Show a helpful warning when people try to call `client.containers()`

People upgrading to docker-py 2.0 without being aware of the new client
API will likely try to call the old `containers()` method. This adds a
helpful warning telling them to use APIClient to get the old API.
Signed-off-by: Aanand Prasad <aanand.prasad@gmail.com>

docker/models/resource.py

@@ -60,6 +60,12 @@ class Collection(object):
         #: is on.
         self.client = client
 
+    def __call__(self, *args, **kwargs):
+        raise TypeError(
+            "'{}' object is not callable. You might be trying to use the old "
+            "(pre-2.0) API - use docker.APIClient if so."
+            .format(self.__class__.__name__))
+
     def list(self):
         raise NotImplementedError
 

tests/unit/client_test.py

 import datetime
 import docker
+from docker.utils import kwargs_from_env
 import os
 import unittest
 
@@ -59,6 +60,16 @@ class ClientTest(unittest.TestCase):
         assert "'Client' object has no attribute 'abcdef'" in s
         assert "this method is now on the object APIClient" not in s
 
+    def test_call_containers(self):
+        client = docker.Client(**kwargs_from_env())
+
+        with self.assertRaises(TypeError) as cm:
+            client.containers()
+
+        s = str(cm.exception)
+        assert "'ContainerCollection' object is not callable" in s
+        assert "docker.APIClient" in s
+
 
 class FromEnvTest(unittest.TestCase):