Why can't I refer to another object in Python class declaration

Question

The following is my demo program,

I want to create a temporary object to use in class declaration

But it said, NameError: name 'tmp' is not defined

I want to know why and how to do this

from dataclasses import dataclass

@dataclass
class A:
    ID:int = 0            
    Flag1:bool = False 
    Flag2:bool = False 

def A_to_dict(a:A):
    return{"ID":a.ID,
            "Flag1":a.Flag1,
            'Flag2': a.Flag2}


class B:    
    tmp = A_to_dict(A())        
    # bvars = [x for x in tmp if type(tmp[x]) is bool]  # NameError: name 'tmp' is not defined 
    bvars = [x for x in A_to_dict(A()) if type(A_to_dict(A())[x]) is bool]  # OK
    

b = B()
print(b.bvars)