[gdal-dev] feature.clone() -> same featuredefn

[Even Rouault]

Le lundi 04 août 2014 11:02:41, Martin Landa a écrit :
> Hi Even,
> 
> 2014-08-02 22:18 GMT+02:00 Even Rouault <even.rouault at mines-paris.org>:
> 
> [...]
> 
> >> 1) copy features from input layer to output layer
> >> 2) modify features in output layer (remove some geometry columns)
> > 
> > You can use the SetFrom() method.
> 
> thanks for hint, I found out that input and output layer somehow share
> FeatureDefn. Here is sample code:
> 
> """
>     # create new layer
>     olayer = ods.CreateLayer(layerName, ilayer.GetSpatialRef(),
>                              geom_type, options)
> 
>     if not olayer:
>         fatal("Unable to create layer '%'" % layerName)
> 
>     # create attributes
>     feat_defn = ilayer.GetLayerDefn()
>     for i in range(feat_defn.GetFieldCount()):
>         olayer.CreateField(feat_defn.GetFieldDefn(i))
> 
>     # create also geometry attributes
>     if not geom_name and \
>             olayer.TestCapability(ogr.OLCCreateGeomField):
>         for i in range(feat_defn.GetGeomFieldCount()):
>             geom_defn = feat_defn.GetGeomFieldDefn(i)
>             if geom_name and geom_defn.GetName() != geom_name:
>                 continue
>             olayer.CreateGeomField(feat_defn.GetGeomFieldDefn(i))
> 
>     print
>     print ilayer, ilayer.GetLayerDefn()
>     print olayer, olayer.GetLayerDefn()
> """
> 
> prints
> 
> <osgeo.ogr.Layer; proxy of <Swig Object of type 'OGRLayerShadow *' at
> 0x7fbf0d95f930> > <osgeo.ogr.FeatureDefn; proxy of <Swig Object of
> type 'OGRFeatureDefnShadow *' at 0x7fbf0d95fae0> >
> <osgeo.ogr.Layer; proxy of <Swig Object of type 'OGRLayerShadow *' at
> 0x7fbf0d95fba0> > <osgeo.ogr.FeatureDefn; proxy of <Swig Object of
> type 'OGRFeatureDefnShadow *' at 0x7fbf0d95fae0> >
> 
> I am not sure why... Thanks for clarification in advance, Martin

Martin,

You cannot deduce if the underlying C++ objects are identical from that. In 
fact what you are seeing is the address of a SWIG "shadow" object, that is 
something whose lifetime is related to the lifetime of the Python objects.

See the below example :

$ python
Python 2.6.6 (r266:84292, Oct 29 2012, 23:09:05) 
[GCC 4.4.3] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> from osgeo import ogr
>>> ds = ogr.GetDriverByName('memory').CreateDataSource('')
>>> lyr1 = ds.CreateLayer('foo')
>>> lyr1.GetLayerDefn()
<osgeo.ogr.FeatureDefn; proxy of <Swig Object of type 'OGRFeatureDefnShadow *' 
at 0x7fd85c0574e0> >
>>> lyr1.GetLayerDefn()
<osgeo.ogr.FeatureDefn; proxy of <Swig Object of type 'OGRFeatureDefnShadow *' 
at 0x7fd847c493f0> >
>>> lyr1.GetLayerDefn()
<osgeo.ogr.FeatureDefn; proxy of <Swig Object of type 'OGRFeatureDefnShadow *' 
at 0x7fd85c0574e0> >
>>> lyr1.GetLayerDefn()
<osgeo.ogr.FeatureDefn; proxy of <Swig Object of type 'OGRFeatureDefnShadow *' 
at 0x7fd847c49450> >
>>> 

The shadow object changes at every execution. In your case, the old address 
for ilayer.GetLayerDefn() was likely "recycled" for olayer.GetLayerDefn()

You can try ilayer.GetLayerDefn().IsSame(olayer.GetLayerDefn()) and it should 
return 0 if their content is different (note that it compares the "content" of 
the layer and field definition, not the equality of pointers)

Best regards,

Even

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