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Custom logger not overwriting root log level

I'm trying to create a custom logger that should log at INFO level. The code looks as follows:
import logging
logger = logging.getLogger(__name__)
# write to stderr
handler = logging.StreamHandler()
handler.setLevel(logging.INFO)
# format log message
formatter = logging.Formatter("%(asctime)s: %(levelname)s:%(name)s - %(message)s")
handler.setFormatter(formatter)
logger.addHandler(handler)
logger.debug("This is a debug message")
logger.info("This is an info message")
logger.warning("This is a warning message")
logger.error("This is an error message")
logger.critical("This is a critical message")
The resulting output is:
2021-07-20 15:24:51,148: WARNING:__main__ - This is a warning message
2021-07-20 15:24:51,149: ERROR:__main__ - This is an error message
2021-07-20 15:24:51,149: CRITICAL:__main__ - This is a critical message
I expected the output to include logs at info level as well but as you can see it's not. However, if I change to handler.setLevel(logging.ERROR) the output looks like this:
2021-07-20 15:27:49,864: ERROR:__main__ - This is an error message
2021-07-20 15:27:49,864: CRITICAL:__main__ - This is a critical message
So if I am being more restrictive than the root logger it overwrites but if I am less restrictive it does not. How do I make the custom logger to log INFO-level logs?

You need to set the log level on the logger instance like this:
import logging
logger = logging.getLogger(__name__)
# write to stderr
handler = logging.StreamHandler()
handler.setLevel(logging.INFO)
logger.setLevel(logging.INFO)
# format log message
formatter = logging.Formatter("%(asctime)s: %(levelname)s:%(name)s - %(message)s")
handler.setFormatter(formatter)
logger.addHandler(handler)
logger.debug("This is a debug message")
logger.info("This is an info message")
logger.warning("This is a warning message")
logger.error("This is an error message")
logger.critical("This is a critical message")

Ok so lee-pai-long's answer fixed it but I read up some more and I now understand why it works like it does.
The logger itself has a logging level which is set by
logger.setLevel(logging.INFO)
The logger can have one or more handlers connected to it, each of which can have its own logging level.
handler.setLevel(logging.INFO)
For example you could have a FileHandler logging at WARNING-level and a StreamHandler logging at INFO-level.
So what caused the bug was that the handler was set to INFO but the logger's level was never set, making it default to the root level which is WARNING.

A couple of additions to the existing answers:
For simple logging configuration like this, use
logging.basicConfig as described in this example in Python
documentation. It automatically logging levels for the root logger and
its handlers.
logging.basicConfig(format='%(levelname)s:%(message)s', level=logging.DEBUG)
To investigate the state of whole logging logger tree (and their
handlers, formatter and filters) use logging_tree. If I add this
in the end of the snippet in the question.
import logging_tree
logging_tree.printout()
It makes very clear that's going on. It's especially helpful in more
complex logging configurations.
<--""
Level WARNING
|
o<--"__main__"
Level NOTSET so inherits level WARNING
Handler Stream <_io.TextIOWrapper name='<stderr>' mode='w' encoding='utf-8'>
Level INFO
Formatter fmt='%(asctime)s: %(levelname)s:%(name)s - %(message)s' datefmt=None

Unexpected behavior of the Python logging module with info and warning messages

I have a question regarding logging module in Python.
If I instantiate a logger and set its level to INFO without adding any handlers, then I expect that info messages will be printed on the screen.
However, in reality, warning messages are printed but info messages are not.
After adding explicitly a handler, both levels are printed.
Precisely, the following small script:
import logging
logger = logging.getLogger(__name__)
logger.setLevel(logging.INFO)
logger.info("This is an info message")
logger.warning("This is a warning message")
print("*** Add a stream handler explicitly")
handler = logging.StreamHandler()
logger.addHandler(handler)
logger.info("This is an info message")
logger.warning("This is a warning message")
gives output
This is a warning message
*** Add a stream handler explicitly
This is an info message
This is a warning message
(checked in Python 3.7.6 and 3.8.2).
I would expect that either no messages are printed without a handler, or both levels are printed after setting level to INFO.

I suggest you use loguru. It's a really simple to use tool and for me it's very intuitive.
You can add a default logger just using this line:
from loguru import logger
If you want to change the logging level, just setup the logger like this:
logger.remove() # This disables the default logger
logger.add(sys.stderr, level="INFO") # This adds a default logger (format and colors included) but at the logging level you need
And that's it. All the scripts that use the loguru logger in your code will be set up like this. By default it outputs to stderr but you can change it however you need.

Python logging issue?

I am writing the below code to log messages into a file:
#!/usr/bin/python3
import logging
## init logger
logger = logging.getLogger(__name__)
## create handler file
handler = logging.FileHandler('mylog3.log')
## set log level
handler.setLevel(logging.INFO)
## add handler to logger
logger.addHandler(handler)
l = [1, 2, 3, 4]
i = 3
logger.info('This is an info message')
logger.warning('This is a warning message')
logger.debug('This is a debug message. I=%d l=%s', i, l)
logger.error('This is an error message. l =%s', l)
logger.fatal('This is a fatal error message')
The problem I am seeing is that no matter what setting I give, it always prints the warning, error and fatal error messages in file. The debug and info messages are never written.
I verified that I am seeing the issue w/ both PyCharm on Windows and also on my Ubuntu linux Python 3. So, it must be that I am doing something wrong. What can it be? I am at a loss!

Both loggers and handlers have levels. You've set the level for the handler, but not for the logger - and that defaults to WARNING. If you do e.g. logger.setLevel(logging.DEBUG), you should see DEBUG and INFO messages as well (if you don't set a level on the handler at all) or INFO messages but not DEBUG messages (if you set the handler's level to logging.INFO).
See this diagram for more info on the information flow in logging.

The debug and info messages are not shown because of the line handler.setLevel(logging.INFO). This causes all messages of level logging.INFO or less (only info and debug) to not be shown. You can replace loggign.INFO with logging.NOTSET to fix this.

Logging setLevel is being ignored

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

logging.info doesn't show up on console but warn and error do

When I log an event with logging.info, it doesn't appear in the Python terminal.
import logging
logging.info('I am info') # no output
In contrast, events logged with logging.warn do appear in the terminal.
import logging
logging.warn('I am warning') # outputs "I am warning"
Is there a environment level change I can to make logging.info print to the console? I want to avoid making changes in each Python file.

The root logger always defaults to WARNING level. Try calling
logging.getLogger().setLevel(logging.INFO)
and you should be fine.

Like #ztyx said that default logger level is WARNING. You have to set it to a lower level
You can do it by using logging.basicConfig and setting logger level:
logging.basicConfig(level=logging.DEBUG)

The above solutions didn't work for me, but the code here did:
# set up logging to file
logging.basicConfig(level=logging.DEBUG,
format='%(asctime)s %(name)-12s %(levelname)-8s %(message)s',
datefmt='%m-%d %H:%M',
filename='/temp/myapp.log',
filemode='w')
# define a Handler which writes INFO messages or higher to the sys.stderr
console = logging.StreamHandler()
console.setLevel(logging.INFO)
# add the handler to the root logger
logging.getLogger('').addHandler(console)
(I omitted parts of the code for the sake of readability)

This will work
import logging
logging.basicConfig()
logger = logging.getLogger(__name__)
logger.setLevel(logging.INFO)
logger.info('its working')

In more recent versions of Python 3 (tested with Python 3.8), console logging requires creating a console handler to correctly show info messages.
The following example is modified from the Configuring Logging example in the Python documentation:
import logging
# create logger
logger = logging.getLogger('__name__')
level = logging.INFO
logger.setLevel(level)
# ----> console info messages require these lines <----
# create console handler and set level to debug
ch = logging.StreamHandler()
ch.setLevel(level)
# add ch to logger
logger.addHandler(ch)
# -----------------------------------------------------
# 'application' code
logger.debug('debug message')
logger.info('info message')
logger.warning('warn message')
logger.error('error message')
logger.critical('critical message')
Running the above code generates the following output:
info message
warn message
error message
critical message
Here is this same code without the console handler.
import logging
# create logger
logger = logging.getLogger('__name__')
level = logging.INFO
logger.setLevel(level)
# 'application' code
logger.debug('debug message')
logger.info('info message')
logger.warning('warn message')
logger.error('error message')
logger.critical('critical message')
Without the console handler, the output does not include the info message:
warn message
error message
critical message
I do not understand why this is the case as it seems unnecessary.

What's the minimum required code for a working module-level logger? I did an experiment (with python version 3.8.6).
The take-away is:
logging.basicConfig() is needed (however, specifying level=... is NOT needed)
it's necessary to configure the root logger: logging.getLogger().setLevel(...)
So, a minimum working example is:
The library/module code does NOT need to configure the logger:
# library/module code: lib.py
import logging
LOGGER = logging.getLogger('x.y.z')
def some_function():
LOGGER.info("hi")
The application code need to configure the logger with 2 lines at minimum:
# Application Code
import logging, lib
logging.basicConfig()
logging.getLogger().setLevel(logging.INFO) # configure root logger
main() # code that will trigger lib
Here's the experiment:
In [1]: import logging
In [2]: lg = logging.getLogger('x.y.z')
In [3]: lg.info(1)
In [4]: logging.basicConfig()
In [5]: lg.info(1)
In [6]: logging.basicConfig(level=logging.INFO)
In [7]: lg.info(1)
In [8]: logging.basicConfig()
In [9]: logging.getLogger().setLevel(logging.INFO)
In [10]: lg.info(1)
INFO:x.y.z:1

For those using absl.logging, the equivalent command is
from absl import logging
logging.set_verbosity(logging.INFO)

If you are using Django to power your server, you just simply need to change the log level in your settings.py file as such:
"handlers": {
"console": {
-- "level": "WARNING",
++ "level": "INFO",
"class": "logging.StreamHandler",
"formatter": "stackdriver",
}
},
More examples in the documentation here:
https://docs.djangoproject.com/en/4.0/topics/logging/#configuring-logging-1

logging.info() will use the root logger for logging.
According to official doc,
if you do not set an explicit handler for the logger, a special handler called lastResort will be used.
See the code here. By default the logging level of lastResort (it is stream handler) is 30.
we can change its level to output info message.
# setting both the logger and handler's level will work as expected.
logger.setLevel(logging.DEBUG)
logging.lastResort.setLevel(logging.DEBUG)
However, this is like a hack and never a encouraged action.
Using logging.basicConfig()
If we want to do logging real quick, we can use the method logging.basicConfig.
logging.basicConfig(level=logging.DEBUG)
This will create the logger. The logger.level will be the level we set here.
A stream handler will be also created, with level NOTSET.
Without a level param, the default level for root logger is WARNING.
reference
logger logging without handler: logger logging without a handler