I am begining with PyQGIS and QGIS in general. I wanted a way to create a layer via python, so i downloaded "Plugin Builder" and starded coding. Here's the code : 

    def activatePlugin (self):
        global limitAer,limitSou ,limitOra , limitENE, limitCha, layer, QgsVectorLayer

        layer = QgsVectorLayer('/Users/fse/Desktop/54_028_380_EXE_CCE_SID_CIR_001_B2/SHAPE/couche_test.shp', 'couche_test', 'ogr')
        
        QgsMapLayerRegistry.instance().addMapLayer(layer)
        
        if not layer.isValid():
            print "Layer was not loaded!"
        else:
            print "Layer was loaded successfully!"

I get a "Layer was not loaded" response and a print in the OGR console telling me that the source data is invalid. 

I have tried all sorts of slashes, backslashes etc... It is basically always the same.