Converting a function into a singleton class that returns one instance

Question

I am doing this project on Python 2.7 and I am trying to convert this function that returns a logging object. I am trying to make sure that I can utilize the same object instance through different python modules by importing it without actually creating new instances? For e.g if I actually import the module where the instance was created originally, each time I import it a new instance gets created. But I want to use the same original instance, throughout all different modules, that was created the first time I ran that module since multiple lines are being printed out inside my log file due to multiple instances. That is why a singleton class in Python 2.7 is required but I am not sure how to convert this function into a singleton class such that it returns one instance and I can use it throughout all my different modules by importing without triggering new instances. The setLogger function creates a logger instance which inputs logs into the file_name log file.

def setLogger(file_name):
    logger = logging.getLogger(__name__)
    if not getattr(logger, 'handler_set', None):
        logger.setLevel(logging.INFO)
        stream_handler = logging.StreamHandler()
        file_handler = logging.FileHandler(file_name)
        formatter = logging.Formatter('%(message)s')
        file_handler.setFormatter(formatter)
        logger.addHandler(file_handler)
        logger.addHandler(stream_handler)
        logger.setLevel(logging.INFO)
        logger.propagate = False
        logger.handler_set = True
    return logger

Answer 1

I don't understand why different loggers are generated when using the same name.

To force a single logger instance, get the logger once and store it in a global variable.

logger = None  # global variable

def setLogger(file_name):
    global logger
    if logger == None:
       print('Creating logger')
       logger = logging.getLogger(__name__)  # only run once
    if not getattr(logger, 'handler_set', None):
        ........
    return logger

+++++++++++++++++++++++++++++++++++++++++++++++

I set up some modules to narrow down the issue.

--- mylogger.py ---

  import logging

  logger = None  # global variable

  def setLogger(file_name="LogOut.log"):
      global logger
      if logger == None:
         print('Creating logger')
         logger = logging.getLogger(__name__)  # only run once
      if not getattr(logger, 'handler_set', None):
          logger.setLevel(logging.INFO)
          stream_handler = logging.StreamHandler()
          file_handler = logging.FileHandler(file_name)
          formatter = logging.Formatter('%(message)s')
          file_handler.setFormatter(formatter)
          logger.addHandler(file_handler)
          logger.addHandler(stream_handler)
          logger.setLevel(logging.INFO)
          logger.propagate = False
          logger.handler_set = True
      return logger

--- modc.py ---

  import mylogger

  def writelog():
      print("Hello from " + __name__)
      print(__name__, 'LogID:', id(mylogger.setLogger()))
      mylogger.setLogger().warning("Hello from " + __name__)

--- modb.py ---

  import mylogger
  import modc

  def writelog():
      print("Hello from " + __name__)
      print(__name__, 'LogID:', id(mylogger.setLogger()))
      mylogger.setLogger().warning("Hello from " + __name__)

  modc.writelog()

--- moda.py ---

  import mylogger

  def writelog():
      print("Hello from " + __name__)
      print(__name__, 'LogID:', id(mylogger.setLogger()))
      mylogger.setLogger().warning("Hello from " + __name__)
      
  import modb
  import modc

  writelog()
  modb.writelog()
  modc.writelog()

I ran moda.py. Here is the output. The logger is created once and every module uses the same instance of the logger. All messages are saved to the same file. Note that I'm using python 3.7.

  Hello from modc
  Creating logger
  modc LogID: 1764613056456
  Hello from modc
  Hello from __main__
  __main__ LogID: 1764613056456
  Hello from __main__
  Hello from modb
  modb LogID: 1764613056456
  Hello from modb
  Hello from modc
  modc LogID: 1764613056456
  Hello from modc