C API: Detect invalid UCS4 characters in debug mode

Currently, PyUnicode_FromKindAndData() and PyUnicodeWriter_WriteUCS4() create an invalid string if a character is not in the [U+0000; U+10ffff] range. Instead, I propose to raise an exception in this case.

Example creating an invalid string:

$ python3.15
>>> import _testcapi, sys
>>> invalid_ucs4 = (0x11_0000).to_bytes(4, byteorder=sys.byteorder)
>>> s = _testcapi.unicode_fromkindanddata(4, invalid_ucs4*5)

>>> list(s)
SystemError: invalid maximum character passed to PyUnicode_New

>>> s[0]
SystemError: invalid maximum character passed to PyUnicode_New

Linked PRs

With 32-bit wchar_t (ex: Unix), PyUnicode_FromWideChar() and PyUnicodeWriter_WriteWideChar() already raise a ValueError if a character is outside the [U+0000; U+10ffff] range.

pythongh-158445: Reject invalid UCS4 in PyUnicode_FromKindAndData()

Other Python and C functions raising an exception if the code pointer is negative or is greater than 0x10ffff:

Python chr() raises ValueError.
PyUnicode_FromOrdinal(), PyUnicode_Fill() and PyUnicodeWriter_WriteChar() raise ValueError.
PyUnicode_FromFormat("%c") raises OverflowError.
PyUnicode_New() raises SystemError.

PyUnicode_WriteChar() raises ValueError if the character is greater than the string maximum character.

pythongh-158445: Detect invalid UCS4 characters in debug mode

gh-158445: Detect invalid UCS4 characters in debug mode (#158502)

Since checking for invalid characters at runtime was rejected, I repurpose the issue to "Detect invalid UCS4 characters in debug mode".

This broke several buildbots.

gh-158445: Add C API tests on build information functions (#158603)

This broke several buildbots.

Oh, I didn't get buildbot failures report on my PR, so I didn't notice. Thanks for the notification.

So test_capi fails on 2 buildbots: Python is built in release mode with assertion using ./configure (...) CFLAGS=-UNDEBUG

ARM64 Raspbian 3.x: https://buildbot.python.org/#/builders/1678/builds/3888
x86 Debian Non-Debug with X 3.x: https://buildbot.python.org/#/builders/1245/builds/9676

I wrote a first PR #158603 to fix support.built_with_c_assertions() (merged). Previously, -UNDEBUG compiler flag was not handled properly.

I wrote a second PR #158604 to fix test_capi.test_unicode.

gh-158445: Skip invalid UCS-4 tests if Python is built with assertions (

My latest change should fix the 2 buildbots.

Thank you!

gh-158445: Allocate memory in the heap in Py_GetVersion() (#158608)

ARM64 Raspbian 3.x: https://buildbot.python.org/#/builders/1678/builds/3888
x86 Debian Non-Debug with X 3.x: https://buildbot.python.org/#/builders/1245/builds/9676

Both workers are back to green.

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C API: Detect invalid UCS4 characters in debug mode #158445

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C API: Detect invalid UCS4 characters in debug mode #158445

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vstinner commented on Sep 29, 2026

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