pyQGIS 3.0 script not running with processing import

Question

I have problems using version 3 of QGIS with Python 3. I followed different tutorials like this one : how to use pyqgis processing runalg clipvectorsbypolygon

But it seems to be working only with QGIS 2.XX and not 3.0. Here is what I try to run :

from qgis.core import *
import PyQt5.QtCore
import sys
sys.path.append('C:/QGIS 3.0/apps/qgis/python/plugins')
qgs = QgsApplication([], True)
qgs.setPrefixPath("C:/QGIS 3.0/apps/qgis", True)
qgs.initQgis()
import processing
qgs.exitQgis()

But I get on PyCharm the error message : "Process finished with exit code 1". I don't understand what is missing in my code. Can you please explain me how to use processing library with independent pyqgis scripts ?

Answer 1

I tested your code and stripped the problem down to the paths you are using. The error seems to be the forward slashes (/) that confuse your windows python installation. You should use backslashes on windows and use them as escaped symbol (eg. double backslash \). I updated your code with the paths of my system (an OSGeo4W64 installation), you can change them to match your directory structure:

from qgis.core import *
import PyQt5.QtCore
import sys
qgs = QgsApplication([], True)
qgs.setPrefixPath("C:\\OSGeo4W64\\apps\\qgis", True) 
#should be C:\\QGIS 3.0\\apps\\qgis
qgs.initQgis()
sys.path.append('C:\\OSGeo4W64\\apps\\qgis\\python\\plugins')
# should be "C:\\QGIS 3.0\\apps\\qgis\\python\\plugins" in your system
import processing
qgs.exitQgis()

Now the script should work. Sidenote: I ran the script from OSGeo4W Shell using the python-qgis command (python interpreter with QGIS paths). In case PyCharm still throws some errors, try running your script there or in PyDev (Eclipse). For reference: There is a similar answer to this one that also shows how to call native c++ algorithms from python.

• dmjf08: I always have the same issue... Ok for the paths but I think my problem comes maybe from the Windows global path. Can you give me what you have in PC path concerning python and qgis ? Thank you for your help.

Answer 2

I solved my problem by adding the following line to my code :

sys.path.append("C:\\QGIS 3.0\\apps\\Python36\\Lib\\site-packages")

In fact when I tried to interpret my python script with PyCharm the problem was that I didn't get any information about my mistake. I ran it with the basic IDLE for Python 3 and it said me ModuleNotFoundError: No module named 'osgeo' which means the link to the library is not done because osgeo is installed with QGIS. The link to the library is what I wrote above.

BUT notice that you could also add this path to your %PYTHONPATH% variable. You mustn't do that because the folder site-packages\ contains many other libraries that can confuse Python executables like pip.