AttributeError: ‘NoneType‘ object has no attribute ‘sort‘

reason

It may be when returning a function , This function returns nothing
For example, one is returned here dfs function , This function itself does not return anything , So wrong reporting

from typing import (
    List,
)
import math
class Solution:
    """
    @param n: An integer
    @return: a list of combination
             we will sort your return value in output
    """
    def dfs(self, n, start, sub_ans, ans):
        if start <= 1:
            if len(sub_ans) > 1:
                ans.append(sub_ans[:])
            return
        for i in range(start, int(math.sqrt(n))+1):
            if n%i == 0:
                sub_ans.append(i)
                self.dfs(n//i, i, sub_ans, ans)
                sub_ans.pop()
        if start <= n:
            sub_ans.append(n)
            self.dfs(1, 1, sub_ans, ans)
            sub_ans.pop()
    def get_factors(self, n: int) -> List[List[int]]:
        # write your code here
        ans = []
        return self.dfs(n, 2, [], ans) #  There is a mistake here 

solve

Call this function directly , Return the previously defined reference passing variables that participate in the function as parameters .
For example, call directly here dfs, return ans.

from typing import (
    List,
)
import math
class Solution:
    """
    @param n: An integer
    @return: a list of combination
             we will sort your return value in output
    """
    def dfs(self, n, start, sub_ans, ans):
        if start <= 1:
            if len(sub_ans) > 1:
                ans.append(sub_ans[:])
            return
        for i in range(start, int(math.sqrt(n))+1):
            if n%i == 0:
                sub_ans.append(i)
                self.dfs(n//i, i, sub_ans, ans)
                sub_ans.pop()
        if start <= n:
            sub_ans.append(n)
            self.dfs(1, 1, sub_ans, ans)
            sub_ans.pop()
    def get_factors(self, n: int) -> List[List[int]]:
        # write your code here
        ans = []
        self.dfs(n, 2, [], ans) #  Call directly 
        return ans #  Return reference transfer variable