V Pythonu nedělám, ale zkusil jsem něco takového:

def flatten(array):
    def inner(array):
        if len(array) == 0:
            return
        
        for i in range(len(array)):
            item = array[i]
            if type(item) == type([]):
                inner(item)
            else:
                result.append(item)
                
    result = []
    inner(array)
    return result

print(flatten([2, 9, [2, 1, 13, 2], 8, [2, 6]]))
print(flatten([[9, [7, 1, 13, 2], 8], [2, 6]]))
print(flatten([[5, [5, [1, 5], 5], 5], [5, 6]]))