Importing multiple layer files like NAS/OSM in a standalone PyQgis script

Question

I am trying to convert a dlm50 NAS file of Rheinlandpfalz (Germany) to shapefile. With ogr2ogr this repeadetly converts Polygons to Lines and whole layers are missing. If I import it into QGIS and use the validate algorithm it works fine though (ogr convert format still fails).

Importing them in QGIS gives a pop-up where I can chose all layers I want to import. I don't find a way to do this in a python script though. Yes I can open them with QgsVectorLayer and ogr driver but I receive one object, which can't be used properly. I had also problems with this with OSM-files and spatialite files which have failed to load with the spatialite driver but succeded with ogr import (may have been to big?).

I know I should try to convert the dlm50 file to PostGIS but it will be the same driver for reading and I am generally interested if I miss something and the import of such files is possible somehow in a standalone python script (not in QGIS Python console). It should also be possible to work with such files without the need to preliminary convert them to other formats?!

So how can I import layers of multiple layer files like NAS/OSM/Spatialite in a standalone python script?

Answer 1

If you want to load all tables from a Spatialite database having a geometry_column, then the following code makes the Job. It uses SQLITE metadata to retrieve the tables and names, which can be loaded as a layer into QGIS.

import sqlite3

# connect to SQLITE database
database = '//vmapl1//VMapL1_125.sl3'
con = sqlite3.connect(database)
cursor = con.cursor()

# select tables having a geometry column
cursor.execute("SELECT f_table_name FROM geometry_columns;")

# retrieve all tables
layernames = cursor.fetchall()

# iterate over all tables and construct layers
for layername in layernames:
    layername = str(layername[0])
    layer = QgsVectorLayer(str.format('%s|layername=%s' % (database, layername)), layername, 'ogr')

    # add layers to legend
    QgsMapLayerRegistry.instance().addMapLayer(layer)

# close db Connection
con.close()

To use OGR to read other formats, like XML-formatted NAS files, a very similar approach may be helpful:

import ogr

# database is the XML file
database = 'E:\\0_Drucken\\e_ben_monzel_dlm50.xml'

# use OGR to get the metadata of the database, like number and names of layers
ds = ogr.Open(database)
layercount = ds.GetLayerCount()
layernames = {i: ds.GetLayerByIndex(i).GetName() for i in range(layercount)}

# import each layer into QGIS
for index, layername in layernames.items():
    layer = QgsVectorLayer(str.format('%s|layerid=%d' % (database, index)), layername, 'ogr')
    QgsMapLayerRegistry.instance().addMapLayer(layer)

This approach has been tested with NAS test data from LVermGeo RLP.