gh-137754: Fix import of zoneinfo if _datetime is not available

Both modules should use the Python implementation in that case.

Author: serhiy-storchaka

Lib/test/test_zoneinfo/test_zoneinfo.py

@@ -12,6 +12,7 @@
 import re
 import shutil
 import struct
+import sys
 import tempfile
 import unittest
 from datetime import date, datetime, time, timedelta, timezone
@@ -1946,6 +1947,21 @@ class CTestModule(TestModule):
     module = c_zoneinfo
 
 
+class MiscTests(unittest.TestCase):
+    def test_pydatetime(self):
+        with CleanImport('zoneinfo', 'zoneinfo._tzpath', 'zoneinfo._zoneinfo',
+                         '_zoneinfo', 'datetime', '_pydatetime', '_datetime'):
+            sys.modules['_datetime'] = None
+            import datetime
+            import zoneinfo
+            zoneinfo.ZoneInfo.clear_cache()
+            tzinfo = zoneinfo.ZoneInfo('Europe/Paris')
+            datetime.datetime(2025, 10, 26, 2, 0, tzinfo=tzinfo)
+            self.assertIn('_pydatetime', sys.modules)
+            self.assertNotIn('_zoneinfo', sys.modules)
+            self.assertIn('zoneinfo._zoneinfo', sys.modules)
+
+
 class ExtensionBuiltTest(unittest.TestCase):
     """Smoke test to ensure that the C and Python extensions are both tested.
 

Lib/zoneinfo/__init__.py

@@ -12,7 +12,10 @@
 
 try:
     from _zoneinfo import ZoneInfo
-except ImportError:  # pragma: nocover
+except (ImportError, AttributeError):  # pragma: nocover
+    # AttributeError: module 'datetime' has no attribute 'datetime_CAPI'.
+    # This happens when the '_datetime' module is not available and the
+    # pure Python implementation is used instead.
     from ._zoneinfo import ZoneInfo
 
 reset_tzpath = _tzpath.reset_tzpath

Misc/NEWS.d/next/Library/2025-08-16-09-02-11.gh-issue-137754.mCev1Y.rst

@@ -0,0 +1,2 @@
+Fix import of the :mod:`zoneinfo` module if the C implementation of the
+:mod:`datetime` module is not available.