[AIRFLOW-592] example_xcom import Error

fix exmple_xcom import error:
AttributeError: 'module' object has no attribute
'python_operator'

Closes apache#1853 from forevernull/master

airflow/example_dags/example_xcom.py

@@ -12,7 +12,8 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 from __future__ import print_function
-import airflow
+from airflow import DAG
+from airflow.operators.python_operator import PythonOperator
 from datetime import datetime, timedelta
 
 seven_days_ago = datetime.combine(
@@ -24,7 +25,7 @@
     'provide_context': True
 }
 
-dag = airflow.DAG(
+dag = DAG(
     'example_xcom',
     start_date=datetime(2015, 1, 1),
     schedule_interval="@once",
@@ -59,13 +60,13 @@ def puller(**kwargs):
     v1, v2 = ti.xcom_pull(key=None, task_ids=['push', 'push_by_returning'])
     assert (v1, v2) == (value_1, value_2)
 
-push1 = airflow.operators.python_operator.PythonOperator(
+push1 = PythonOperator(
     task_id='push', dag=dag, python_callable=push)
 
-push2 = airflow.operators.python_operator.PythonOperator(
+push2 = PythonOperator(
     task_id='push_by_returning', dag=dag, python_callable=push_by_returning)
 
-pull = airflow.operators.python_operator.PythonOperator(
+pull = PythonOperator(
     task_id='puller', dag=dag, python_callable=puller)
 
 pull.set_upstream([push1, push2])