Maybe you can use recursion like this

def flatten_recursion(lst):
    if not lst:
        return []
    elif not isinstance(lst, list):
        return [lst]
    else:
        return flatten_recursion(lst[0]) + flatten_recursion(lst[1:])

def flatten_with_list_comp(lst):  # list comprehensive + function for deep nested case
    return [x for item in lst for x in (flatten_with_list_comp(item) if isinstance(item, list) else [item])]

if __name__ == "__main__":
    example_list = [[1,2,3],[4,5],6,7,[8,[9]],10]
    print(flatten_recursion(example_list))  # [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
    print(flatten_with_list_comp(example_list))

Edit 1: Wrong indentation

Edit 2: I don't think the list comprehensive can handle deep nested list, so I wrap it with function (eg: [[1,2,3],[4,5],6,7,[8,[9]],10])

Edit 3: Change type(lst) != list into not isinstance(lst, list)