title: DAG Import Errors (ModuleNotFound) in airflow 2.1.0
body: <p>I am completely new to apache airflow and trying to use  fresh airflow 2.1.0 correctly. Webserver and scheduler are working fine (I suppose. Cause I can list dags, I can interact with UI). If it`s important, I run <strong>airflow under docker container centos 7 based.</strong></p>
  <p>I`ve stuck with importing dags.</p>
  <p>I have installed required modules, but have errors.</p>
  <p>Errors:</p>
  <pre><code>Broken DAG: [/opt/airflow/dags/GWS_churn_prediction_load_data/gws_main_dag.py] Traceback (most recent call last): File &quot;/opt/airflow/dags/GWS_churn_prediction_load_data/gws_main_dag.py&quot;, line 11, in &lt;module&gt; from airflow.operators.postgres_operator import PostgresOperator File &quot;/usr/local/lib/python3.6/site-packages/airflow/operators/postgres_operator.py&quot;, line 23, in &lt;module&gt; from airflow.providers.postgres.operators.postgres import Mapping, PostgresOperator # noqa ModuleNotFoundError: No module named 'airflow.providers.postgres'
  Broken DAG: [/opt/airflow/dags/EDW/EDW_000003/test.py] Traceback (most recent call last): File &quot;&lt;frozen importlib._bootstrap&gt;&quot;, line 219, in _call_with_frames_removed File &quot;/opt/airflow/dags/EDW/EDW_000003/test.py&quot;, line 7, in &lt;module&gt; df2 = df.withColumn(&quot;AUDIT_RK&quot;,col(&quot;AUDIT_ID&quot;).cast(&quot;Integer&quot;)+1)#.show(false) NameError: name 'col' is not defined
  Broken DAG: [/opt/airflow/dags/EDW/EDW_000003/EDW_000003_AUDIMEX.py] Traceback (most recent call last): File &quot;&lt;frozen importlib._bootstrap&gt;&quot;, line 219, in _call_with_frames_removed File &quot;/opt/airflow/dags/EDW/EDW_000003/EDW_000003_AUDIMEX.py&quot;, line 3, in &lt;module&gt; from airflow.contrib.example_dags.libs.helper import print_stuff ModuleNotFoundError: No module named 'airflow.contrib.example_dags'
  Broken DAG: [/opt/airflow/dags/GWS_churn_prediction_model/gws_model_main_dag.py] Traceback (most recent call last): File &quot;/usr/local/lib/python3.6/site-packages/airflow/models/baseoperator.py&quot;, line 179, in apply_defaults result = func(self, *args, **kwargs) File &quot;/usr/local/lib/python3.6/site-packages/airflow/models/baseoperator.py&quot;, line 511, in __init__ &quot;arguments were:\n**kwargs: {k}&quot;.format(c=self.__class__.__name__, k=kwargs, t=task_id), airflow.exceptions.AirflowException: Invalid arguments were passed to PythonSensor (task_id: check_new_week). Invalid arguments were: **kwargs: {'provide_context': True}
  </code></pre>
  <p>I installed <code>apache-airflow-providers-postgres</code>. This didn't help. I installed all required providers` modules (apache-airflow-providers-ssh, -apache-spark (this required for project), -ftp, -imap, -ssh and so on).</p>
  <p>Airflow.cfg has a line with a path where I import dags: <code>/opt/airflow/dags</code>, but there is no dags (ofc, it`s cause dag import errors).</p>
  <ol>
  <li>Why do i see <code>airflow.providers.postgres</code> error? <strong>I've installed providers package</strong>. What is wrong?</li>
  <li>How can i solve problem with <code>example_dags</code>? What is this module exactly? I haven't find this in PyPi.</li>
  </ol>
  <p>Any help and hints will be appreciated.</p>