Mohammed,
That's good question and I'm working on a project that requires me to do similar but mine is frequency.
I believe that once a code is run and your receiver is running that changing a variable is not possible.
However I have a workaround, I am using a file as a source which will be either blank (so no action takes place) or a frequency value, this triggers a reset of python and then reads in the value as a variable.
This works quite well in my case but may not suit all applications. My receiver only needs to be on when I trigger it from the transmitter, it can't be used if you're looking for a constant FFT plot.
Hope this helps and if there are any suggestions for a more refined solution that would be great.
Brian