Pcbnew python GetNetClasses() missing default

I am trying to get the netclass from two design and compare them.
import pcbnew
board=pcbnew.GetBoard()
nc=board.GetNetClasses()
len(nc)

return to me 0

If I manually add a netclass from the GUI, it will return 1.
How can I get the default netclass setting for clearance?
And how can I change the default netclass of one board in python?
Thanks