Redirect Python 'print' output to Logger

问题:

I have a Python script that makes use of 'Print' for printing to stdout. I've recently added logging via Python Logger and would like to make it so these print statements go to logger if logging is enabled. I do not want to modify or remove these print statements.

I can log by doing 'log.info("some info msg")'. I want to be able to do something like this:

if logging_enabled:
  sys.stdout=log.info
print("test")

If logging is enabled, "test" should be logged as if I did log.info("test"). If logging isn't enabled, "test" should just be printed to the screen.

Is this possible? I know I can direct stdout to a file in a similar manner (see: redirect prints to log file)

回答1:

You have two options:

1. Open a logfile and replace sys.stdout with it, not a function:

   log = open("myprog.log", "a")
   sys.stdout = log
   
   >>> print("Hello")
   >>> # nothing is printed because it goes to the log file instead.
   

2. Replace print with your log function:

   # If you're using python 2.x, uncomment the next line
   #from __future__ import print_function
   print = log.info
   
   >>> print("Hello!")
   >>> # nothing is printed because log.info is called instead of print
   

回答2:

One more method is to wrap the logger in an object that translates calls to write to the logger's log method.

Ferry Boender does just this, provided under the GPL license in a post on his website:

import logging
import sys

class StreamToLogger(object):
   """
   Fake file-like stream object that redirects writes to a logger instance.
   """
   def __init__(self, logger, log_level=logging.INFO):
      self.logger = logger
      self.log_level = log_level
      self.linebuf = ''

   def write(self, buf):
      for line in buf.rstrip().splitlines():
         self.logger.log(self.log_level, line.rstrip())

logging.basicConfig(
   level=logging.DEBUG,
   format='%(asctime)s:%(levelname)s:%(name)s:%(message)s',
   filename="out.log",
   filemode='a'
)

stdout_logger = logging.getLogger('STDOUT')
sl = StreamToLogger(stdout_logger, logging.INFO)
sys.stdout = sl

stderr_logger = logging.getLogger('STDERR')
sl = StreamToLogger(stderr_logger, logging.ERROR)
sys.stderr = sl

This allows you to easily route all output to a logger of your choice. If needed, you can save sys.stdout and/or sys.stderr as mentioned by others in this thread before replacing it if you need to restore it later.

回答3:

Of course, you can both print to the standard output and append to a log file, like this:

# Uncomment the line below for python 2.x
#from __future__ import print_function

import logging

logging.basicConfig(level=logging.INFO, format='%(message)s')
logger = logging.getLogger()
logger.addHandler(logging.FileHandler('test.log', 'a'))
print = logger.info

print('yo!')

回答4:

You really should do that the other way: by adjusting your logging configuration to use print statements or something else, depending on the settings. Do not overwrite print behaviour, as some of the settings that may be introduced in the future (eg. by you or by someone else using your module) may actually output it to the stdout and you will have problems.

There is a handler that is supposed to redirect your log messages to proper stream (file, stdout or anything else file-like). It is called StreamHandler and it is bundled with logging module.

So basically in my opinion you should do, what you stated you don't want to do: replace print statements with actual logging.

回答5:

A much simpler option,

import logging, sys
logging.basicConfig(filename='path/to/logfile', level=logging.DEBUG)
logger = logging.getLogger()
sys.stderr.write = logger.error
sys.stdout.write = logger.info