Liste de la liste d'exclusion

a = [1,2]
b = [2,3]
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# single line
excluded = list(set(a) - set(b))

>>> excluded
[1]

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# Or define function:
def set_approach(a, b):
    return list(set(a) - set(b))

>>> set_approach(a, b)
[1]
Benja