Add new gotcha

Hi,

While running tests on my code I was doing something like:

class Data:
    mylist = [1,2,3,4]


def test_function():
    local_list = Data.mylist
    local_list+=new_list

    # Do stuff with 

and there are multiple tests using mylist. This is going to modify the list instead of creating a new one, an unexpected behavior of +=. This will let one test talk to the tests happening later. The safer approach is:

class Data:
    mylist = [1,2,3,4]


def test_function():
    local_list =new_list + Data.mylist

    # Do stuff with 

here + will copy the list. Could you please add this case?

Cheers.

issue 中没有指定文件、测试或入口点。首先在 guide 中找到关于易错点或列表变更的部分，然后添加这个 += 与 + 的案例；当示例和更安全的替代方案出现在适当的文档部分时，就算完成。