Fix swallowed TypeError in async generator wrapper fallback

[Parth-001-git]

Summary

Replaces broad except TypeError fallback logic with explicit sys.version_info checks when handling async generator task creation.

This prevents unrelated user TypeError exceptions from being silently swallowed inside _ContextPreservedAsyncGeneratorWrapper.__anext__.

Reproduction

The issue can be reproduced using the async generator example from #12520, where a user-side TypeError was previously swallowed and iteration completed silently.

Fix

• Replaced runtime except TypeError fallback with explicit Python version branching.
• Preserves Python 3.10 compatibility behavior.
• Allows genuine user exceptions to propagate correctly.

Greptile Summary

This PR fixes a long-standing bug where a broad except TypeError catch in _ContextPreservedAsyncGeneratorWrapper.__anext__ and aclose silently swallowed any TypeError raised by the user's generator, replacing it with an explicit sys.version_info >= (3, 11) branch that reflects the true Python version boundary at which asyncio.create_task gained its context keyword argument.

• Core fix: User-thrown TypeErrors inside async generators now propagate correctly instead of being silently caught and re-routed through the Python 3.10 fallback path.
• Scope: Both aclose and __anext__ receive the same fix; the sys module is added at the top of the module (compliant with the repo import rule).
• Style nit: The version check predicate is duplicated in two methods and import sys is inserted ahead of import asyncio, breaking the existing alphabetical import order.

Confidence Score: 4/5

Safe to merge — the changed code correctly replaces runtime exception catching with an explicit Python version check, and behavior on both code paths is preserved.

The logic change is accurate: asyncio.create_task's context keyword was introduced in Python 3.11, so the version guard is the right boundary. Error propagation through the outer except block is unchanged. The only remaining notes are style-level: the duplicated version predicate and the out-of-order import sys line.

No files require special attention; both affected methods in langfuse/_client/observe.py received the same treatment consistently.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A["__anext__ called"] --> B{sys.version_info >= 3.11?}
    B -- Yes --> C["asyncio.create_task(generator.__anext__(), context=self.context)"]
    B -- No --> D["self.context.run(asyncio.create_task, generator.__anext__())"]
    C --> E{Exception?}
    D --> E
    E -- StopAsyncIteration --> F["_finalize() + re-raise"]
    E -- Exception / CancelledError --> G["_finalize_with_error(e) + re-raise"]
    E -- No exception --> H["Append item if capture_output"]
    H --> I["return item"]

    J["aclose called"] --> K{sys.version_info >= 3.11?}
    K -- Yes --> L["asyncio.create_task(generator.aclose(), context=self.context)"]
    K -- No --> M["self.context.run(asyncio.create_task, generator.aclose())"]
    L --> N{Exception?}
    M --> N
    N -- Yes --> O["_finalize_with_error(error) + re-raise"]
    N -- No --> P["_finalize()"]

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Prompt To Fix All With AI

Fix the following 2 code review issues. Work through them one at a time, proposing concise fixes.

---

### Issue 1 of 2
langfuse/_client/observe.py:1-2
The `sys.version_info >= (3, 11)` predicate is evaluated on every `aclose` and `__anext__` call and duplicated in two places. A single module-level constant avoids both the minor repetition and makes the intent obvious to future readers.

```suggestion
import sys
import asyncio

# asyncio.create_task(context=...) was added in Python 3.11
_TASK_CONTEXT_KWARG_SUPPORTED = sys.version_info >= (3, 11)
```

### Issue 2 of 2
langfuse/_client/observe.py:1-5
Standard library imports are ordered alphabetically throughout the existing block (`asyncio`, `contextvars`, `inspect`, `os`). Inserting `sys` at position 1 breaks that ordering; it should follow `os`.

```suggestion
import asyncio
import contextvars
import inspect
import os
import sys
```

_{Reviews (1): Last reviewed commit: "Fix swallowed TypeError in async generat..." | Re-trigger Greptile}

Context used:

• Rule used - Move imports to the top of the module instead of p... (source)

Learned From
langfuse/langfuse-python#1387

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