utils.py

    import site
    if pythonPlugins is None:
        pythonPlugins = []
    assert isinstance(pythonPlugins, list)

    if isinstance(qgisResourceDir, str) and os.path.isdir(qgisResourceDir):
        import importlib, re
        modules = [m for m in os.listdir(qgisResourceDir) if re.search(r'[^_].*\.py', m)]
        modules = [m[0:-3] for m in modules]
        for m in modules:
            mod = importlib.import_module('qgisresources.{}'.format(m))
            if "qInitResources" in dir(mod):
                mod.qInitResources()


    envVar = os.environ.get('QGIS_PLUGINPATH', None)
    if isinstance(envVar, list):
        pythonPlugins.extend(re.split('[;:]', envVar))

    # make plugin paths available to QGIS and Python
    os.environ['QGIS_PLUGINPATH'] = ';'.join(pythonPlugins)
    os.environ['QGIS_DEBUG'] = '1' if qgisDebug else '0'
    for p in pythonPlugins:
        sys.path.append(p)

    if isinstance(QgsApplication.instance(), QgsApplication):

        return QgsApplication.instance()

    else:

        if PATH_QGIS is None:
            # find QGIS Path
            if sys.platform == 'darwin':
                # search for the QGIS.app
                import qgis, re
                assert '.app' in qgis.__file__, 'Can not locate path of QGIS.app'
                PATH_QGIS_APP = re.split(r'\.app[\/]', qgis.__file__)[0] + '.app'
                PATH_QGIS = os.path.join(PATH_QGIS_APP, *['Contents', 'MacOS'])

                if not 'GDAL_DATA' in os.environ.keys():
                    os.environ['GDAL_DATA'] = r'/Library/Frameworks/GDAL.framework/Versions/Current/Resources/gdal'

                QApplication.addLibraryPath(os.path.join(PATH_QGIS_APP, *['Contents', 'PlugIns']))
                QApplication.addLibraryPath(os.path.join(PATH_QGIS_APP, *['Contents', 'PlugIns', 'qgis']))


            else:
                # assume OSGeo4W startup
                PATH_QGIS = os.environ['QGIS_PREFIX_PATH']

        assert os.path.exists(PATH_QGIS)

        qgsApp = QgsApplication([], True)
        qgsApp.setPrefixPath(PATH_QGIS, True)
        qgsApp.initQgis()

        def printQgisLog(tb, error, level):
            if error not in ['Python warning']:
                print(tb)

        QgsApplication.instance().messageLog().messageReceived.connect(printQgisLog)

        return qgsApp


def createCRSTransform(src, dst):
    assert isinstance(src, QgsCoordinateReferenceSystem)
    assert isinstance(dst, QgsCoordinateReferenceSystem)
    t = QgsCoordinateTransform()
    t.setSourceCrs(src)
    t.setDestinationCrs(dst)
    return t

if __name__ == '__main__':
    #nice predecessors

    qgsApp = initQgisApplication()
    se = SpatialExtent.world()
    assert nicePredecessor(26) == 25
    assert nicePredecessor(25) == 25
    assert nicePredecessor(23) == 20
    assert nicePredecessor(999) == 950
    assert nicePredecessor(1001) == 1000
    assert nicePredecessor(1.2) == 1.0      #
    assert nicePredecessor(0.8) == 0.5
    assert nicePredecessor(0.2) == 0.1
    assert nicePredecessor(0.021) == 0.01
    assert nicePredecessor(0.0009991) == 0.0005