Deprecate `UserList.__add__` when the argument is not a sequence

[GasinAn]

Bug report

Bug description:

from collections import UserList

my_userlist = UserList([1, 2, 3])
my_dict = {'a': 10, 'b': 20}

print(my_userlist + my_dict)
print(my_dict + my_userlist)
my_userlist += my_dict
print(my_userlist)

my_userlist = UserList([1, 2, 3])
my_set = {5, 4}

print(my_userlist + my_set)
print(my_set + my_userlist)
my_userlist += my_set
print(my_userlist)

In Lib/collections/__init__.py, it will do self.__class__(self.data + list(other)), self.__class__(list(other) + self.data) and self.data += list(other). However, it allows other to by any iterable object, even if other is a dict object or a set object. Should this be fixed?

CPython versions tested on:

3.12

Operating systems tested on:

Windows

[picnixz]

I don't think we can change this behavior, it would be backward incompatible. While plain lists don't support + for arbitrary iterables, UserList does. If you want to change this behavior, please open https://discuss.python.org/c/ideas/6 to discuss the rationale and why (but you really need convincing arguments for that). I don't think it hurts to have more feature on this type compared to bare lists so I'm going to close it for now.

We could deprecate that behavior in favor of only supporting real sequences but I don't think it's worth the change.

cc @rhettinger