[Qgis-user] QgsVectorFileWriter

[Lionel Roubeyrie]

Hi Carson,

Le mercredi 21 janvier 2009 à 16:48 +0000, Carson Farmer a écrit :
> > In this way, it would be better to save in a temporary shapefile first,
> > and if the process finishes successfully, remove the destination
> > shapefile and copy the temporary shp to the new location.
> >   
> hmm, I don't  know about this...
> Firstly, it's only useful to do this if you're concerned about the 
> initial file, but since you appear to be writing to a tmp directory, 
> this probably isn't all that critical...
Sometimes I don't want to get 2 versions of a shp and select the source
shp to be overwritten. If something goes wrong and the source is deleted
first, all is lost...
> 
> Secondly, this creates the possibility that if something goes wrong, you 
> have TWO useless files lying about on your system (one you were trying 
> to replace, and the other that didn't finish writing properly).
A simple test where if a problem occurs, we delete the tmp file...

> 
> The better implementation would probably be to use the memory provider 
> to create a temporary QgsVectorLayer in memory, and if the process 
> finishes successfully, remove the old version and write the memory layer 
> to file. Of course, this only works if you're working with relatively 
> 'small' layers, if you have huge vector layers (> 500,000 features) then 
> you're probably better off just writing directly to file...
> 
> mem_layer = QgsVectorLayer('Point', 'layer_name', 'memory')
> 
> you'll have to double check the above, this is just from 'memory' ;-)
It may be a solution :-)

> 
> Carson
> 
-- 
Lionel Roubeyrie
chargé d'études
LIMAIR - La Surveillance de l'Air en Limousin
http://www.limair.asso.fr