Moved network_mode default to create_host_config ; small fix to preserve blank host_config in start if no option is provided

docker/client.py

@@ -780,7 +780,7 @@ class Client(clientbase.ClientBase):
             publish_all_ports=publish_all_ports, links=links, dns=dns,
             privileged=privileged, dns_search=dns_search, cap_add=cap_add,
             cap_drop=cap_drop, volumes_from=volumes_from, devices=devices,
-            network_mode=network_mode, restart_policy=restart_policy,
+            network_mode=network_mode or '', restart_policy=restart_policy,
             extra_hosts=extra_hosts, read_only=read_only, pid_mode=pid_mode,
             ipc_mode=ipc_mode, security_opt=security_opt, ulimits=ulimits
         )

docker/utils/utils.py

@@ -428,6 +428,8 @@ def create_host_config(
 
     if network_mode:
         host_config['NetworkMode'] = network_mode
+    elif network_mode is None:
+        host_config['NetworkMode'] = 'default'
 
     if restart_policy:
         host_config['RestartPolicy'] = restart_policy
@@ -615,16 +617,6 @@ def create_container_config(
         if volumes_from is not None:
             raise errors.InvalidVersion(message.format('volumes_from'))
 
-    # NetworkMode must be present and valid in host config from 1.20 onwards
-    if compare_version('1.20', version) >= 0:
-        if host_config is None:
-            host_config = {'NetworkMode': 'default'}
-        else:
-            if 'NetworkMode' not in host_config:
-                host_config['NetworkMode'] = 'default'
-            elif host_config['NetworkMode'] == '':
-                host_config['NetworkMode'] = 'default'
-
     return {
         'Hostname': hostname,
         'Domainname': domainname,