[GIS] different way to build qgis python standalone application

pyqgispythonqgis

There seems to be 2 different ways to start a Python QGIS standalone script.
The official tutorial is using :

from qgis.core import *
# supply path to where is your qgis installed
QgsApplication.setPrefixPath("/path/to/qgis/installation", True)
# load providers
QgsApplication.initQgis()

But I have also seen code looking like this :

from qgis.core.contextmanagers import qgisapp
with qgisapp() as app:
    #do stuff

For example here and here.

I didn't see any reference to the latter method on the official doc.

Does anybody know if there is a difference between the two ?

Best Answer

If you look at the source code for the context manager qgisapp:

https://github.com/qgis/QGIS/blob/master/python/core/contextmanagers.py

you'll see it is just a neat wrapper for running code inside a QgsApplication context.

Following what Nathan W does with his Qgis python is usually a good idea, so I'd go with this.

Related Solutions

[GIS] How to Setup Python to recognize QGIS

It looks like you have a similar problem to one I had. Though the specifics of the paths are different I believe that a similar solution can work. See my solution in this post.

What is happening is that it is not just the PYTHONPATH which is read but also the PATH variable. On load QGIS appends the paths of the osgeo binaries to the path, which means they get read last. That means the ArcGIS location is found first and Windows looks no further.

What you don't want to do is muck up your ArcGIS install by playing with the main PATH variable (where there will be a reference to the Arc Python directory). However, if you tell QGIS to reverse the sys.path variable as the first thing it does, it will read the osgeo paths first and ignore the Arc Python instalation. This change is only apparent within the specific Python session (i.e. it has no effect on the actual PATH or PYTHONPATH variables stored on your computer and neither is it permanent, so will be safe as far as ArcGIS is concerned and less dangerous than editting your PATH variable as this could end up breaking Arc with the same problem but the other way around).

Depending on your install, you might want to change my solution to something like (based on what I read in your question):

import sys
sys.path.extend([r"C:\Quantum_GIS_Lisboa\apps\qgis\python",r"C:\Quantum_GIS_Lisboa\apps\qgis\bin"])
sys.path.reverse()

If you have used osgeo4w to install QGIS this will be unnecessary, but you could do it anyway as a belt-and-braces approach - it won't hurt). I suspect you will also need to add the lines at the start of the set_qgis_01.py file (I did mine in the GDALTOOLS.py file because that's where my conflict manifested itself).

QGIS Standalone Applications – Using Environment Variables in QGIS

You have to start the Qt event loop using:

app.exec_()

I would remove QgsApplication.exitQgis(). It should ideally be handled in a slot that is connected to lastWindowClosed() signal of the application.

Best Answer

Related Solutions

[GIS] How to Setup Python to recognize QGIS

QGIS Standalone Applications – Using Environment Variables in QGIS

Related Question