python33, dictionary, NoneType' object has no attribute 'append'

[gintare]

I am not able to initialize empty dictionary in python33.
I have tried:

D=dict()
D={}

Later i want to insert values using:

D.setdefault(num).append(ansdb2.Rname)

In both cases i am getting error "NoneType' object has no attribute 'append'"

[gintare]

It should be empty brackets as a second parameter to setdefault:

D.setdefault(num, []).append(ansdb2.Rname)