add driver for a shapekey via python? how can i add a driver for a shapekey via python? 

shape_keys.animation_data.driver_button_add(True)


or

obj=bpy.context.active_object
obj.driver_add('key_blocks["Eye_left"].value')


The Problem is that it can't find the shape key and shape keys are getting their driver with a different method; so I'm a bit confused how it can be added by python correctly.
Sorry I'm new to Python. Thanks for your help!
import bpy

obj = bpy.context.active_object
driver = obj.data.shape_keys.key_blocks['Key 1'].driver_add("value")