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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"`typing.Union` does not support attribute assignment post gh-105511","articleBody":"# Bug report\n\n### Bug description:\n\nBefore #105511, `typing.Union` is implemented in Python, and custom attributes can be assigned to a `Union` variable.\n\n- #105511\n\n```python\nta = Union[int, str]\nta.__some_attribute__ = (int, str)\n```\n\nHowever, after #105511, `typing.Union` is now implemented in C, and no attributes can be assigned to it. The `UnionType` does not allow subclassing; users cannot bypass this using subclassing.\n\n```python\n$ ipython\nPython 3.14.0a6+ experimental free-threading build (heads/main:1755157207c, Apr  6 2025, 02:21:10) [Clang 17.0.0 (clang-1700.0.13.3)]\nType 'copyright', 'credits' or 'license' for more information\nIPython 9.0.2 -- An enhanced Interactive Python. Type '?' for help.\nTip: Use `object?` to see the help on `object`, `object??` to view it's source\n\nIn [1]: import optree\n\nIn [2]: optree.PyTree[int]\n/Users/PanXuehai/Projects/optree/env/lib/python3.14t/site-packages/IPython/core/interactiveshell.py:3111: SyntaxWarning: 'return' in a 'finally' block\n  return result\n╭────────────────────────────────────────────────────────────────────── Traceback (most recent call last) ───────────────────────────────────────────────────────────────────────╮\n│ in \u003cmodule\u003e:1                                                                                                                                                                  │\n│                                                                                                                                                                                │\n│ /Users/PanXuehai/Projects/cpython/.pydev/lib/python3.14t/typing.py:398 in inner                                                                                                │\n│                                                                                                                                                                                │\n│    395 │   │   @functools.wraps(func)                                                                                                                                          │\n│    396 │   │   def inner(*args, **kwds):                                                                                                                                       │\n│    397 │   │   │   try:                                                                                                                                                        │\n│ ❱  398 │   │   │   │   return _caches[func](*args, **kwds)                                                                                                                     │\n│    399 │   │   │   except TypeError:                                                                                                                                           │\n│    400 │   │   │   │   pass  # All real errors (not unhashable args) are raised below.                                                                                         │\n│    401 │   │   │   return func(*args, **kwds)                                                                                                                                  │\n│                                                                                                                                                                                │\n│ /Users/PanXuehai/Projects/optree/optree/typing.py:254 in __class_getitem__                                                                                                     │\n│                                                                                                                                                                                │\n│   251 │   │   │   Deque[recurse_ref],  # type: ignore[valid-type]                                                                                                              │\n│   252 │   │   │   CustomTreeNode[recurse_ref],  # type: ignore[valid-type]                                                                                                     │\n│   253 │   │   ]                                                                                                                                                                │\n│ ❱ 254 │   │   pytree_alias.__pytree_args__ = item  # type: ignore[attr-defined]                                                                                                │\n│   255 │   │                                                                                                                                                                    │\n│   256 │   │   # pylint: disable-next=no-member                                                                                                                                 │\n│   257 │   │   original_copy_with = pytree_alias.copy_with  # type: ignore[attr-defined]                                                                                        │\n╰────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────╯\nAttributeError: 'typing.Union' object has no attribute '__pytree_args__' and no __dict__ for setting new attributes\n\nIn [3]: class Foo(type(int | str)):\n   ...:     pass\n   ...:     \n╭────────────────────────────────────────────────────────────────────── Traceback (most recent call last) ───────────────────────────────────────────────────────────────────────╮\n│ in \u003cmodule\u003e:1                                                                                                                                                                  │\n╰────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────╯\nTypeError: type 'typing.Union' is not an acceptable base type\n```\n\nRepro:\n\n```bash\n./python -m pip install -v optree\n./python -c 'import optree; optree.PyTree[int]'\n```\n\nSource: https://github.com/metaopt/optree/blob/v0.15.0/optree/typing.py#L246-L254\n\ncc @JelleZijlstra\n\n### CPython versions tested on:\n\nCPython main branch\n\n### Operating systems tested on:\n\nmacOS\n\n\u003c!-- gh-linked-prs --\u003e\n### Linked PRs\n* gh-132146\n* gh-132157\n\u003c!-- /gh-linked-prs --\u003e\n","author":{"url":"https://github.com/XuehaiPan","@type":"Person","name":"XuehaiPan"},"datePublished":"2025-04-05T18:44:48.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":18},"url":"https://github.com/132139/cpython/issues/132139"}