`typing.Union` does not support attribute assignment post gh-105511

[XuehaiPan]

Bug report

Bug description:

Before #105511, typing.Union is implemented in Python, and custom attributes can be assigned to a Union variable.

• gh-105499: Merge typing.Union and types.UnionType #105511

ta = Union[int, str]
ta.__some_attribute__ = (int, str)

However, 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.

$ ipython
Python 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)]
Type 'copyright', 'credits' or 'license' for more information
IPython 9.0.2 -- An enhanced Interactive Python. Type '?' for help.
Tip: Use `object?` to see the help on `object`, `object??` to view it's source

In [1]: import optree

In [2]: optree.PyTree[int]
/Users/PanXuehai/Projects/optree/env/lib/python3.14t/site-packages/IPython/core/interactiveshell.py:3111: SyntaxWarning: 'return' in a 'finally' block
  return result
╭────────────────────────────────────────────────────────────────────── Traceback (most recent call last) ───────────────────────────────────────────────────────────────────────╮
│ in <module>:1                                                                                                                                                                  │
│                                                                                                                                                                                │
│ /Users/PanXuehai/Projects/cpython/.pydev/lib/python3.14t/typing.py:398 in inner                                                                                                │
│                                                                                                                                                                                │
│    395 │   │   @functools.wraps(func)                                                                                                                                          │
│    396 │   │   def inner(*args, **kwds):                                                                                                                                       │
│    397 │   │   │   try:                                                                                                                                                        │
│ ❱  398 │   │   │   │   return _caches[func](*args, **kwds)                                                                                                                     │
│    399 │   │   │   except TypeError:                                                                                                                                           │
│    400 │   │   │   │   pass  # All real errors (not unhashable args) are raised below.                                                                                         │
│    401 │   │   │   return func(*args, **kwds)                                                                                                                                  │
│                                                                                                                                                                                │
│ /Users/PanXuehai/Projects/optree/optree/typing.py:254 in __class_getitem__                                                                                                     │
│                                                                                                                                                                                │
│   251 │   │   │   Deque[recurse_ref],  # type: ignore[valid-type]                                                                                                              │
│   252 │   │   │   CustomTreeNode[recurse_ref],  # type: ignore[valid-type]                                                                                                     │
│   253 │   │   ]                                                                                                                                                                │
│ ❱ 254 │   │   pytree_alias.__pytree_args__ = item  # type: ignore[attr-defined]                                                                                                │
│   255 │   │                                                                                                                                                                    │
│   256 │   │   # pylint: disable-next=no-member                                                                                                                                 │
│   257 │   │   original_copy_with = pytree_alias.copy_with  # type: ignore[attr-defined]                                                                                        │
╰────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────╯
AttributeError: 'typing.Union' object has no attribute '__pytree_args__' and no __dict__ for setting new attributes

In [3]: class Foo(type(int | str)):
   ...:     pass
   ...:     
╭────────────────────────────────────────────────────────────────────── Traceback (most recent call last) ───────────────────────────────────────────────────────────────────────╮
│ in <module>:1                                                                                                                                                                  │
╰────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────╯
TypeError: type 'typing.Union' is not an acceptable base type

Repro:

./python -m pip install -v optree
./python -c 'import optree; optree.PyTree[int]'

Source: https://github.com/metaopt/optree/blob/v0.15.0/optree/typing.py#L246-L254

cc @JelleZijlstra

CPython versions tested on:

CPython main branch

Operating systems tested on:

macOS

Linked PRs

• gh-132139: Document that you can no longer set attributes on unions #132146
• gh-132139: 3.14 what's new: elaborate on why you can no longer set Union attributes #132157

[picnixz]

Was it designed to support custom attributes? is it a documented behavior? (I don't know so I'm just asking if this was deliberate)

[JelleZijlstra]

Yeah, don't do that. Setting arbitrary attributes on Union objects (or any other typing objects) was never supported and if you do that sort of thing, be prepared for your code to break.

That said, we could allow setting arbitrary attributes again by adding a managed __dict__ to union objects, at a cost of increasing memory usage for every other user of unions. I'd be curious to hear opinions on whether that's worth it.

[picnixz]

at a cost of increasing memory usage for every other user of unions

How much of a cost are we talking about? I think you mentioned something about the fact that unions' size is smaller (or was it larger?) but I can't find the issue (I think I read this yesterday somewhere). If we're only talking about a few bytes when it's not used, then why not. But if we're doubling the union sizes I think it's maybe not worth.

Now, are there other projects for which this could be needed? if there is enough usage, and from large libraries, then maybe it's a possibility? By the way, for what reason do you actually need unions to support assignment?