Finding polygon self-intersection with PyQGIS on QGIS 3.4

Question

My script:

from qgis.core import *
import qgis.utils
import processing, os, sys, glob

polys = QgsVectorLayer("/Users/john/shapefile/layer.shp","intersection","ogr")
output = "/Users/john/shapefile/intersection.shp"
processing.run("saga:polygonselfintersection", polys, output)

Generates the return message:

if param not in parameters or parameters[param.name()] is None:
TypeError: argument of type 'QgsVectorLayer' is not iterable

If you want to use 'saga:polygonselfintersection' algorithm you need to get main name parameters by using processing.algorithmHelp('saga:polygonselfintersection') command at Python Console. Please, see my answer. — xunilk, Jan 09 '19 at 17:56

score 2 · Accepted Answer · answered Apr 16 '19 at 08:58

Tested in QGIS 3.6.0. I also needed to check some layers for its intersecting features and found that for me raw geometry loop works much faster than saga algorithm. In my case I had three polygon layers which I had to check. Here is a code:

import time
import processing
layers_list = iface.mapCanvas().layers()

start = time.time()
for lyr in layers_list:
    tech_layer = QgsVectorLayer("MultiPolygon?crs=EPSG:102014" , "tech_test", "memory")
    features = [f for f in lyr.getFeatures()]
    for n in features:
        geom = n.geometry()
        if any(geom.overlaps(init.geometry())==True for init in features if n.id()!=init.id()):
            tech_layer.dataProvider().addFeatures([n])
end = time.time()
elapsed_time = end - start  
et_str_1 = time.strftime("%H:%M:%S", time.gmtime(elapsed_time)) 

start = time.time()
for lyr in layers_list:
    out = processing.run('saga:polygonselfintersection', {'POLYGONS':lyr, 'INTERSECT': 'memory:tmp'})
end = time.time()
elapsed_time = end - start  
msec_str = str(int(elapsed_time%1*60))
et_str_2 = time.strftime("%H:%M:%S", time.gmtime(elapsed_time))

print ("Raw geometry check: \t{}".format(et_str_1))
print ("SAGA geometry check: \t{}".format(et_str_2))

Here are the results:

Raw geometry check:     00:00:13
SAGA geometry check:    00:00:24

In general this might not be too significant here but in case of checking much more layers raw geometry loop can be faster.

score 1 · Answer 2 · answered Jan 09 '19 at 17:53

1

If you want to use 'saga:polygonselfintersection' algorithm you need to get mandatory name parameters by using following command at Python Console:

processing.algorithmHelp('saga:polygonselfintersection')

After that, your code, adapted for my paths in a Linux system, it will be modified to:

import processing    
polys = QgsVectorLayer("/home/zeito/pyqgis_data/layer.shp","intersection","ogr")
output = "/home/zeito/pyqgis_data/intersection.shp"
processing.runAndLoadResults("saga:polygonselfintersection", 
                            {'POLYGONS':polys, 
                             'INTERSECT':output})

I created layer.shp and it looks as at following image:

After running above code, layer resulting (Intersection) was automatically added to Map Canvas:

and its behavior is as expected.

answered Jan 09 '19 at 17:53

xunilk

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after processing finished I want to change layers name from intersection to another. How to ? – p wat Jan 10 '19 at 09:38
By code or manually? In last case, you can do that by placing cursor on layer name (layers panel), click on mouse right button and select 'Rename layer' at contextual menu. – xunilk Jan 11 '19 at 22:11
In my case have many layers so I want to use script to change layers name after processing finished. – p wat Jan 13 '19 at 01:56

Finding polygon self-intersection with PyQGIS on QGIS 3.4

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