Fix durableClient binding FunctionLoadError on Python 3.14 (PEP 649/749)

[nachogarcia]

Problem

On Python 3.14, a function app that uses @app.durable_client_input(...) fails to start: the durableClient binding fails type validation in the Functions Python worker, raising FunctionLoadError, and the host returns 503. The same app works on Python ≤3.13.

Fixes #596.

Root cause

DFApp._add_rich_client makes the worker's binding type-check pass by forcing the client parameter's annotation to str:

user_code.__annotations__[parameter_name] = str

…and then wraps the user function with @functools.wraps(user_code) into df_client_middleware.

PEP 649 / PEP 749 (Python 3.14) changed how annotations are stored: functions now carry an __annotate__ callable, and functools.WRAPPER_ASSIGNMENTS copies __annotate__ instead of the __annotations__ dict. The copied __annotate__ lazily re-derives the original DurableOrchestrationClient annotation, overriding the str patch. The worker reads the wrapper's annotation as DurableOrchestrationClient, durableClient binding validation fails, and startup 503s.

On Python ≤3.13 there is no __annotate__, so wraps copied the already-patched __annotations__ dict and everything worked.

Fix

After the @wraps wrapper is created, re-apply the str override on the wrapper and drop its __annotate__, so both the dict-based reader and the __annotate__-based reader (used by the worker on 3.14) agree:

_ann = dict(getattr(df_client_middleware, "__annotations__", {}))
_ann[parameter_name] = str
df_client_middleware.__annotations__ = _ann
if hasattr(df_client_middleware, "__annotate__"):
    df_client_middleware.__annotate__ = None

The pre-wrap patch is kept for the ≤3.13 path. The block is a guarded no-op on ≤3.13 (hasattr(__annotate__) is False there). Runtime behavior is unchanged: the rich DurableOrchestrationClient is still constructed from the starter string and passed to user code — only the annotation the worker inspects is forced to str.

Testing

• New regression test test_durable_client_input_annotation_overridden_to_str asserts the built wrapper's client-parameter annotation reads as str via both __annotations__ and (on 3.14) annotationlib.get_annotations(..., format=Format.FORWARDREF). It fails on 3.14 without the fix and passes with it.
• Full suite: 310 passed on Python 3.14.6.
• Decorator tests also pass on Python 3.13.7 (no regression on the pre-PEP-649 path).
• flake8 ./azure/ clean.

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