The answer was given already by either Sivasubramaniam Arunachalam or ch3ka.
I am just adding a performances view of the answer.
dicttest={}
dicttest['ligne1']={'ligne1.1':'test','ligne1.2':'test8'}
%timeit dicttest['ligne1']['ligne1.1']
%timeit dicttest.get('ligne1').get('ligne1.1')
gives us :
112 ns ± 29.7 ns per loop (mean ± std. dev. of 7 runs, 10000000 loops each)
235 ns ± 9.82 ns per loop (mean ± std. dev. of 7 runs, 1000000 loops each)