UserString subtypes parameterless `__init__`

Description

Subtypes of UserString cannot be initialized without a seq argument as the builtin str can.

Reproduction

from collections import UserString

class MyString(UserString):
    pass

>>> str()
''
>>> MyString()
Traceback (most recent call last):
  File "<python-input-2>", line 1, in <module>
    MyString()
    ~~~~~~~~^^
TypeError: UserString.__init__() missing 1 required positional argument: 'seq'

Workaround

from collections import UserString

class MyString(UserString):
    def __init__(self, seq: object = str()) -> None:
        super().__init__(seq)

CPython versions tested on:

3.13

Operating systems tested on:

Linux

從 collections.UserString 入口開始，在 Python 3.13 上重現無參數建構範例。檢查 UserString 的初始化行為，並驗證沒有自訂 init 的子類別可以像 str() 一樣被具現化，同時現有的序列初始化仍然正確。