From 6105d7ea13c8fc59f7d5f04791de9552f30f3169 Mon Sep 17 00:00:00 2001
From: themylogin <themylogin@gmail.com>
Date: Mon, 23 Feb 2026 15:14:29 +0100
Subject: [PATCH 1/3] Fix AttributeError: 'ClientConnectorCertificateError'
 object has no attribute '_os_error'.

ClientConnectorCertificateError is a subclass of ClientConnectorError and should have all of its
attributes (LSP). However, when I try to access the os_error attribute, I get the mentioned exception.
---
 CHANGES/12136.bugfix.rst        |  2 ++
 CONTRIBUTORS.txt                |  1 +
 aiohttp/client_exceptions.py    | 10 +++++++++-
 tests/test_client_exceptions.py |  9 +++++++++
 4 files changed, 21 insertions(+), 1 deletion(-)
 create mode 100644 CHANGES/12136.bugfix.rst

diff --git a/CHANGES/12136.bugfix.rst b/CHANGES/12136.bugfix.rst
new file mode 100644
index 00000000000..14ad7edf326
--- /dev/null
+++ b/CHANGES/12136.bugfix.rst
@@ -0,0 +1,2 @@
+``ClientConnectorCertificateError.os_error`` no longer raises :exc:`AttributeError`
+-- by :user:`themylogin`.
diff --git a/CONTRIBUTORS.txt b/CONTRIBUTORS.txt
index 7c5613648ca..487691f688f 100644
--- a/CONTRIBUTORS.txt
+++ b/CONTRIBUTORS.txt
@@ -392,6 +392,7 @@ Vladimir Kamarzin
 Vladimir Kozlovski
 Vladimir Rutsky
 Vladimir Shulyak
+Vladimir Vinogradenko
 Vladimir Zakharov
 Vladyslav Bohaichuk
 Vladyslav Bondar
diff --git a/aiohttp/client_exceptions.py b/aiohttp/client_exceptions.py
index af83a42705e..8c5bf7c9a3e 100644
--- a/aiohttp/client_exceptions.py
+++ b/aiohttp/client_exceptions.py
@@ -342,10 +342,18 @@ class ClientConnectorSSLError(*ssl_error_bases):  # type: ignore[misc]
 class ClientConnectorCertificateError(*cert_errors_bases):  # type: ignore[misc]
     """Response certificate error."""
 
+    _conn_key: ConnectionKey
+
     def __init__(
         self, connection_key: ConnectionKey, certificate_error: Exception
     ) -> None:
-        self._conn_key = connection_key
+        if isinstance(certificate_error, cert_errors + (OSError,)):
+            # ssl.CertificateError has errno and strerror, so we should be fine
+            os_error = certificate_error
+        else:
+            os_error = OSError()
+
+        super().__init__(connection_key, os_error)
         self._certificate_error = certificate_error
         self.args = (connection_key, certificate_error)
 
diff --git a/tests/test_client_exceptions.py b/tests/test_client_exceptions.py
index ed87b6d50f1..a7bf4db05ef 100644
--- a/tests/test_client_exceptions.py
+++ b/tests/test_client_exceptions.py
@@ -201,6 +201,15 @@ def test_str(self) -> None:
             " [Exception: ('Bad certificate',)]"
         )
 
+    def test_oserror(self) -> None:
+        certificate_error = OSError(1, "Bad certificate")
+        err = client.ClientConnectorCertificateError(
+            connection_key=self.connection_key, certificate_error=certificate_error
+        )
+        assert err.os_error == certificate_error
+        assert err.errno == 1
+        assert err.strerror == "Bad certificate"
+
 
 class TestServerDisconnectedError:
     def test_ctor(self) -> None:

From fa9ed38a266426495e56572a4a7224a564781d53 Mon Sep 17 00:00:00 2001
From: Sam Bull <git@sambull.org>
Date: Thu, 26 Feb 2026 22:40:18 +0000
Subject: [PATCH 2/3] Add comment

---
 aiohttp/client_exceptions.py | 1 +
 1 file changed, 1 insertion(+)

diff --git a/aiohttp/client_exceptions.py b/aiohttp/client_exceptions.py
index 8c5bf7c9a3e..ff163b78229 100644
--- a/aiohttp/client_exceptions.py
+++ b/aiohttp/client_exceptions.py
@@ -345,6 +345,7 @@ class ClientConnectorCertificateError(*cert_errors_bases):  # type: ignore[misc]
     _conn_key: ConnectionKey
 
     def __init__(
+        # TODO: If we require ssl in future, this can become ssl.CertificateError
         self, connection_key: ConnectionKey, certificate_error: Exception
     ) -> None:
         if isinstance(certificate_error, cert_errors + (OSError,)):

From 481c13e046227c996d887151a94330187c5a174c Mon Sep 17 00:00:00 2001
From: "pre-commit-ci[bot]"
 <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Date: Thu, 26 Feb 2026 22:40:57 +0000
Subject: [PATCH 3/3] [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci
---
 aiohttp/client_exceptions.py | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/aiohttp/client_exceptions.py b/aiohttp/client_exceptions.py
index ff163b78229..3b66c7fdcd2 100644
--- a/aiohttp/client_exceptions.py
+++ b/aiohttp/client_exceptions.py
@@ -346,7 +346,9 @@ class ClientConnectorCertificateError(*cert_errors_bases):  # type: ignore[misc]
 
     def __init__(
         # TODO: If we require ssl in future, this can become ssl.CertificateError
-        self, connection_key: ConnectionKey, certificate_error: Exception
+        self,
+        connection_key: ConnectionKey,
+        certificate_error: Exception,
     ) -> None:
         if isinstance(certificate_error, cert_errors + (OSError,)):
             # ssl.CertificateError has errno and strerror, so we should be fine