Can only concatenate list (not "str") to list. What is the problem?

Question

Python

I made a bot, it was necessary to convert a two-dimensional list into a one-dimensional one . I
used:

def combing_list(self, task:list):
  result = sum(task, [])
  return result

users = self.combing_list(users)

And converted the type, and found out the type (list is always passed), but an error occurs.
TypeError: can only concatenate list (not "str") to list on the second line. What is the problem?

Answer 1

What's on the list?
For your code to work, task must be not just a list, but a list of lists. If there is a line in it, then you will get exactly this error.
Heaps , it might be easier to use itertools.chain.from_iterable() to solve your problem .

Answer 2

What do you want to achieve? If you add lists, then use append/extend.

Answer 3

The fact that there is no such operation
sum(list1, list2)
. What did you want to do? If we add the data of another to one list, then
list1.extend(list2)