Calculate centroids using PyQGIS

Question

I am trying to calculate the centroids of polygons using PyQGIS.

When I calculate the centroids with the interface (Vector --> Geometry Tools --> Polygon Centroids) everything works out.

However, I fail to reproduce the same using the python console in QGIS (version 2.18.16).

Following the history output after conducting the operation manually as well as the QGIS documentation, I use this line of code:

processing.runalg("qgis:polygoncentroids", shapefile, "memory:")

This produces the error:

NameError: name 'processing' is not defined

What am I doing wrong here?

Answer 1

To solve your problem you should only import the module before executing the command (usually this is not necessary).

import processing

If you want to use QGIS 3 to calculate the centroids of a polygon layer you can execute the following command:

processing.run("native:centroids", {'INPUT':'D:/polygons.shp','ALL_PARTS':False,'OUTPUT':'memory:'})

If you have a MultiPolygon layer and for each part you want to calculate its centroid you can execute the following command:

processing.run("native:centroids", {'INPUT':'D:/polygons.shp','ALL_PARTS':True,'OUTPUT':'memory:'})

Answer 2

You must Import the processing library

import processing
shapefile = "c:/tar/polygon.shp"
processing.runandload("qgis:polygoncentroids", shapefile, "memory")