When I am working on a project, I find that the same message is printed twice. The following minimal code can reproduce this issue:
Source code for logger.py:
import sys
import logging
class MyLogger:
def __init__(self, name):
self.logger = logging.getLogger(name)
self.logger.setLevel(logging.DEBUG)
stream_handler = logging.StreamHandler(sys.stderr)
formatter = logging.Formatter(
"[%(asctime)s] [%(name)s] [%(levelname)s] %(message)s", "%Y-%m-%d %H:%M:%S"
)
stream_handler.setFormatter(formatter)
stream_handler.setLevel(logging.DEBUG)
self.logger.addHandler(stream_handler)
# self.logger.propagate = False
def info(self, message):
self.logger.info("{}".format(message))
my_logger = MyLogger("Logging debug")
Source code for main.py:
import logging
from logger import my_logger
root_logger = logging.getLogger()
stream_handler = logging.StreamHandler()
root_logger.addHandler(stream_handler)
my_logger.info("demo")
my_logger.info("another test")
When we run main.py, we can see that each log message is printed twice on the
console:
[2020-06-19 21:56:53] [Logging debug] [INFO] demo
demo
[2020-06-19 21:56:53] [Logging debug] [INFO] another test
another test
But why? The reason is that, when we use logging.getLogger() without any
argument, we will get the root logger.
my_logger in logger.py is thus a child logger of this root logger. By
default, child logger messages will be propagated to the logger in the upper
hierarchy.
We have two options: (1) Disable propagation from child logger (2) Do not use root logger.
Option 1: In the logger.py, uncomment the line self.logger.propagate = False to make sure that child logger does not propagate its message to the
root logger.
Option 2: Simply do not use root logger in main.py. For example, we can get
another logger by using another_logger = logging.getLogger('main'). In this
way, logging message from my_logger have no relationship with
another_logger. So the logging messages are printed only once.