How to create dissolved buffer layer with pyqgis?

Question

I have a line vector layer with a field name of "buffer_distance". I want create a dissolved buffer layer based on it's field. I write below code:

from qgis.utils import iface
from qgis.analysis import QgsGeometryAnalyzer 
mc = iface.mapCanvas() 
layer = mc.currentLayer()
Distance_Field = layer.fieldNameIndex("buffer_distance")
QgsGeometryAnalyzer().buffer(layer, "/tmp/buffer.shp", False, True, Distance_Field)

when run this code in QGIS 2.18 python console, It creates a new shapefile with all table fields of input layer but there is not any feature. What is problem?

Answer 1

You have several issues in your code.

1. Name field too long.
2. You don't need index name field as parameter. You need attribute value.
3. Wrong parameters number and wrong order.

So, following code works as expected:

from qgis.utils import iface
from qgis.analysis import QgsGeometryAnalyzer 
mc = iface.mapCanvas() 
layer = mc.currentLayer()
feat = layer.getFeatures().next()
att = feat.attribute('buf_dist')
QgsGeometryAnalyzer().buffer(layer,
 "/tmp/buffer.shp",
 att,
 False,
 False,
 -1)

After running it at Python Console of QGIS, I got desired buffer (retrieved from "/tmp/buffer.shp"):

enter image description here

as it can see at above image.

Editing Note:

In this case I would use QgsGeometry class because it also has a buffer method. Following code does the work:

mc = iface.mapCanvas() 
layer = mc.currentLayer()
feats = [ feat for feat in layer.getFeatures() ]
buffers = [ feat.geometry().buffer(feat.attribute('buf_dist'), -1).exportToWkt()
 for feat in feats ]
epsg = layer.crs().postgisSrid()
uri = "Polygon?crs=epsg:" + str(epsg) + "&field=id:integer""&index=yes"
mem_layer = QgsVectorLayer(uri,
 'buffers',
 'memory')
prov = mem_layer.dataProvider()
feats = [ QgsFeature() for i in range(len(buffers)) ]
for i, feat in enumerate(feats):
 feat.setAttributes([i])
 feat.setGeometry(QgsGeometry.fromWkt(buffers[i]))
prov.addFeatures(feats)
QgsMapLayerRegistry.instance().addMapLayer(mem_layer) 

I tried it out with line layer of following image:

enter image description here

and it works as expected.

• HMadadi: Hi @xunilk, I used your code but it created equal buffer size for each feature while in my field there are different values.
• HMadadi: It seems "att" in QgsGeometryAnalyzer is first value that used to create buffer size for all features.
• xunilk: In this case I would use QgsGeometry class because it also has a buffer method. Please, see my Editing Note.
• HMadadi: in second code Is it possible to dissolve buffers?
• xunilk: Yes, it's possible. You can use gdalogr:dissolvepolygons, qgis:dissolve or grass:v.dissolve processing methods. I recommend qgis:dissolve because it has less parameters.