Python 3.14 regression in supported types for | syntax for typing.Union and strings

[onerandomusername]

Bug report

Bug description:

On Python 3.14, it seems that pipe unions with typing.Union and str objects raise a TypeError. This is a regression from Python 3.13 behaviour, where Union could be or'd with a str to create a Union with a ForwardRef.

Python 3.13 behaviour

>>> from typing import Union
>>> Union[int, str] | 'tuple'
typing.Union[int, str, ForwardRef('tuple')]

Python 3.14 behaviour

>>> from typing import Union
>>> Union[int, str] | 'tuple'
Traceback (most recent call last):
  File "<python-input-1>", line 1, in <module>
    Union[int, str] | 'tuple'
    ~~~~~~~~~~~~~~~~^~~~~~~~~
TypeError: unsupported operand type(s) for |: 'typing.Union' and 'str'

Version information

>>> sys.version_info
sys.version_info(major=3, minor=14, micro=0, releaselevel='final', serial=0)

CPython versions tested on:

3.14

Operating systems tested on:

Linux

Linked PRs

• gh-140348: Fix using | on unusual objects plus Unions #140383

[xuantengh]

bisect to dc6d66f

[xuantengh]

The error comes due to str is treated as not unionable:

cpython/Objects/unionobject.c

Lines 243 to 261 in 5d0abb6

	static int
	is_unionable(PyObject *obj)
	{
	if (obj == Py_None ||
	PyType_Check(obj) ||
	_PyGenericAlias_Check(obj) ||
	_PyUnion_Check(obj) ||
	Py_IS_TYPE(obj, &_PyTypeAlias_Type)) {
	return 1;
	}
	return 0;
	}
	PyObject *
	_Py_union_type_or(PyObject* self, PyObject* other)
	{
	if (!is_unionable(self) || !is_unionable(other)) {
	Py_RETURN_NOTIMPLEMENTED;
	}

[vstinner]

cc @JelleZijlstra

[bswck]

Reproducible in 3.15:

Python 3.15.0a1+ (heads/main:38d4b436ca7, Oct 20 2025, 20:42:58) [GCC 15.2.1 20250808 (Red Hat 15.2.1-1)] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> from typing import Union
>>> Union[int, str] | 'complex'
Traceback (most recent call last):
  File "<python-input-1>", line 1, in <module>
    Union[int, str] | 'complex'
    ~~~~~~~~~~~~~~~~^~~~~~~~~~~
TypeError: unsupported operand type(s) for |: 'typing.Union' and 'str'

@ZeroIntensity (mentorship) I'd add labels: 3.14, 3.15

[diegorusso]

Also xdsl package might be affected:

$ python
Python 3.15.0a1+ (heads/main-dirty:237dca52ba1, Oct 22 2025, 12:29:48) [GCC 13.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> from xdsl import dialects
Traceback (most recent call last):
  File "<python-input-0>", line 1, in <module>
    from xdsl import dialects
  File "/tmp/pyperformance.kRffc2sdK/cpython/build/venv/lib/python3.15/site-packages/xdsl/dialects/__init__.py", line 5, in <module>
    from xdsl.utils.dialect_loader import IRDLDialectFinder
  File "/tmp/pyperformance.kRffc2sdK/cpython/build/venv/lib/python3.15/site-packages/xdsl/utils/dialect_loader.py", line 12, in <module>
    from xdsl.utils.dialect_stub import DialectStubGenerator
  File "/tmp/pyperformance.kRffc2sdK/cpython/build/venv/lib/python3.15/site-packages/xdsl/utils/dialect_stub.py", line 9, in <module>
    import xdsl.irdl
  File "/tmp/pyperformance.kRffc2sdK/cpython/build/venv/lib/python3.15/site-packages/xdsl/irdl/__init__.py", line 3, in <module>
    from .attributes import *  # noqa: TID251
    ^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/tmp/pyperformance.kRffc2sdK/cpython/build/venv/lib/python3.15/site-packages/xdsl/irdl/attributes.py", line 409, in <module>
    AttrConstraint[AttributeInvT]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    | AttributeInvT
    ~~~~~~~~~~~~~~~
    | type[AttributeInvT]
    ~~~~~~~~~~~~~~~~~~~~~
    | "TypeForm[AttributeInvT]"
    ^~~~~~~~~~~~~~~~~~~~~~~~~~~
TypeError: unsupported operand type(s) for |: 'typing.Union' and 'str'