Switch backend detection to use entrypoints.

docs/versionhistory.rst

@@ -3,6 +3,11 @@ Version history
 
 This library adheres to `Semantic Versioning 2.0 <http://semver.org/>`_.
 
+**UNRELEASED**
+
+- Switched backend loading to use
+  `import entrypoints <https://setuptools.pypa.io/en/latest/userguide/entry_point.html>`_.
+
 **4.6.2**
 
 - Fixed regression caused by (`#807 <https://github.com/agronholm/anyio/pull/807>`_)

pyproject.toml

@@ -67,6 +67,10 @@ doc = [
 [project.entry-points]
 pytest11 = {anyio = "anyio.pytest_plugin"}
 
+[project.entry-points."anyio.backends"]
+trio = "anyio._backends._trio:backend_class"
+asyncio = "anyio._backends._asyncio:backend_class"
+
 [tool.setuptools_scm]
 version_scheme = "post-release"
 local_scheme = "dirty-tag"

src/anyio/_core/_eventloop.py

@@ -5,7 +5,7 @@
 import threading
 from collections.abc import Awaitable, Callable, Generator
 from contextlib import contextmanager
-from importlib import import_module
+from importlib.metadata import EntryPoint, entry_points
 from typing import TYPE_CHECKING, Any, TypeVar
 
 import sniffio
@@ -18,13 +18,38 @@
 if TYPE_CHECKING:
     from ..abc import AsyncBackend
 
+
+def find_backends() -> dict[str, EntryPoint]:
+    """
+    Loads the available backends from setuptools entrypoints.
+    """
+
+    backends: dict[str, EntryPoint]
+
+    # for some reason, in 3.12 and above, EntryPoints.__getitem__ was changed to return a
+    # *single* entrypoint, rather than a list.
+
+    if sys.version_info < (3, 12):
+        eps = entry_points()
+        found_backends = eps["anyio.backends"]
+        backends = {ep.name: ep for ep in found_backends}
+    else:
+        eps = entry_points(group="anyio.backends")
+        backends = {}
+        for ep in eps:
+            backends[ep.name] = ep
+
+    return backends
+
+
 # This must be updated when new backends are introduced
 BACKENDS = "asyncio", "trio"
 
 T_Retval = TypeVar("T_Retval")
 PosArgsT = TypeVarTuple("PosArgsT")
 
 threadlocals = threading.local()
+available_backends: dict[str, EntryPoint] = find_backends()
 loaded_backends: dict[str, type[AsyncBackend]] = {}
 
 
@@ -60,7 +85,7 @@ def run(
 
     try:
         async_backend = get_async_backend(backend)
-    except ImportError as exc:
+    except (ImportError, KeyError) as exc:
         raise LookupError(f"No such backend: {backend}") from exc
 
     token = None
@@ -124,8 +149,8 @@ def current_time() -> float:
 
 
 def get_all_backends() -> tuple[str, ...]:
-    """Return a tuple of the names of all built-in backends."""
-    return BACKENDS
+    """Return a tuple of the names of all available backends."""
+    return tuple(available_backends.keys())
 
 
 def get_cancelled_exc_class() -> type[BaseException]:
@@ -161,6 +186,6 @@ def get_async_backend(asynclib_name: str | None = None) -> type[AsyncBackend]:
     try:
         return loaded_backends[asynclib_name]
     except KeyError:
-        module = import_module(f"anyio._backends._{asynclib_name}")
-        loaded_backends[asynclib_name] = module.backend_class
-        return module.backend_class
+        loaded_backend: type[AsyncBackend] = available_backends[asynclib_name].load()
+        loaded_backends[asynclib_name] = loaded_backend
+        return loaded_backend

tests/test_eventloop.py

@@ -9,7 +9,7 @@
 from pytest import MonkeyPatch
 from pytest_mock.plugin import MockerFixture
 
-from anyio import run, sleep_forever, sleep_until
+from anyio import get_all_backends, run, sleep_forever, sleep_until
 
 pytestmark = pytest.mark.anyio
 fake_current_time = 1620581544.0
@@ -48,6 +48,13 @@ async def async_add(x: int, y: int) -> int:
     assert result == 3
 
 
+def test_find_builtin_backends() -> None:
+    backends = get_all_backends()
+    assert len(backends) >= 2
+    assert any(x == "trio" for x in backends)
+    assert any(x == "asyncio" for x in backends)
+
+
 class TestAsyncioOptions:
     def test_debug(self) -> None:
         async def main() -> bool: