[gdal-dev] Is it possible to add new fields to a shapefile and populate then?

[Stephen Woodbridge]

On 4/3/2017 11:37 PM, jratike80 wrote:
> Stephen Woodbridge wrote
>> Hi All,
>>
>> I have a shapefile(s) and I want to read the features, generate some
>> metrics about each feature and then add them to the that feature. I'm
>> using python and one obvious way to to in effect create a new shapefile
>> with the columns I need to add, then copy the existing shapefile to the
>> new one adding the additional metrics.
>>
>> I wondering if I can add the new columns to the shapefile, then do a
>> read and update.
>>
>> Pros and cons to these approaches? and how to do the later one if
>> possible?
>>
>> I looked at the python gdal/ogr cookbook and didn't this specific
>> example. But I thinking of something along the lines of:
>>
>> 1. open the dataset for read/write
>> 2. layer.CreateField() to add the new fields
>> 3. loop through the features in the layer
>> 4.    and use feature.SetField()
>> 5.    and feature = None to commit the changes
>>
>> will this work?
>>
>> Thanks,
>>    -Steve
>
> Hi,
>
> You can demonstrate that it is possible with ogrinfo:
>
> ogrinfo  -sql "alter table test add column foo integer" test.shp
> ogrinfo  -dialect sqlite -sql "update test set foo=2" test.shp

Hi Jukka,

Ahh, good point, I always seem to forget that fact.
I have gotten the python code to add the new fields, but the data is not 
getting written to the file.

     # open the shapefile
     driver = ogr.GetDriverByName('ESRI Shapefile')
     dataSource = driver.Open(infile, 1) # open for rw
     if dataSource is None:
         print "ERROR: could not open '%s' as shapefile!" % (infile)
         sys.exit(1)

     layer = dataSource.GetLayer()
     layer.CreateField(ogr.FieldDefn("area",     ogr.OFTReal))


     for feature in layer:
         geom = feature.GetGeometryRef()
         feature.SetField("area", geom.GetArea())
         feature = None

     dataSource = None

In the cookbook example for "Create a New Shapefile and Add Data" there 
is a call to:

layer.CreateFeature(feature)

but I do not see an equivalent call for:

layer.UpdateFeature(feature)

So How do I update the feature and force the data to get written to the 
shapefile. I'm presuming the CreateFeature will append a new record 
which is not what I want.

-Steve

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