Lately I’ve been spending time porting a bigger plugin from QGIS 2.8 to 3 while maintaining 2.8 compatibility.
You can find it at https://github.com/opengisch/qgis2compat/
and https://plugins.qgis.org/plugins/qgis2compat/
One code to rule them all.
My target was to have to edit the source code as little as possible to simulate a lazy or busy coder that has to upgrade his/her plugins.
Lots of work has already gone into 2.14 to support PyQt 4 and 5 with the same code (Kudos to jef-n and mkuhn).

# QGIS before 2.14
from PyQt4
# QGIS 2.14 and after
from qgis.PyQt

The qgis python package will then use the appropriate PyQt for you. But not all can be fixed in QGIS itself and this is why QGIS2compat can help you.

Use cases

QGIS2compat targets two main use cases.

PyQt compat

If you still need to rely on QGIS < 2.14 writing from qgis.PyQt will not work for you as the qgis.PyQt package is simply not there. And this is one of the two use case where QGIS2compat can help you. This feature is complete.

QGIS 2-3 API compatibility

The other use case of QGIS2compat plugin is the availability of a QGIS API compatibility layer which lets you write your code for QGIS 3 API and it will take care of adapting it to the QGIS 2 API. This feature is an ongoing work in progress since we are in the middle of API breakage period. So we do need your help to keeping adding new apicompat fixes (see below).

Usage

In your plugin’s __init__.py you should put something like the example
below. This will pick the QGIS PyQt compatibility layer which is
available since QGIS 2.14 or fall back to qgis2compat.
Also if you are in QGIS >= 2.14 and QGIS < 3 it will run the apicompat
package which will take care of the Python api changes between QGIS 2
and QGIS 3.

import os
import qgis.utils
# noinspection PyPep8Naming
def classFactory(iface):  # pylint: disable=invalid-name
    """Load MyPlugin class from file MyPlugin.
    :param iface: A QGIS interface instance.
    :type iface: QgsInterface
    """
    plugin_name = os.path.dirname(__file__).split(os.path.sep)[-1]
    plugin_name = qgis.utils.pluginMetadata(plugin_name, 'name')
    try:
        # qgis.PyQt is available in QGIS >=2.14
        from qgis.PyQt.QtCore import qVersion
        # qgis.utils.QGis is available in QGIS < 3
        if hasattr(qgis.utils, 'QGis'):
            import qgis2compat.apicompat
            qgis2compat.log('apicompat used in %s' % plugin_name)
    except ImportError:
        try:
            # we are in QGIS < 2.14
            import qgis2compat
            import qgis2compat.apicompat
            qgis2compat.log('PyQt and apicompat used in %s' % plugin_name)
        except ImportError:
            import traceback
            message = ('The Plugin %s uses the QGIS2compat plugin. '
                       'Please install it with the plugin manager it and '
                       'restart QGIS. For more information read '
                       'https://opengis.ch/qgis2compat' %
                       plugin_name)
            traceback.print_exc()
            raise ImportError(message)
    from .myplugin import MyPlugin
    return MyPlugin(iface)

in each module where you do PyQt imports you should use the following
structure.

from qgis.PyQt.QtCore import QFileDialog

This will guarantee that the imports come from the most appropriate and
up-to-date place and gives you PyQt4 and PyQt5 support for QGIS >= 2.8.

Updating your plugin

This can be done automatically by the 2to3 tool included in QGIS sourcecode.
Please note that it is not the plain 2to3 python tool and can be found
at https://github.com/qgis/QGIS/blob/master/scripts/2to3
This tool will fix many (probably not all) issues with your code and make it
compatible with Python 3.
After running 2to3, update your __init__.py as explained above.
once done, it is time to run your tests (which you of course have written
before migrating) and fix the minor glitches that might have appeared.

Adding new apicompat fixes

To add a new api compatibility fix, just create (or add to an existing one) a
new module in apicompat and import it in __init__.py__ like it is done for
the qgsvectorlayer.py
As QGIS2compat works on a fairly low level, we require unit tests for each
fix to be included in each pull request.

Need professional help?

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