Creating our own block from python

Hello GNU world , 

    In my previous projects i have created flow graphs using existing blocks . Right now i am planning to create my own block using python code . Can anyone guide me after creating python block how to convert it into a custom block on gnu radio ? Do we need to create xml file for that ? Can anyone guide me how a new block s developed from .py code ?

Thanks and regards ,

Ranganathan Sampathkumar