Iterative selection of features and export to shapefile using PyQGIS

Question

I'm working with QGIS 3.10 A Coruña on MacOS 10.13 environment. I have a vector polygon layer and I need to select features with a given attribute value and export them as a new shapefile. I'm a newbie in these matters and haven't get any further than

layer = qgis.utils.iface.mapCanvas().currentLayer()
request = QgsFeatureRequest(QgsExpression("CODNUT2 = 'ES24'"))
features = layer.getFeatures(request)

1. What should be the next steps to generate the shapefile with those selected features?
2. How to make it an iterative process to generate new shapefiles for every value in the CODNUT2 attribute?

Answer 1

You can use the following script. Just change folder.

layer = qgis.utils.iface.mapCanvas().currentLayer()

field_name = "CODNUT2"
idx = layer.fields().indexOf(field_name)
unique_values = layer.uniqueValues(idx)

folder = "/path/to/folder"

for value in unique_values:
    file_path = f"{folder}/_{value}.shp"
    layer.selectByExpression(f"{field_name}='{value}'")
    
    QgsVectorFileWriter.writeAsVectorFormat(layer,
                                            file_path,
                                            'utf-8',
                                            layer.crs(),
                                            "ESRI Shapefile",
                                            onlySelected=True
                                            )

Answer 2

I would use two processing tools to achieve your problem: qgis:listuniquevalues to get the unique values and native:saveselectedfeatures to save the selected feature.

layer = iface.activeLayer()

# Find the unique values for the given field
res1 = processing.run("qgis:listuniquevalues",
                      {'INPUT': layer,
                       'FIELDS': ['CODNUT2'],
                       'OUTPUT': 'TEMPORARY_OUTPUT',
                       'OUTPUT_HTML_FILE': 'TEMPORARY_OUTPUT'})
distinct_value = res1['OUTPUT']

# Loop trough the unique values
for feat in distinct_value.getFeatures():
    attr_value = feat['CODNUT2']  # distinct value 

    # Select the objects with the unique value
    layer.selectByExpression(f"CODNUT2='{attr_value}'")

    # Exported selected objects to shp
    processing.run("native:saveselectedfeatures",
                   {'INPUT': layer,
                    'OUTPUT': f'C:/Users/ncipa/Downloads/test_selectedtest_{attr_value}.shp'})

Answer 3

There is a better approach for what you want to do with processing and the native splitvectorlayer algorithm.

Just do :

import processing
params = {'INPUT':iface.activeLayer(),
'FIELD':'CODNUT2',
'FILE_TYPE':0,
'OUTPUT':'/path/to/folder'}
processing.run("native:splitvectorlayer", params)

Put 'FILE_TYPE':1 if you want shpfile

To get the help :

processing.algorithmHelp('native:splitvectorlayer') #in the console

Answer 4

You can use following code for obtaining a memory layer with your request:

qgis.utils.iface.mapCanvas().currentLayer()

request = QgsFeatureRequest().setFilterExpression (u"CODNUT2 = 'ES24'")

selected_feats = layer.getFeatures(request)

attr = [ feat.attributes() for feat in selected_feats ]
idx = [ ele[0] for ele in attr ]
val_req = [ ele[1] for ele in attr ]

iterator = layer.getFeatures(request)

feats = [ feat for feat in iterator ]

epsg = layer.crs().postgisSrid()

uri = "Polygon?crs=epsg:" + str(epsg) + "&field=id:integer""&field=CODNUT2:string""&index=yes"

mem_layer = QgsVectorLayer(uri, 
                           'mem_layer', 
                           'memory')

prov = mem_layer.dataProvider()

for i, feat in enumerate(feats):
    feat.setAttributes([idx[i], val_req[i]])
    mem_layer.addFeature(feat)

prov.addFeatures(feats)

QgsProject.instance().addMapLayer(mem_layer)

For testing it, I created 'CODNUT2' field in one shapefile and arbitrarily put in it some 'ES24' values and other different.

After running above code, result was as expected. Memory layer has features with desired request and corresponding ids.