AttributeError for python 3.14: 'typing.Union' object has no attribute __discriminator__ and no __dict__ for setting new attributes #1

[lencshu]

Please using Annotated[Union] with Python 3.14

Describe the bug

When casting:

cast(CachedDiscriminatorType, union).__discriminator__ = details

With union being:

anthropic.types.raw_message_start_event.RawMessageStartEvent | anthropic.types.raw_message_delta_event.RawMessageDeltaEvent | anthropic.types.raw_message_stop_event.RawMessageStopEvent | anthropic.types.raw_content_block_start_event.RawContentBlockStartEvent | anthropic.types.raw_content_block_delta_event.RawContentBlockDeltaEvent | anthropic.types.raw_content_block_stop_event.RawContentBlockStopEvent

To Reproduce

I’m getting an error when running the MCP tool call

  File "/app/.venv/lib/python3.14/site-packages/anthropic/_streaming.py", line 185, in __aiter__
    async for item in self._iterator:
        yield item
  File "/app/.venv/lib/python3.14/site-packages/anthropic/_streaming.py", line 214, in __stream__
    yield process_data(data=data, cast_to=cast_to, response=response)
          ~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/app/.venv/lib/python3.14/site-packages/anthropic/_base_client.py", line 641, in _process_response_data
    return cast(ResponseT, construct_type(type_=cast_to, value=data))
                           ~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/app/.venv/lib/python3.14/site-packages/anthropic/_models.py", line 518, in construct_type
    discriminator = _build_discriminated_union_meta(union=type_, meta_annotations=meta)
  File "/app/.venv/lib/python3.14/site-packages/anthropic/_models.py", line 688, in _build_discriminated_union_meta
    cast(CachedDiscriminatorType, union).__discriminator__ = details
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AttributeError: 'typing.Union' object has no attribute '__discriminator__' and no __dict__ for setting new attributes

CachedDiscriminatorType should be an Annotated rather than Union directly

OS
macos

Python version
Python v3.14.0

Library version
anthropic v0.72.0

[Natim]

Refs openai/openai-python#2705

[Natim]

🧩 The Root Cause

Starting with Python 3.14, typing.Union (and also types.UnionType for | syntax) is now fully immutable — you can no longer attach arbitrary attributes to them (they have no __dict__).
Hence this line fails with:

cast(CachedDiscriminatorType, union).__discriminator__ = details

With the error:

AttributeError: 'typing.Union' object has no attribute '__discriminator__' and no __dict__ for setting new attributes

This behavior worked in ≤ 3.13 because Union objects allowed dynamic attributes, but that’s gone.

[Natim]

✅ The Correct Fix

We can’t store metadata on the Union type itself anymore.
Instead, we can keep a sidecar cache (a WeakKeyDictionary) keyed by the union type.

Here’s how you can safely patch both OpenAI and Anthropic implementations:

Step 1 – Create a global cache

Add this near the top of the module (just after CachedDiscriminatorType is defined):

import weakref

# Global cache since typing.Union objects are now immutable (Python 3.14+)
_DISCRIMINATOR_CACHE: "weakref.WeakKeyDictionary[type, DiscriminatorDetails]" = weakref.WeakKeyDictionary()

Step 2 – Update _build_discriminated_union_meta

Replace

if isinstance(union, CachedDiscriminatorType):
    return union.__discriminator__

With

cached = _DISCRIMINATOR_CACHE.get(union)
if cached is not None:
    return cached

Step 3 - Update the ___discriminator__ set

Replace

cast(CachedDiscriminatorType, union).__discriminator__ = details
return details

With

_DISCRIMINATOR_CACHE[union] = details
return details

[Natim]

🧠 Why This Works

• WeakKeyDictionary lets cached entries disappear when their type objects are garbage-collected.
• Works across all supported Python versions.
• Preserves existing “caching” semantics without mutating Union objects.