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You should add logging statements to your code to record important events that occur. Each logging statement added to the log file should have a timestamp, and be traceable to a given code file and line number.

sessions.py
Python 
import logging

def establish_session(user_id):
  logging.info("Establishing session for user: %s", user_id)

  user = find_user(user_id)

  if not user:
    logging.error("Unable to find user %s!", user_id)
    raise UserNotFoundException(user_id)

  login_user(user)

  logging.info("Established session for user: %s", user_id)
  logging.debug("Last logged in: %s", user.last_login_time)

  return user
tail -f ./application.log
12:59:41.2 INFO  sessions.py:4  Establishing session for user: 3892
12:59:41.8 ERROR sessions.py:9  Unable to find user 3892!
12:59:43.2 INFO  sessions.py:4  Establishing session for user: 94012
12:59:43.8 INFO  sessions.py:19 Established session for user: 94012
12:59:43.8 DEBUG sessions.py:21 Last logged in: 3 days ago