fix: read the tornado global live instead of an import-time snapshot

releasenotes/notes/fix-tornado-global-desync-8c2d40fa71b9e35a.yaml

@@ -0,0 +1,7 @@
+---
+fixes:
+  - |
+    Setting ``tenacity.tornado = None`` -- the usual way a downstream test
+    suite forces the non-tornado code path -- raised ``AttributeError: 'NoneType'
+    object has no attribute 'gen'`` from the ``@retry`` decorator. Tornado
+    availability is read live again rather than from an import-time snapshot.

tenacity/__init__.py

@@ -89,10 +89,13 @@
 except ImportError:
     tornado = None  # type: ignore[assignment]
 
-# mypy resolves `tornado` to the module (it only ever sees the `try` branch),
-# so testing the module object for truthiness reads as an always-true check.
-# Keep the availability answer in a plain bool instead.
-_HAS_TORNADO = tornado is not None
+
+def _has_tornado() -> bool:
+    # A function, not a module-level constant: test suites force the
+    # non-tornado path by setting `tenacity.tornado = None`, so the answer has
+    # to be computed from the live global every time it is asked for.
+    return tornado is not None
+
 
 if t.TYPE_CHECKING:
     if sys.version_info >= (3, 11):
@@ -786,7 +789,7 @@ def wrap(f: t.Callable[P, R]) -> _RetryDecorated[P, R]:
         ):
             r = AsyncRetrying(*dargs, **dkw)
         elif (
-            _HAS_TORNADO
+            _has_tornado()
             and hasattr(tornado.gen, "is_coroutine_function")
             and tornado.gen.is_coroutine_function(f)
         ):
@@ -801,7 +804,7 @@ def wrap(f: t.Callable[P, R]) -> _RetryDecorated[P, R]:
 
 from tenacity.asyncio import AsyncRetrying  # noqa: E402
 
-if _HAS_TORNADO:
+if _has_tornado():
     from tenacity.tornadoweb import TornadoRetrying
 
 

tests/test_tornado.py

@@ -15,9 +15,11 @@
 import unittest
 from collections.abc import Generator
 from typing import Any
+from unittest import mock
 
 from tornado import gen, testing
 
+import tenacity
 from tenacity import RetryError, retry, stop_after_attempt, tornadoweb
 
 from .test_tenacity import NoIOErrorAfterCount
@@ -73,6 +75,17 @@ def retryable(thing: NoIOErrorAfterCount) -> None:
         finally:
             gen.is_coroutine_function = old_attr
 
+    def test_tornado_set_to_none(self) -> None:
+        # Forcing the non-tornado path by nulling the module global is how
+        # downstream suites exercise installs without tornado.
+        with mock.patch.object(tenacity, "tornado", None):
+
+            @retry(stop=stop_after_attempt(1))
+            def retryable() -> int:
+                return 1
+
+            assert retryable() == 1
+
 
 if __name__ == "__main__":
     unittest.main()