When I run the below piece of code, the logging is working fine. But when I comment #1) and #2) under setup_logger the log is not displayed.
What does #1) and #2) do here?
import logging
import sys
def get_logger(name):
print('get_logger -- ', name)
log = logging.getLogger("hello.{}".format(name))
return log
def setup_logger():
print('setup_logger')
root = logging.getLogger("")
root.setLevel(logging.ERROR)
handler = logging.StreamHandler(sys.stdout)
handler.setFormatter(logging.Formatter(
fmt="%(asctime)s - %(name)s - %(levelname)s - %(message)s"
))
root.addHandler(handler)
logger = logging.getLogger("hello") #1
logger.setLevel(logging.DEBUG) #2
LOG = get_logger(__name__)
setup_logger()
print(LOG)
def main():
LOG.debug('hello')
if __name__ == '__main__':
main()
The first line you marked gets a logger with given name. If there currently is no logger with the name you provided, a new one will be created (Also known as Singleton). You can read more about logging.getLogger in the Python documentation :
Return a logger with the specified name or, if name is None, return a logger
which is the root logger of the hierarchy. If
specified, the name is typically a dot-separated hierarchical name
like ‘a’, ‘a.b’ or ‘a.b.c.d’. Choice of these names is entirely up to
the developer who is using logging.
All calls to this function with a given name return the same logger instance.
This means that logger instances never need to be
passed between different parts of an application.
The second line you marked sets the loglevel on the logger instance. If you set the loglevel to DEBUG, all messages you log will be printed to the console / file. If you set it to INFO, all messages except for
debug messages will be logged. And so on. Quote from the docs again:
Sets the threshold for this logger to level. Logging messages which
are less severe than level will be ignored; logging messages which
have severity level or higher will be emitted by whichever handler or
handlers service this logger, unless a handler’s level has been set to
a higher severity level than level.
When a logger is created, the level is set to NOTSET (which causes all
messages to be processed when the logger is the root logger, or
delegation to the parent when the logger is a non-root logger). Note
that the root logger is created with level WARNING.
If you still have questions, I can edit my post to answer them.
I am trying to change the loglevel of a python logger. But it does not seem to work:
import logging
#50 CRITICAL
#40 ERROR
#30 WARNING <-- default
#20 INFO
#10 DEBUG
# 0 NOTSET
logger = logging.getLogger('a')
logger.setLevel(logging.DEBUG)
print('log level', logger.getEffectiveLevel())
logger.debug('Debug') # 10
logger.info('Info') # 20
logger.warning('Warning') # 30
logger.error('Error') # 40
The output is:
log level 10
Warning
Error
But should be:
log level 10
Debug
Info
Warning
Error
Why is that and what did I do wrong?
The problem is that the default logging handler handles only the default logging levels.
You have to add a logging handler that does the actual logging.
If you need just console logging, you could use:
logger = logging.getLogger('a')
logger.addHandler(logging.StreamHandler())
logger.setLevel(logging.DEBUG)
...
This gets you:
log level 10
Debug
Info
Warning
Error
logging.basicConfig() adds a handler with some basic formatting, but usually you will define your own format for logging.
One way to solve this is to set the basic config before logging the messages. Like this:
logging.basicConfig()
Although you've set the level of the logger, the message has to pass through a handler which has a log level also. The rule is that the most restrictive log level wins. There are several solutions. Broadly you can use basicConfig to setup the logging root and it will be inherited by child loggers like "a". Or set a handler for "a".
import logging
#50 CRITICAL
#40 ERROR
#30 WARNING <-- default
#20 INFO
#10 DEBUG
# 0 NOTSET
# option - set a basic config for root logger and its descendents
#logging.basicConfig(level=logging.DEBUG)
logger = logging.getLogger('a')
logger.setLevel(logging.DEBUG)
# option = create a handler for this logger
handler = logging.StreamHandler()
handler.setLevel(logging.DEBUG)
logger.addHandler(handler)
print('log level', logger.getEffectiveLevel())
logger.debug('Debug') # 10
logger.info('Info') # 20
logger.warning('Warning') # 30
logger.error('Error') # 40
The below code is copied from the documentation. I am supposed to be able to see all the info logs. But I don't. I am only able to see the warn and above even though I've set setLevel to INFO.
Why is this happening? foo.py:
import logging
logger = logging.getLogger(__name__)
logger.setLevel(logging.DEBUG)
logger.debug('debug message')
logger.info('info message')
logger.warn('warn message')
logger.error('error message')
logger.critical('critical message')
Output:
workingDirectory$ python foo.py
warn message
error message
critical message
Where did the info and debug messages go??
Replace the line
logger.setLevel(logging.DEBUG)
with
logging.basicConfig(level=logging.DEBUG, format='%(message)s')
and it should work as expected. If you don't configure logging with any handlers (as in your post - you only configure a level for your logger, but no handlers anywhere), you'll get an internal handler "of last resort" which is set to output just the message (with no other formatting) at the WARNING level.
Try running logging.basicConfig() in there. Of note, I see you mention INFO, but use DEBUG. As written, it should show all five messages. Swap out DEBUG with INFO, and you should see four messages.
import logging
logging.basicConfig()
logger = logging.getLogger(__name__)
logger.setLevel(logging.DEBUG)
logger.debug('debug message')
logger.info('info message')
logger.warn('warn message')
logger.error('error message')
logger.critical('critical message')
edit: Do you have logging set up elsewhere in your code already? Can't reproduce the exact behavior you note with the specific code provided.
As pointed by some users, using:
logging.basicConfig(level=logging.DEBUG, format='%(message)s')
like written in the accepted answer is not a good option because it sets the log level for the root logger, so it may lead to unexpected behaviours (eg. third party libraries may start to log debug messages if you set loglevel=logging.DEBUG)
In my opinion the best solution is to set log level just for your logger, like this:
import logging
logger = logging.getLogger('MyLogger')
handler = logging.StreamHandler()
formatter = logging.Formatter('%(asctime)s - %(name)s - %(levelname)s - %(message)s')
handler.setFormatter(formatter)
logger.addHandler(handler)
logger.setLevel(logging.DEBUG)
Not really intuitive solution, but is necessary if you want to set log level only for 'MyLogger' and leave the root logger untouched.
So, why is logging.basicConfig(level=logging.DEBUG, format='%(message)s') setting the log level globally?
Well, actually it doesn't. As said, it's just changing the configuration of the root logger and, as described in the python documentation:
Loggers should NEVER be instantiated directly, but always through the
module-level function logging.getLogger(name). Multiple calls to
getLogger() with the same name will always return a reference to the
same Logger object.
So, logging.basicConfig is creating a StreamHandler with a default Formatter and adding it to the root logger.
The point is that if any other library is using the "root logger", you're going to set that log level for that library too so it can happen that you start to see debug logs from third party libraries.
This is why I think it's better to create your own logger and set your own formatters and handlers, so you can leave the root logger untouched.
This is technically also an "answer", because it can "solve" the problem. BUT I definitely DO NOT like it. It is not intuitive, and I lost 2+ hours over it.
Before:
import logging
logger = logging.getLogger('foo')
logger.setLevel(logging.INFO)
logger.info('You can not see me')
# Or you can just use the following one-liner in command line.
# $ python -c "import logging; logger = logging.getLogger('foo'); logger.setLevel(logging.INFO); logger.info('You can not see me')"
After:
import logging
logging.debug('invisible magic') # <-- magic
logger = logging.getLogger('foo')
logger.setLevel(logging.INFO)
logger.info('But now you can see me')
# Or you can just use the following one-liner in command line.
$ python -c "import logging; logging.debug('invisible magic'); logger = logging.getLogger('foo'); logger.setLevel(logging.INFO); logger.info('But now you see me')"
PS: Comparing it to the current chosen answer, and #Vinay-Sajip's explanation, I can kind of understand why. But still, I wish it was not working that way.
If you want this to work WITHOUT basicConfig, you have to first set up the lowest possible level you'll log onto the logger. Since the logger sets a minimum threshold, handlers which have a lower threshold but belong to the same logger won't get those lower threshold messages since they're ignored by the logger in the first place. Intuitive, but not obvious.
We start by doing this:
lgr = logging.getLogger(name)
lgr.setLevel(logging.DEBUG)
Then, set up the handlers with the different levels you need, in my case I want DEBUG logging on stdout and INFO logging to a rotating file, so I do the following:
rot_hndlr = RotatingFileHandler('filename.log',
maxBytes=log_size,
backupCount=3)
rot_hndlr.setFormatter(formatter)
rot_hndlr.setLevel(logging.INFO)
lgr.addHandler(rot_hndlr)
stream_hndlr = logging.StreamHandler()
stream_hndlr.setFormatter(stream_formatter)
lgr.addHandler(stream_hndlr)
Then, to test, I do this:
lgr.debug("Hello")
lgr.info("There")
My stdout (console) will look like this:
Hello
There
and my filename.log file will look like this:
There
In short, change the level in logging.basicConfig will influence the global settings.
You should better set level for each logger and the specific handler in the logger.
The following is an example that displays all levels on the console and only records messages >= errors in log_file.log. Notice the level for each handler is different.
import logging
# Define logger
logger = logging.getLogger('test')
# Set level for logger
logger.setLevel(logging.DEBUG)
# Define the handler and formatter for console logging
consoleHandler = logging.StreamHandler() # Define StreamHandler
consoleHandler.setLevel(logging.DEBUG) # Set level
concolsFormatter = logging.Formatter('%(name)s - %(levelname)s - %(message)s') # Define formatter
consoleHandler.setFormatter(concolsFormatter) # Set formatter
logger.addHandler(consoleHandler) # Add handler to logger
# Define the handler and formatter for file logging
log_file = 'log_file'
fileHandler = logging.FileHandler(f'{log_file}.log') # Define FileHandler
fileHandler.setLevel(logging.ERROR) # Set level
fileFormatter = logging.Formatter('%(asctime)s - %(name)s - %(levelname)s - %(message)s') # Define formatter
fileHandler.setFormatter(fileFormatter) # Set formatter
logger.addHandler(fileHandler) # Add handler to logger
# Test
logger.debug('This is a debug')
logger.info('This is an info')
logger.warning('This is a warning')
logger.error('This is an error')
logger.critical('This is a critical')
Console output:
# Test
test - DEBUG - This is a debug
test - INFO - This is an info
test - WARNING - This is a warning
test - ERROR - This is an error
test - CRITICAL - This is a critical
File log_file.log content:
2021-09-22 12:50:50,938 - test - ERROR - This is an error
2021-09-22 12:50:50,938 - test - CRITICAL - This is a critical
To review your logger's level:
logger.level
The result should be one of the following:
10 # DEBUG
20 # INFO
30 # WARNING
40 # ERROR
50 # CRITICAL
To review your handlers's levels:
logger.handlers
[<StreamHandler stderr (DEBUG)>,
<FileHandler ***/log_file.log (ERROR)>]
The accepted answer does not work for me on Win10, Python 3.7.2.
My solution:
logging.basicConfig(level=logging.DEBUG)
logger = logging.getLogger(__name__)
logger.setLevel(logging.DEBUG)
It's order sensitive.
You have to set the basicConfig of the root logger to DEBUG, then you can set the level of your individual loggers to more restrictive levels.
This is not what I expected. Here is what I had to do:
#!/usr/bin/env python3
import logging
# by default this is WARNING. Leaving it as WARNING here overrides
# whatever setLevel-ing you do later so it seems they are ignored.
logging.basicConfig(level=logging.DEBUG)
l = logging.getLogger(__name__)
l.setLevel(level=logging.INFO)
# if I hadn't called basicConfig with DEBUG level earlier,
# info messages would STILL not be shown despite calling
# setLevel above. However now debug messages will not be shown
# for l because setLevel set it to INFO
l.warning('A warning message will be displayed')
l.info('A friendly info message will be displayed')
l.debug('A friendly debug message will not be displayed')
Most of the answers that I've found for this issue uses the basicConfig of the root logger.
It's not helpful for those who intend to use multiple independent loggers that were not initialised with basicConfig. The use of basicConfig implies that the loglevels of ALL loggers will be changed. It also had the unfortunate side effect of generating duplicate logs.
So I tried over several days experimenting with different ways to manipulate the loglevels and came up with one that finally worked.
The trick was to not only change the log levels of all the handlers but also the all the handlers of the parent of the logger.
def setLevel(self, infoLevel):
# To dynamically reset the loglevel, you need to also change the parent levels as well as all handlers!
self.logger.parent.setLevel(infoLevel)
for handler in self.logger.parent.handlers:
handler.setLevel(infoLevel)
self.logger.setLevel(infoLevel)
for handler in self.logger.handlers:
handler.setLevel(infoLevel)
The inspiration came from the fact that the basicConfig changes the root logger settings, so I was trying to do the same without using basicConfig.
For those that are interested, I did a little Python project on Github that illustrates the different issues with setting loglevel of the logger (it works partially), proves the SLogger (Sample Logger) implementation works, and also illustrates the duplicate log issue with basicConfig when using multiple loggers not initialised with it.
https://github.com/FrancisChung/python-logging-playground
TLDR: If you're only interested in a working sample code for the logger, the implentation is listed below
import logging
CRITICAL = 50
FATAL = CRITICAL
ERROR = 40
WARNING = 30
WARN = WARNING
INFO = 20
DEBUG = 10
NOTSET = 0
class SLogger():
"""
SLogger : Sample Logger class using the standard Python logging Library
Parameters:
name : Name of the Logger
infoLevel : logging level of the Logger (e.g. logging.DEBUG/INFO/WARNING/ERROR)
"""
def __init__(self, name: str, infoLevel=logging.INFO):
try:
if name is None:
raise ValueError("Name argument not specified")
logformat = '%(asctime)s %(levelname)s [%(name)s %(funcName)s] %(message)s'
self.logformat = logformat
self.name = name.upper()
self.logger = logging.getLogger(self.name)
self.logger.setLevel(infoLevel)
self.add_consolehandler(infoLevel, logformat)
except Exception as e:
if self.logger:
self.logger.error(str(e))
def error(self, message):
self.logger.error(message)
def info(self, message):
self.logger.info(message)
def warning(self, message):
self.logger.warning(message)
def debug(self, message):
self.logger.debug(message)
def critical(self, message):
self.logger.critical(message)
def setLevel(self, infoLevel):
# To dynamically reset the loglevel, you need to also change the parent levels as well as all handlers!
self.logger.parent.setLevel(infoLevel)
for handler in self.logger.parent.handlers:
handler.setLevel(infoLevel)
self.logger.setLevel(infoLevel)
for handler in self.logger.handlers:
handler.setLevel(infoLevel)
return self.logger.level
def add_consolehandler(self, infoLevel=logging.INFO,
logformat='%(asctime)s %(levelname)s [%(name)s %(funcName)s] %(message)s'):
sh = logging.StreamHandler()
sh.setLevel(infoLevel)
formatter = logging.Formatter(logformat)
sh.setFormatter(formatter)
self.logger.addHandler(sh)
Create object the right way, e.g. inspired by Google:
import logging
formatter = logging.Formatter('%(asctime)s %(threadName)s: %(message)s')
log = logging.getLogger(__name__)
log.setLevel(logging.DEBUG)
handler = logging.StreamHandler()
handler.setFormatter(formatter)
log.addHandler(handler)
log.debug('debug message')
log.info('info message')
log.warn('warn message')
log.error('error message')
log.critical('critical message')
2022-11-22 23:17:59,342 MainThread: debug message
2022-11-22 23:17:59,342 MainThread: info message
2022-11-22 23:17:59,342 MainThread: warn message
2022-11-22 23:17:59,342 MainThread: error message
2022-11-22 23:17:59,342 MainThread: critical message
As pointed out by #ManuelFedele, logging.basicConfig is not a good solution.
#VinaySajip explained that the setLevel is ignored because the logger is using the internal handler "of last resort", whose level is set to WARNING.
This explanation was also helpful:
The Handler.setLevel() method, just as in logger objects, specifies the lowest severity that will be dispatched to the appropriate destination. Why are there two setLevel() methods? The level set in the logger determines which severity of messages it will pass to its handlers. The level set in each handler determines which messages that handler will send on.
So a good solution is to add a handler to the logger, with the appropriate level:
import logging
logger = logging.getLogger(__name__)
logger.setLevel(logging.DEBUG)
ch = logging.StreamHandler()
ch.setLevel(logging.DEBUG) # or whatever level should be displayed on the console
logger.addHandler(ch)
Output
>>> logger.debug('debug message')
debug message
>>> logger.info('info message')
info message
>>> logger.warn('warn message')
<stdin>:1: DeprecationWarning: The 'warn' method is deprecated, use 'warning' instead
warn message
>>> logger.error('error message')
error message
>>> logger.critical('critical message')
critical message
It seems recently there were some changes of python logging package. Some code work previously doesn't work now. And I am confused. My python version is Python 2.7.15.
The first example I don't understand is that below only prints "WARNING:root:hello from warn". If I understand correctly, "logging.info" actually calls the root logger, and root logger default to warn level. So the first "hello from info" is ignored, which is fine. But why the second "hello from info" is also not printed?
import logging
logging.info("hello from info")
logging.warn("hello from warn")
logging.basicConfig(level=logging.INFO)
logging.info("hello from info")
The second question is the logging level of Handler versus logger. If we set the log level for both the handler and logger, which one is working? Or what if we just set level for Handler? Take the example as below. We already set the log level for StreamHandler, but the "hello from info" is not printed to stdout. Only the "hello from warn" (Besides, it is not "WARNING:t:hello from warn"). Why is that?
import logging
ch = logging.StreamHandler()
ch.setLevel(logging.INFO)
logger = logging.getLogger('t')
logger.addHandler(ch)
logger.info("hello from info")
logger.warn("hello from warn")
But why the second "hello from info" is also not printed?
Because
logging.info / warn / error / debug call logging.basicConfig under the hood. Example:
def info(msg, *args, **kwargs):
if len(root.handlers) == 0:
basicConfig()
root.info(msg, *args, **kwargs)
logging.basicConfig does not do anything if the root logger is already configured. Quote from the docs:
This function does nothing if the root logger already has handlers configured for it.
So, in your code, the root logger is configured with WARN level when logging.info("hello from info") is executed. The subsequent call of logging.basicConfig(level=logging.INFO) has no effect.
Rule of thumb: configure the loggers (no matter if manually or via logging.basicConfig()) as early as possible in your code.
If we set the log level for both the handler and logger, which one is working?
Both! Logger level and handler level are two different stages of filtering records. The logger level defines what records are actually passed to its handlers, while the handler level defines what records will be handled by the particular handler. Examples:
logger.setLevel(logging.INFO)
handler.setLevel(logging.ERROR)
logger.addHandler(handler)
logger.info('spam')
Since logger has level INFO, it will process spam record and pass it to its handlers. However, handler has level ERROR, so the record will not be processed by handler.
logger.setLevel(logging.WARN)
handler.setLevel(logging.DEBUG)
logger.addHandler(handler)
logger.info('spam')
Now the handler will process almost any record, including spam record since its level is INFO, thus greater than DEBUG. However, the handler will never receive spam to process because the logger will not process it, thus not passing spam to its handlers.
logger.setLevel(logging.INFO)
h1 = logging.StreamHandler()
h1.setLevel(logging.CRITICAL)
h2 = logging.FileHandler('some.log')
h2.setLevel(logging.DEBUG)
logger.addHandler(h1)
logger.addHandler(h2)
logger.info('spam')
Now the logger has two handlers, h1 printing records to terminal, h2 writing them to file. The logger will pass only records of level INFO or greater to its handlers. However, you will see only records with level CRITICAL in terminal, but all records in log file.
I created the following script. Could any of you explain to me why the output is like what shows below
Source
import logging
logger = logging.getLogger()
logger.setLevel(logging.DEBUG)
print('debug', logger.isEnabledFor(logging.DEBUG))
print('info', logger.isEnabledFor(logging.INFO))
print('warning', logger.isEnabledFor(logging.WARNING))
print('error', logger.isEnabledFor(logging.ERROR))
logger.debug('debug')
logger.info('info')
logger.warning('warning')
logger.error('error')
logging.debug('debug')
logging.info('info')
logging.warning('warning')
logging.error('error')
Output
debug True
info True
warning True
error True
warning
error
DEBUG:root:debug
INFO:root:info
WARNING:root:warning
ERROR:root:error
Specifically
what is the difference between logger.info and logging.info here
how come that logger.isEnabledFor(logging.DEBUG) is True while logger.debug('debug') does not show anything
how come that logger.info has no output but logging.info has
A few things to clarify:
Default log level for root logger is WARNING
Root logger is not initialized if you do nothing, that is, without any handlers or formatter set up:
>>> import logging
>>> logging.root.handlers
[]
Okay, but you found out the problem: when logging level set to DEBUG, the root logger is not working as expected. Debug messages are ignored. With the same not configured root logger, warning messages output normally. Why is that?
Keep in mind we don't have any handler for root logger right now. But looking into the code, we do see:
if (found == 0):
if lastResort:
if record.levelno >= lastResort.level:
lastResort.handle(record)
elif raiseExceptions and not self.manager.emittedNoHandlerWarning:
sys.stderr.write("No handlers could be found for logger"
" \"%s\"\n" % self.name)
self.manager.emittedNoHandlerWarning = True
Which means, we have a lastResort for backup if no handler is found. You can refer to the definition of lastResort, it is initialized with logging level WARNING. Meanwhile, debug messages don't have this backup so they are ignored when no handler is set.
For your questions:
These two loggers are identical, since the root logger is returned when getLogger() receives no arguments.
See below:
Logger.isEnabledFor(lvl)
Indicates if a message of severity lvl would
be processed by this logger. This method checks first the module-level
level set by logging.disable(lvl) and then the logger’s effective
level as determined by getEffectiveLevel().
Calling any logging functions in logging module will initialize the root logger with basicConfig() which adds a default handler, so that the subsequent calls on logger will also work.
What you should do is, use logging.basicConfig() to set up a default handler for root logger and messages will be output according to the logger level and message level.
getLogger creates an instance of Logger class if argument name is added. Otherwise it returns root logger. So in this case the program is using the common logger as functions logging.debug, logging.info, logging.warning, logging.info