>>> a = []
>>> a += "hello"
>>> a
['h', 'e', 'l', 'l', 'o']
strings are iterable and a whole lot of things in python are happy to accept an iterable as input.
But when you do that, then you’re not giving it a string, you’re giving it some iterable of characters.
So, consider what you asked for to happen. In my example, if I wanted to add a string to the list, then I would append the string to the list.