How to do a buffer operation for a polygon drawn by the user using python in QGIS

I have a polygon drawn on the map canvas. Now, I want to do buffer for certain distance around that polygon.I can see the below operation in QGIS in which it uses a layer as input parameter instead i want the polygon as an input in the buffer.How should this be accomplished in QGIS using python.

processing.runalg("qgis:fixeddistancebuffer","layer-name-you-need",0.1,10,False,"Path-to-file-to-store")