Only create the default network if at least one service needs it.

Signed-off-by: Daniel Nephin <dnephin@docker.com>

tests/unit/project_test.py

@@ -7,6 +7,8 @@ from .. import unittest
 from compose.const import LABEL_SERVICE
 from compose.container import Container
 from compose.project import Project
+from compose.service import ContainerNet
+from compose.service import Net
 from compose.service import Service
 
 
@@ -263,6 +265,32 @@ class ProjectTest(unittest.TestCase):
         service = project.get_service('test')
         self.assertEqual(service.net.mode, 'container:' + container_name)
 
+    def test_uses_default_network_true(self):
+        web = Service('web', project='test', image="alpine", net=Net('test'))
+        db = Service('web', project='test', image="alpine", net=Net('other'))
+        project = Project('test', [web, db], None)
+        assert project.uses_default_network()
+
+    def test_uses_default_network_custom_name(self):
+        web = Service('web', project='test', image="alpine", net=Net('other'))
+        project = Project('test', [web], None)
+        assert not project.uses_default_network()
+
+    def test_uses_default_network_host(self):
+        web = Service('web', project='test', image="alpine", net=Net('host'))
+        project = Project('test', [web], None)
+        assert not project.uses_default_network()
+
+    def test_uses_default_network_container(self):
+        container = mock.Mock(id='test')
+        web = Service(
+            'web',
+            project='test',
+            image="alpine",
+            net=ContainerNet(container))
+        project = Project('test', [web], None)
+        assert not project.uses_default_network()
+
     def test_container_without_name(self):
         self.mock_client.containers.return_value = [
             {'Image': 'busybox:latest', 'Id': '1', 'Name': '1'},