How to extract list from nested inconsistent dictionary (Python)

Question

I have a nested dictionary which looks something like this:

dict_a={a:{
           a1:{
               a1_1:[list1],
               a1_2:[list2]},
        b:{
           b1:[list3]},
        c:[list4]}

So, as it is visible, the nested dictionary is very inconsistent in terms of number of elements and keys. How do I just access the lists and modify them and put it back to the dictionary?