Python: fix(python): handle callable class middleware safely in _determine_middleware_type (#6697)

[Henry Su (hsusul)]

Motivation & Context

Passing a callable class instance (a class implementing __call__ with fewer than 2 parameters or an unresolvable middleware type) as middleware caused _determine_middleware_type() to raise an unhandled AttributeError: 'BadMiddleware' object has no attribute '__name__' during exception string formatting, masking the intended MiddlewareException.

Description & Review Guide

• What are the major changes?

  • Updated _determine_middleware_type() in python/packages/core/agent_framework/_middleware.py to safely resolve the middleware display name using getattr(middleware, "__name__", type(middleware).__name__) instead of accessing middleware.__name__ directly.
  • Added unit test suite TestCallableClassMiddlewareErrorHandling in python/packages/core/tests/core/test_middleware_with_agent.py covering parameter count mismatch, decorator/annotation mismatch, and undetermined type errors on callable class instances.

• What is the impact of these changes?

  • Callable class instances passed as middleware now properly raise clear, informative MiddlewareException errors with the class name rather than crashing with an AttributeError.

• What do you want reviewers to focus on?

  • Exception handling in _determine_middleware_type and regression test assertions.

Related Issue

Fixes #6697

Contribution Checklist

• The code builds clean without any errors or warnings
• All unit tests pass, and I have added new tests where possible
• The PR follows the Contribution Guidelines
• This PR is linked to an issue and there is no other open PR for this issue (see Related Issue above).
• This is not a breaking change.

[Copilot]

Pull request overview

This PR fixes a Python middleware classification edge case where passing a callable class instance (i.e., an object implementing __call__) could trigger an AttributeError while formatting an error message in _determine_middleware_type(), masking the intended MiddlewareException. The update makes middleware name resolution robust for non-function callables and adds regression tests to ensure clear, informative exceptions are raised.

Changes:

• Resolve middleware display names safely via getattr(middleware, "__name__", type(middleware).__name__) to avoid AttributeError for callable class instances.
• Keep _determine_middleware_type() behavior intact while ensuring all error paths can format messages reliably.
• Add unit tests covering callable-class middleware failures: insufficient parameters, decorator/annotation type mismatch, and undetermined type.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
python/packages/core/agent_framework/_middleware.py	Safely computes a middleware display name and uses it in all exception messages within _determine_middleware_type().
python/packages/core/tests/core/test_middleware_with_agent.py	Adds regression tests ensuring callable class instances produce MiddlewareException messages containing the class name (no AttributeError).

[github-actions]

Python Test Coverage Report •

File	Stmts	Miss	Cover	Missing
packages/core/agent_framework
_middleware.py	401	17	95%	61, 64, 69, 369, 910, 926, 928, 930, 1063, 1066, 1093, 1095, 1231, 1235, 1417, 1421, 1491
TOTAL	45583	4479	90%

Python Unit Test Overview

Tests	Skipped	Failures	Errors	Time
9410	34 💤	0 ❌	0 🔥	2m 7s ⏱️

[Eduard van Valkenburg (eavanvalkenburg)]

the mistaken signature are caught by type checkers (which is good), so we need to explicitly ignore those, see the failing check. Henry Su (@hsusul)

[Henry Su (hsusul)]

Thanks for pointing that out! Updated the test suite in commit 2d716dce8 to annotate the test middleware lists as list[Any] and set _middleware_type directly on the test class.