diff --git a/docker/api/client.py b/docker/api/client.py
index 6e567b161d..65b5baa967 100644
--- a/docker/api/client.py
+++ b/docker/api/client.py
@@ -83,6 +83,12 @@ class APIClient(
             configuration.
         user_agent (str): Set a custom user agent for requests to the server.
     """
+
+    __attrs__ = requests.Session.__attrs__ + ['_auth_configs',
+                                              '_version',
+                                              'base_url',
+                                              'timeout']
+
     def __init__(self, base_url=None, version=None,
                  timeout=DEFAULT_TIMEOUT_SECONDS, tls=False,
                  user_agent=DEFAULT_USER_AGENT, num_pools=DEFAULT_NUM_POOLS):
diff --git a/docker/transport/npipeconn.py b/docker/transport/npipeconn.py
index db059b445a..ab9b90480a 100644
--- a/docker/transport/npipeconn.py
+++ b/docker/transport/npipeconn.py
@@ -69,6 +69,11 @@ def _get_conn(self, timeout):
 
 
 class NpipeAdapter(requests.adapters.HTTPAdapter):
+
+    __attrs__ = requests.adapters.HTTPAdapter.__attrs__ + ['npipe_path',
+                                                           'pools',
+                                                           'timeout']
+
     def __init__(self, base_url, timeout=60,
                  pool_connections=constants.DEFAULT_NUM_POOLS):
         self.npipe_path = base_url.replace('npipe://', '')
diff --git a/docker/transport/ssladapter.py b/docker/transport/ssladapter.py
index 31f45fc459..8fafec3550 100644
--- a/docker/transport/ssladapter.py
+++ b/docker/transport/ssladapter.py
@@ -25,6 +25,10 @@
 class SSLAdapter(HTTPAdapter):
     '''An HTTPS Transport Adapter that uses an arbitrary SSL version.'''
 
+    __attrs__ = HTTPAdapter.__attrs__ + ['assert_fingerprint',
+                                         'assert_hostname',
+                                         'ssl_version']
+
     def __init__(self, ssl_version=None, assert_hostname=None,
                  assert_fingerprint=None, **kwargs):
         self.ssl_version = ssl_version
diff --git a/docker/transport/unixconn.py b/docker/transport/unixconn.py
index 978c87a1bf..3565cfb629 100644
--- a/docker/transport/unixconn.py
+++ b/docker/transport/unixconn.py
@@ -50,6 +50,11 @@ def _new_conn(self):
 
 
 class UnixAdapter(requests.adapters.HTTPAdapter):
+
+    __attrs__ = requests.adapters.HTTPAdapter.__attrs__ + ['pools',
+                                                           'socket_path',
+                                                           'timeout']
+
     def __init__(self, socket_url, timeout=60,
                  pool_connections=constants.DEFAULT_NUM_POOLS):
         socket_path = socket_url.replace('http+unix://', '')