Here's my solution, it works very well however compiler throws an error and I'm unable to go further

Code:

left_join = lambda phrases: ",".join(phrases).replace("right", "left")

Error:

Your result:"___checkio___type___,values"
Right result:"left,left,left,stop"
Fail:left_join({"___checkio___type___":"tuple","values":["left","right","left","stop"]})