[Qgis-developer] calling createEmptyDataSource from python

Tim Sutton tim at linfiniti.com
Fri Jun 6 18:40:38 EDT 2008

Hi Stefanie

Another thing:

+bool createEmptyDataSource(const QString& provider,
+                          const QString& filename,
+                          const QString& fileformat,
+                          const QString& enc,
+                          QGis::WKBTYPE geometrytype,
+                          const std::list<std::pair<QString, QString>
>& attributes) {

In Qt its not really necessary to pass by reference like this since Qt
uses implicit sharing / copy one write which basically means if you
pass a Qt object to a function, it does not make a copy of the object
until the receiving class tries to change it. You can read more here:


Removing the reference modifier from function parameters creates in my
opinion cleaner, easier to read code and is more consistent with the
Qt4 programming style which was are at least striving to achieve on
public parts of the API. In a similar vein its preferred to use QList
over std::list on public methods, and QPair over std::pair.

I wonder if it doesnt make sense to wrap the functionality you have
kindly provided as a method in qgsvectorfilewriter and update qgsapp
to defer to that method rather. Basically, your method above
implemented in qgsvectorfilewriter would provide a convenience method
that removes the need to building a qgsfieldmap first. That approach
would seem to conform to Martins (I think good) suggestion while
providing you the convenience you were looking for.

>   * createEmptyDataSource not only makes file.shp, but also file.dbf,
> file.prj, and file.shx.  I don't know what they are, but they sound
> important.  :-)

QgsVectorFileWriter will do this for you too. One of the unit tests I
have written illustrates the use of QgsVectorFileWriter if you look
under tests/src/core.

Keep those patches coming ! :-)



2008/6/6 Stefanie Tellex <stefie10 at media.mit.edu>:
> Hi Martin,
> I didn't know about QgsVectorFileWriter.  How come qgisapp.cpp doesn't use
> it to create a new layer?  I was looking there to figure out how to make a
> new empty data source, and saw it was doing fancy stuff to call
> createEmptyDataSource in the ogr library.
> I think my patch is better for three reasons:
>  * It seems like it would be more work to call QgsVectorFileWriter from
> python than it would be to use the patch I submitted to call
> createEmptyDataSource from python.  With the patch I submitted,
> createEmptyDataSource gets passed a list of tuples specifying the fields,
> while QgsVectorFileWriter wants a QgsFieldMap, and has to be opened and
> closed to save the shapefile.
>   * my patch refactors createEmptyDataSource into a function in
> QgsVectorDataProvider, which cleans up the code in qgisapp, makes it easier
> to change qgis app to use createEmptyDataSource with other providers, and
> makes it so python and qgisapp use the same mechanism for making new empty
> data sources.
>   * createEmptyDataSource not only makes file.shp, but also file.dbf,
> file.prj, and file.shx.  I don't know what they are, but they sound
> important.  :-)
> Thanks,
> Stefanie
> Martin Dobias wrote:
>> On Thu, Jun 5, 2008 at 4:34 AM, Stefanie Tellex <stefie10 at media.mit.edu>
>> wrote:
>>> Hi,
>>> Is there a way to call the ogr provider's createEmptyDataSource function
>>> from python?  I could try using ctypes, but I wasn't sure if it would
>>> work
>>> with the sip interface.  Combining two ways of talking to C/C++ from
>>> python
>>> seems hairy.
>> Hi Stefanie,
>> what about using QgsVectorFileWriter instead? Or is there anything
>> that you'd like to use and it's not available?
>> Bye
>> Martin
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Tim Sutton
QGIS Project Steering Committee Member - Release Manager
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