title: Airflow + python logging module doesn't write to log file
body: <p>Trying to write "hello world" into an airflow log (airflow 1.10.3). Based on the SO solutions presented <a href="https://stackoverflow.com/questions/40120467/adding-logs-to-airflow-logs">here</a> and <a href="https://stackoverflow.com/questions/40120474/writing-to-airflow-logs">here</a> I should be able to just <code>import logging</code> and <code>logging.info('hello world')</code>. That doesn't seem to work for me.</p>
  
  <pre><code>import logging
  from datetime import datetime, timedelta
  from airflow import DAG
  from airflow.operators.python_operator import PythonOperator
  
  default_args = {
      'owner': 'benten',
      'depends_on_past': False,
      'start_date': datetime(2019, 7, 25),
      'email_on_failure': False,
      'retries': 1,
      'retry_delay': timedelta(minutes=1),
      }
  
  def logging_is_fun():
      logging.debug("hellow world")
      logging.info("hello world")
      logging.critical("hello world")
      return None
  
  with DAG('fun_logs', schedule_interval='45 * * * *', default_args=default_args) as dag:
      log_task = PythonOperator(python_callable=logging_is_fun, task_id='log_test_task')
  </code></pre>
  
  <p>I trigger the dag manually and the task executes with no problems. But alas when I check the logs all I see is this:</p>
  
  <p><code>*** Reading local file: /home/ubuntu/airflow/logs/fun_logs/log_test_task/2019-08-31T19:22:49.653712+00:00/1.log</code></p>
  
  <p><strong>Where are my amazing "hello world" statements?</strong> I don't expect to see all of them, given my log level setting. I do expect to see the critical message, though. </p>
  
  <p>My airflow.cfg has the following in it (all default settings to the best of my knowledge):</p>
  
  <pre><code># The folder where airflow should store its log files
  # This path must be absolute
  base_log_folder = /home/ubuntu/airflow/logs
  
  # Airflow can store logs remotely in AWS S3, Google Cloud Storage or Elastic Search.
  # Users must supply an Airflow connection id that provides access to the storage
  # location. If remote_logging is set to true, see UPDATING.md for additional
  # configuration requirements.
  remote_logging = False
  remote_log_conn_id =
  remote_base_log_folder =
  encrypt_s3_logs = False
  
  # Logging level
  logging_level = WARN
  fab_logging_level = WARN
  
  # Logging class
  # Specify the class that will specify the logging configuration
  # This class has to be on the python classpath
  # logging_config_class = my.path.default_local_settings.LOGGING_CONFIG
  logging_config_class =
  
  # Log format
  log_format = [%%(asctime)s] {%%(filename)s:%%(lineno)d} %%(levelname)s - %%(message)s
  simple_log_format = %%(asctime)s %%(levelname)s - %%(message)s
  
  # Log filename format
  log_filename_template = {{ ti.dag_id }}/{{ ti.task_id }}/{{ ts }}/{{ try_number }}.log
  log_processor_filename_template = {{ filename }}.log
  dag_processor_manager_log_location = /home/ubuntu/airflow/logs/dag_processor_manager/dag_processor_manager.log
  </code></pre>