Fixed import error when exceptiongroup isn't available

[agronholm]

Fixes #2227. Closes #2229. This is an alternate version of the linked PR.

Summary

Last minute changes to the AnyIO 4.0 compatibility PR inadvertently caused an ImportError on Pythons earlier than 3.11 when the exceptiongroup back-port wasn't installed. This makes the import optional and thus restores compatibility with AnyIO 3.x.

Checklist

• I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.

[agronholm]

I also went ahead and updated the pinned AnyIO version as v4.0.0rc1 is out now.

[fluix-dev]

This would also need something like the following patch to fix the tests for anyio < 4 which #2228 raises.

From 917d1c135635d6d9e5aec83633673ec21a529933 Mon Sep 17 00:00:00 2001
From: Steven Guikal <void@fluix.one>
Date: Sun, 30 Jul 2023 17:18:40 -0400
Subject: [PATCH] Fix wsgi test for AnyIO versions before 4.0.0

---
 tests/middleware/test_wsgi.py | 15 ++++++++++-----
 1 file changed, 10 insertions(+), 5 deletions(-)

diff --git a/tests/middleware/test_wsgi.py b/tests/middleware/test_wsgi.py
index ad39754..5b2508c 100644
--- a/tests/middleware/test_wsgi.py
+++ b/tests/middleware/test_wsgi.py
@@ -2,6 +2,7 @@ import sys
 
 import pytest
 
+from importlib.metadata import version
 from starlette.middleware.wsgi import WSGIMiddleware, build_environ
 
 if sys.version_info < (3, 11):  # pragma: no cover
@@ -69,11 +70,15 @@ def test_wsgi_exception(test_client_factory):
     # The HTTP protocol implementations would catch this error and return 500.
     app = WSGIMiddleware(raise_exception)
     client = test_client_factory(app)
-    with pytest.raises(ExceptionGroup) as exc:
-        client.get("/")
-
-    assert len(exc.value.exceptions) == 1
-    assert isinstance(exc.value.exceptions[0], RuntimeError)
+    if version("anyio") < "4":
+        with pytest.raises(RuntimeError):
+            client.get("/")
+    else:
+        with pytest.raises(ExceptionGroup) as exc:
+            client.get("/")
+
+        assert len(exc.value.exceptions) == 1
+        assert isinstance(exc.value.exceptions[0], RuntimeError)
 
 
 def test_wsgi_exc_info(test_client_factory):
-- 
2.41.0

[agronholm]

So do you want me to keep 4.0.0rc1 as the pinned AnyIO version? If so, how do I prove that it also passes on 3.x?

[fluix-dev]

I'm not a contributor so I'm not sure what compatibility guarantees are made or how tests are run. Applying my patch means distributions that have AnyIO < 4 in their repositories (aka everyone since 4.0.0 is only a pre-release) don't need to modify tests.

[agronholm]

Curious – the coverage fails even on AnyIO 3.7.1. Maybe the test changes themselves caused this?

[Kludex]

Closed by mistake.

[agronholm]

The only reason coverage fails here is that the code base is only tested against AnyIO 3 now, and tripping the missing line requires AnyIO 4. Should I add a # pragma: no cover there?

Ideally we would test against both but that may complicate the test suite too much.

[Kludex]

Yes, add the pragma, please.