CODE EXAMPLE FOR PYTHON

pafy python documentation

import pafy

# callback function, this callback simply prints the bytes received,
# ratio downloaded and eta.
def mycb(total, recvd, ratio, rate, eta):
    print(recvd, ratio, eta)

p = pafy.new("cyMHZVT91Dw")
ba = p.getbestaudio()
filename = ba.download(quiet=True, callback=mycb)