A very very simple flow graph

#bring in blocks from the main gnu radio package
from gnuradio import gr

#create the top block
tb = gr.top_block()

#create a signal source
src = gr.null_source(1) #1 indicates the data stream size

#create a signal sink
sink = gr.null_sink(1)

#connect the source to the sink
tb.connect(src, sink)

#run the flow graph
tb.run()

This flow graph does nothing. The null source spits out zeros, and the null sink takes

a data stream and does nothing with it.

Also, there is nothing in the flow graph to control the data rate, the 

CPU usage should climb to 100% 

Source : Josh Knows