Tristan Tristan - 4 years ago 216
Python Question

Custom logger with time stamp in python

I have lots of code on a project with print statements and wanted to make a quick a dirty logger of these print statements and decided to go the custom route. I managed to put together a logger that prints both to the terminal and to a file (with the help of this site), but now I want to add a simple time stamp to each statement and I am running into a weird issue.

Here is my logging class.

class Logger(object):
def __init__(self, stream):
self.terminal = stream
self.log = open("test.log", 'a')

def write(self, message):
self.terminal.write(self.stamp() + message)
self.log.write(self.stamp() + message)

def stamp(self):
d =
string = d.strftime("[%H:%M:%S] ")
return string

Notice the stamp method that I then attempt to use in the write method.

When running the following two lines I get an unexpected output:

sys.stdout = Logger(sys.stdout)
print("Hello World!")


[11:10:47] Hello World![11:10:47]

This what the output also looks in the log file, however, I see no reason why the string that I am adding appends to the end. Can someone help me here?

See answer below. However, for quicker reference the issue is using "print()" in general; replace it with sys.stdout.write after assigning the variable.

Also use "logging" for long-term/larger projects right off the bat.

Answer Source

It calls the .write() method of your stream twice because in cpython print calls the stream .write() method twice. The first time is with the object, and the second time it writes a newline character. For example look at line 138 in the pprint module in cpython v3.5.2

def pprint(self, object):
    self._format(object, self._stream, 0, 0, {}, 0)
    self._stream.write("\n")  # <- write() called again!

You can test this out:

>>> from my_logger import Logger  # has your Logger class
>>> import sys
>>> sys.stdout = Logger(stream=sys.stdout)
>>> sys.stdout.write('hi\n')
[14:05:32] hi

You can replace print(<blah>) everywhere in your code using sed.

$ for mymodule in *.py; do
> sed -i -E "s/print\((.+)\)/LOGGER.debug(\1)/" $mymodule
> done

Check out Python's Logging builtin module. It has pretty comprehensive logging including inclusion of a timestamp in all messages format.

import logging
FORMAT = '%(asctime)-15s %(message)s'
DATEFMT = '%Y-%m-%d %H:%M:%S'
logging.basicConfig(format=FORMAT, datefmt=DATEFMT)
logger = logging.getLogger(__name__)

logger.debug('message: %s', 'message')

This outputs 2016-07-29 11:44:20 message: message to stdout. There are also handlers to send output to files. There is a basic tutorial, an advanced tutorial and a cookbook of common logging recipes.

There is an example of using simultaneous file and console loggers in the cookbook.

import logging

LOGGER = logging.getLogger(__name__)  # get logger named for this module
LOGGER.setLevel(logging.DEBUG)  # set logger level to debug

# create formatter
LOG_DATEFMT = '%Y-%m-%d %H:%M:%S'
LOG_FORMAT = ('\n[%(levelname)s/%(name)s:%(lineno)d] %(asctime)s ' +
              '(%(processName)s/%(threadName)s)\n> %(message)s')
FORMATTER = logging.Formatter(LOG_FORMAT, datefmt=LOG_DATEFMT)

CH = logging.StreamHandler()  # create console handler
CH.setLevel(logging.DEBUG)  # set handler level to debug
CH.setFormatter(FORMATTER)  # add formatter to ch
LOGGER.addHandler(CH)  # add console handler to logger

FH = logging.FileHandler('myapp.log')  # create file handler
FH.setLevel(logging.DEBUG)  # set handler level to debug
FH.setFormatter(FORMATTER)  # add formatter to fh
LOGGER.addHandler(FH)  # add file handler to logger

LOGGER.debug('test: %s', 'hi')

This outputs:

[DEBUG/__main__:22] 2016-07-29 12:20:45 (MainProcess/MainThread)
> test: hi

to both console and file myapp.log simultaneously.

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