Disallow creating new sessions if client exits context

CHANGES.rst

@@ -1,6 +1,10 @@
 Changes
 -------
 
+3.0.0 (2025-11-30)
+^^^^^^^^^^^^^^^^^^^
+* BREAKING: forbid creating loose ``ClientSession`` when ``AioBaseClient`` exits context
+
 2.26.0 (2025-11-27)
 ^^^^^^^^^^^^^^^^^^^
 * bump botocore dependency specification

aiobotocore/__init__.py

@@ -1 +1 @@
-__version__ = '2.26.0'
+__version__ = '3.0.0'

aiobotocore/httpsession.py

@@ -62,7 +62,9 @@ def __init__(
 
         # TODO: handle socket_options
         # keep track of sessions by proxy url (if any)
-        self._sessions: dict[Optional[str], aiohttp.ClientSession] = {}
+        self._sessions: Optional[
+            dict[Optional[str], aiohttp.ClientSession]
+        ] = None
         self._verify = verify
         self._proxy_config = ProxyConfiguration(
             proxies=proxies, proxies_settings=proxies_config
@@ -100,13 +102,17 @@ def __init__(
         # request so don't need proxy manager
 
     async def __aenter__(self):
-        assert not self._sessions
+        assert self._sessions is None
+        self._sessions = {}
 
         return self
 
     async def __aexit__(self, exc_type, exc_val, exc_tb):
+        assert self._sessions is not None, 'Session was never entered'
         self._sessions.clear()
         await self._exit_stack.aclose()
+        # Make _sessions unusable once context is exited
+        self._sessions = None
 
     def _get_ssl_context(self):
         return create_urllib3_context()

tests/test_httpsession.py

@@ -0,0 +1,20 @@
+import botocore
+import pytest
+
+
+async def test_cannot_create_client_sessions_outside_context(session):
+    s3_client_context = session.create_client(
+        's3',
+        'us-west-2',
+        aws_secret_access_key="xxx",
+        aws_access_key_id="xxx",
+    )
+
+    async with s3_client_context as s3_client:
+        pass
+
+    with pytest.raises(
+        botocore.exceptions.HTTPClientError,
+        match="'NoneType' object has no attribute 'get'",
+    ):
+        await s3_client.list_buckets()