CODEWITHSUNDEEP
pythonpython-3.x
Ting Sun
Ting Sun

Reputation: 173

Returning list.append() throws "TypeError: unsupported operand type(s)" in recursive function

I try to write a recursive function to return the longest increasing subsequence, but error occur says "TypeError: unsupported operand type(s) for +: 'NoneType' and 'NoneType'"

def helper(cur_seq, seq, cur_i, result):
    if len(seq) == cur_i:
        return result.append(cur_seq)
    else:
        next_i = cur_i + 1
        if len(cur_seq) == 0 or seq[cur_i] > cur_seq[-1]:
            temp = cur_seq.copy()
            temp1 = cur_seq.copy()
            temp.append(seq[cur_i])
            return helper(temp, seq, next_i, result) + helper(temp1, seq, next_i, result)
        else:
            return helper(cur_seq.copy(), seq, next_i, result)


def longest_sub_sequence(seq):
    cur_seq = []

    result = helper(cur_seq, seq, 0, [])

    max_length = 0
    for i in result:
        if len(i) > max_length:
            max_length = len(i)


    return max_length


if __name__ == "__main__":

    seq = [0, 8, 4, 12, 2, 10, 6, 14, 1, 9, 5, 13, 3, 11, 7, 15]
    y = longest_sub_sequence(seq)
    print(y)

Upvotes: 1

Views: 89

Answers (1)

cs95
cs95

Reputation: 402483

list.append is an in-place operation. For example,

l = [1, 2, 3]
result = l.append(4)

print(result)
# None

print(l)
# [1, 2, 3, 4]

append returns None, modifying l in-place. This means that return result.append(cur_seq) in your function will return None, and the two recursive calls result in None + None which gives you the TypeError.

The fix would be to append first, return later.

def helper(cur_seq, seq, cur_i, result):
    if len(seq) == cur_i:
        result.append(cur_seq)
        return result
    ...

Upvotes: 2

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