Assertion failure from StringIO.__setstate__

[devdanzin]

Crash report

What happened?

It's possible to abort the interpreter by calling StringIO.__setstate__ with a non-string initial_value:

python -c "from io import StringIO; StringIO().__setstate__((None, '', 0, {}))"
python: Objects/unicodeobject.c:2542: as_ucs4: Assertion `PyUnicode_Check(string)' failed.
Aborted (core dumped)

Interestingly, on a non-debug build passing an int as initial_value gives an error message saying that None should be a valid value:

python -c "from io import StringIO; StringIO().__setstate__((1, '', 0, {}))"
Traceback (most recent call last):
  File "<string>", line 1, in <module>
    from io import StringIO; StringIO().__setstate__((1, '', 0, {}))
                             ~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^
TypeError: initial_value must be str or None, not int

Found using fusil by @vstinner.

CPython versions tested on:

3.13, 3.14, CPython main branch

Operating systems tested on:

Linux

Output from running 'python -VV' on the command line:

Python 3.14.0a2+ (heads/main:0af4ec3, Nov 20 2024, 21:45:19) [GCC 13.2.0]