GeezerVison is showing github.com — `typing.Union` does not support attribute assignment post gh-105511 · Issue #132139 · python/cpython
typing.Union does not support attribute assignment post gh-105511 #132139
DescriptionBug report
Bug description:
Before #105511, typing.Union is implemented in Python, and custom attributes can be assigned to a Union variable.
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
Activitychanged the title [-][3.14 Regression] `Union` variable does not allow attribute assignment anymore[/-] [+][3.14 Regression] `typing.Union` variable does not allow attribute assignment anymore[/+] on Apr 5, 2025picnixz commented on Apr 5, 2025 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 commented on Apr 5, 2025 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 commented on Apr 5, 2025
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? changed the title [-][3.14 Regression] `typing.Union` variable does not allow attribute assignment anymore[/-] [+]`typing.Union` variable does not allow attribute assignment anymore post gh-105511[/+] on Apr 5, 2025changed the title [-]`typing.Union` variable does not allow attribute assignment anymore post gh-105511[/-] [+]`typing.Union` variable does not allow attribute assignment post gh-105511[/+] on Apr 5, 2025changed the title [-]`typing.Union` variable does not allow attribute assignment post gh-105511[/-] [+]`typing.Union` does not support attribute assignment post gh-105511[/+] on Apr 5, 2025added a commit that references this issue on Apr 6, 2025gh-132139: Document that you can no longer set attributes on unions (#… added a commit that references this issue on Apr 6, 2025gh-132139: 3.14 what's new: elaborate on why you can no longer set `U… albanD commented on Jul 12, 2025 Another interesting side effect of this is for "public API specifications".
Systems like sphinx considers an API public in the current file if the API's __module__ attribute is set to the current file.
We use this in PyTorch in particular to define types, make them public API from sphinx point of view to properly document. An example is here: https://github.com/pytorch/pytorch/blob/aacb9440791f4e9567e2123f19bb062e75c2d242/torch/ao/quantization/__init__.py#L34-L36
The logic at
|
static const char* const cls_attrs[] = { |
|
"__module__", // Required for compatibility with typing module |
|
NULL, |
|
}; |
makes the behavior a bit surprising as accessing the __module__ attribute works but trying to set it fails with 'typing.Union' object has no attribute '__module__' and no __dict__ for setting new attributes (the attribute actually does exist).
Curious to hear what you think here?
Are we misguided on trying to set the __module__ attribute as a marker of public APIs for the Union object? Or we should try and find a way to enable this behavior? XuehaiPan commented on Jul 12, 2025 ContributorAuthor More actions XuehaiPan commented on Jul 12, 2025 ContributorAuthor More actions
The logic at
cpython/Objects/unionobject.c
Lines 400 to 403 in c7d24b8
static const char* const cls_attrs[] = {
"module", // Required for compatibility with typing module
NULL,
};
makes the behavior a bit surprising as accessing the __module__ attribute works but trying to set it fails with 'typing.Union' object has no attribute '__module__' and no __dict__ for setting new attributes (the attribute actually does exist).
__module__ is a class attribute. You cannot assign it via an instance.
albanD commented on Jul 14, 2025
If it is only related to the docs build, I have worked around this by adding a custom typehint_formmatter and saving the Union objects in a weakrefset:
The problem is that it's not just that. We also have programmatic checks to ensure that things in the __all__ field matches with the __module__. But since I cannot change the __module__, I have to remove the Union from the __all__ field and thus import * from that module changes behavior.
Also given how it's defined you can't subclass Union to add a __dict__ to it either. JelleZijlstra commented on Jul 16, 2025 To recapitulate why I am not interested in changing this:
- This was never supported or documented in any way.
- Reliance on this misfeature would have interacted poorly with the cache that existed before 3.14; if multiple modules each create say a
Union[int, str] and set its __module__, they would have clobbered each other.
- This only worked for some kinds of types supported by the type system; for example, unions created through the
| syntax and types.GenericAlias don't support attribute assignments; setting a __module__ attribute on an Annotated object sets its for all Annotated objects.
- Adding this now would require adding more memory usage to the union object, so all users would have to pay extra in memory usage.
As an alternative, I'd suggest using the type statement introduced in Python 3.12. Type aliases created through this feature automatically get the __module__ they were created in.
If you need to support 3.11 and earlier, you can use:
if sys.version_info >= (3, 12):
exec("type Alias = A | B")
else:
Alias = Union[A, B]
Alias.__module__ = __name__
Reacted by Alex Waygood, albanD and Antonio Spadaro albanD commented on Jul 16, 2025 Thanks for the in-depth answer. This all makes sense.
If you need to support 3.11 and earlier, you can use:
I'm afraid that doesn't work as the "type" statement will through an error in <=3.11 even if not executed:
$ cat foo.py
import sys
if sys.version_info >= (3, 12):
type Alias = A | B
else:
Alias = Union[A, B]
Alias.__module__ = __name__
$ python3.10 foo.py
File "/home/albandes/local/pytorch/3.10_release_source/foo.py", line 4
type Alias = A | B
^^^^^
SyntaxError: invalid syntax
$ python3.13 foo.py
$ That is a great feature that would definitely solve our issue otherwise! JelleZijlstra commented on Jul 16, 2025 Sorry yes, you'd need to use exec. I was thinking about that but forgot to write it when I wrote the comment :). Fixed above. albanD commented on Jul 17, 2025 Thanks for the quick answer @JelleZijlstra , given that exec makes our security people jumpy, I was trying to go with the following. Any concern with that approach?
if sys.version_info >= (3, 12):
from typing import TypeAliasType
Alias = TypeAliasType("Alias", A | B)
else:
Alias = Union[A, B]
Alias.__module__ = __name__Reacted by Jelle Zijlstra
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