Reference class variable in a comprehension of another class variable

This may be a simple question, but I'm having trouble making a unique search for it.

I have a class that defines a static dictionary, then attempts to define a subset of that dictionary, also statically.

So, as a toy example:

class example(object): 
    first_d = {1:1,2:2,3:3,4:4} 
    second_d = dict((k,first_d[k]) for k in (2,3))

This produces NameError: global name 'first_d' is not defined

How should I be making this reference? It seems this pattern works in other cases, eg:

class example2(object):
    first = 1
    second = first + 1