I have two plugins in the QGIS plugin repository, and with the release of QGIS 3 looming it was time to upgrade them for QGIS 3.

There is a short guide by the QGIS dev team that is a good starting point at:
https://github.com/qgis/QGIS/wiki/Plugin-migration-to-QGIS-3

But I had not done any development on these plugins for a while so a more step by step guide was useful, so hopefully, write the guide for the first plugin and follow it step by step for the second.

I am working on Windows, with OSGeo4W.

Before we start we will need to insure a couple of extras are installed through the OSGeo4w Installer:
Desktop:
qgis-dev
Libs:
python-future

Assuming GitHUB is the repo.
In git shell:

There is a conversion script for QGIS plugins provided by the QGIS devs in the main repo.

UPDATE:
The qgis2to3 packages can be found on pip now:

https://github.com/opengisch/qgis2to3/

We can download just the scripts folder using the following link:
https://minhaskamal.github.io/DownGit/#/home?url=https://github.com/qgis/QGIS/tree/master/scripts

Extract that into a location of your choice.

Then we can run the 2to3 script from the OSGeo4W console (cd to the folder you extracted the script to):

This will print out changes that need to be made to convert from QGIS2 to QGIS3.

My first run resulted in many lines of:

So my plugins line of:

Was impossible to convert with the tool, since I was not 100% sure what I needed from the QtCore library (I was young when I wrote the plugin). I commented out the line for the plugin in QGIS 2.8, booted up QGIS 2.8 and tried running the plugin.

So python errors:
NameError: global name ‘QVariant’ is not defined
NameError: global name ‘Qt’ is not defined
Later. I ended up expanding my other import from QtCore to:

Running the 2to3 script again looked ok, with a number of changes required. These changes can be applied with –w flag:

For the next step I booted up my favourite IDE PyCharm. I created a bat file that launched PyCharm with the QGIS dev environmental variables. So copying the “python-qgis-dev.bat” file from:

I changed the final line of:

To:

Then from File> Settings> Project:> Project Interpreter> Set to “C:OSGeo4W64appsPython36python.exe”

It takes a while to update the interpreter.

I only had 2 errors, both for:
QgsMapLayerRegistry.instance().addMapLayer(vl)

There is a list of API breaks between QGIS 2 and QGIS 3 at:
https://qgis.org/api/api_break.html

Looks like QgsMapLayerRegistry was moved to QgsProject. So I edit it to:

Then we can edit the metadata.txt to QGIS 3:
qgisMinimumVersion=3.0

And increase the version number.

Then we need to recompile the icon and ui for Python3 and QT5.

I was struggling a bit with the environmental variables to get it working, and ended up using a great batch script form StackExchange:
https://gis.stackexchange.com/questions/260743/how-to-compile-qtdesigner-user-interface-ui-and-resource-qrc-files-with-qg

So create the .bat file and run it in the folder of you plugin (editing where needed). Note: Your resources_rc may be called resource_rc or something slightly different.

Move the plugin folder to:
C:\Users\USERNAME\AppData\Roaming\QGIS\QGIS3\profiles\default\python\plugins\

Boot up QGIS2.99/3.

I had a few more issues.

It seems QGIS 3 deals with the icon slightly differently.

icon_rc.py is no longer needed, and it seems was not used on my other plugin either.

So I removed the reference to it in the main python script:
from . import icon_rc

I still had some errors.

AttributeError: module ‘qgis.PyQt.QtGui’ has no attribute ‘QDialog’

It seems QDialog has moved to PyQt.QtWidgets.

So in my multi_ring_buffer_dialog.py file I needed to change some lines:

Add:

Change:
QtGui.QDialog
to:
QDialog
In the two instances in that file.

Working plugin!

Commit the changes back to the repo. Cd to the directory in git shell.

Zip the plugin up.
Upload to https://plugins.qgis.org/plugins/

Second plugin:
Same issue with import *
1 error with QgsMapLayerRegistry
My resources_rc file was called resource_rc so the batch script needed to be edited to:
call pyrcc5 resources.qrc -o resource_rc.py
Same issues with QtGui.QDialog

Now time for some improvements.