Migrate from asyncio.coroutines._is_coroutine to inspect.markcoroutinefunction() for Python 3.16+ compatibility #86

Background

In promising/promising_function.py, PromisingFunction uses the following private asyncio marker to make asyncio.iscoroutinefunction() recognize it as a coroutine function:

_is_coroutine = asyncio.coroutines._is_coroutine

This is an internal, undocumented implementation detail of asyncio. While it works correctly today (Python 3.11–3.14), asyncio.iscoroutinefunction() has been deprecated as of Python 3.14 and is slated for removal in Python 3.16.

The stable, public replacement is inspect.markcoroutinefunction(), introduced in Python 3.12.

Suggested approach

Since the project currently supports Python 3.10+, a compatibility shim is needed:

import inspect
import sys

if sys.version_info >= (3, 12):
    inspect.markcoroutinefunction(PromisingFunction)
else:
    PromisingFunction._is_coroutine = asyncio.coroutines._is_coroutine

Or, equivalently, apply inspect.markcoroutinefunction() unconditionally when available, with the private sentinel as a fallback for Python < 3.12.

Impact

• No current breakage — the existing code works on all supported Python versions.
• Action needed before Python 3.16 support is required, when asyncio.iscoroutinefunction() and the _is_coroutine sentinel may be removed.

References

• Raised in PR PromisingContext::get_trace() #79 (comment: PromisingContext::get_trace() #79 (comment))
• Reported by @teremterem
• CPython issue: Deprecate asyncio.iscoroutinefunction python/cpython#122858
• CPython PR: gh-94912: Added marker for non-standard coroutine function detection python/cpython#99247