates an extra field, like "Area_n", and overwrites the existing Area field. Maybe folks should add some safety code like:

ldef = lyr.GetLayerDefn()
if ldef.GetFieldIndex("Area") != -1:
    print "'Area' field already defined"
    ... # exit or overwrite logic