[gdal-dev] Shapefile read/write

[Howard Butler]

Me too :)  I seem to remember making the same mistake(s) the last time  
I tried to do something similar...

Howard


On Dec 4, 2007, at 11:01 AM, Mullins, Steven wrote:

> Frank,
>
> Works perfect!  Now I understand OGR's data handling better too.
>
> Many thanks,
>
> Steve
>
> -----Original Message-----
> From: Frank Warmerdam [mailto:warmerdam at pobox.com]
> Sent: Tue, December 04, 2007 11:26 AM
> To: Mullins, Steven
> Cc: gdal-dev at lists.osgeo.org
> Subject: Re: FW: [gdal-dev] Shapefile read/write
>
>
> Mullins, Steven wrote:
>> Frank,
>>
>> Thanks for the reply.  It runs without an error, but is not  
>> updating the shapefile on disk.  My Python code is below.  Perhaps  
>> I am not opening the file correctly.  I can't find any good example  
>> to go by.  Every example I found copies the schema and data to a  
>> new file using recursion over the fields.  I also found an old list  
>> email (2005) that stated that shapefile write-backs did not work in  
>> OGR at that time.  A shapefile write-back would be ideal for me, if  
>> It can be done.  Do any errors jump out at you?
>>
>> Thanks,
>>
>> Steve
>>
>> ###############################################
>> #!/usr/local/bin/python
>> # read SHP file and update attributes per coordinates
>>
>> import ogr
>> import osr
>> import string
>>
>> ###############################################
>> # Data Source
>> data_source = ogr.Open('./shapefiles83/rainfalls-active/rainfalls- 
>> active.shp')
>
> Steven,
>
> Thi is your first problem.  You need to open the file in update
> mode.  Try:
>
> data_source = ogr.Open('./shapefiles83/rainfalls-active/rainfalls- 
> active.shp',1)
>
> But also the following never actually sends the modified feature
> back to the layer.
>
>> while current_feature is not None:
>>    current_point = current_feature.GetGeometryRef()
>>    current_feature.SetField ( 0, current_point.GetX())
>>    current_feature.SetField ( 1, current_point.GetY())
>>    current_feature = layer_0.GetNextFeature()
>> layer_0.SyncToDisk()
>> data_source.Destroy()
>
> Try adding:
>
>       layer.SetFeature( current_feature )
>
> after the two SetField calls.  The SetFeature() call tells the
> layer to write this feature back to disk.  It is often unclear to
> people, but the feature object is not a direct proxy for the data
> in the datastore.  It is a relatively freestanding feature in
> memory that you can manipulate and use.  If you want changes to it
> to be reflected in the file, then you need to explicitly request it.
>
> Best regards,
> -- 
> --------------------------------------- 
> +--------------------------------------
> I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
> light and sound - activate the windows | http://pobox.com/~warmerdam
> and watch the world go round - Rush    | President OSGeo, http://osgeo.org
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