Develop Reference

Python Logging from Library

Hi this is hopefully a basic question. I have a python library with a lot of logger messages such as this:
log = logging.getLogger(__name__)
log.info("blah")
log.warning("blah")
...
Then I have a separate code that imports and runs this library. In that code, I added this, thinking it would cause all logging messages to go to this file:
log = logging.getLogger(__name__)
fh = logging.FileHandler("/some/file/location/log.txt")
log.addHandler(fh)
This does successfully pass all log messages in that script to direct to that file, but the logging messages from the library aren't being passed along. I don't want to specify the file path from within the library, that doesn't make much sense, I want it specified in the code that runs the library. Most of the examples I'm seeing show imports happening with parent/child modules, but what about one module that calls a completely different module? Does my library need to accept a logger as an argument to use, or can I use the logging module to handle this?
Thanks.

Looks like you are creating two instances of the Logger class. Only the instance in your script is being configured to write to the file location.
Each time the 'getLogger' method is called, it provides a reference to the Logger with the specified name. If a Logger with that name doesn't exist, a new one is created. Note that in Python, __name__ specifies the module name. Since you are calling the library from a script, I'd assume you have two separate modules, hence two different Loggers.
For a quick-and-dirty approach, you can use:
import my_library
log = logging.getLogger(my_library.__name__)
fh = logging.FileHandler("/some/file/location/log.txt")
log.addHandler(fh)
Where my_library is your newly defined library. This will provide the logger which the library instantiated, instead of creating a new one.
Another approach would be to define a module-level function like this:
# In your script
import my_library
log_location = "/some/file/location/log.txt"
my_library.set_log_location(log_location)
# In your newly defined library
def set_log_location(path):
log = logging.getLogger(__name__)
fh = logging.FileHandler("/some/file/location/log.txt")
log.addHandler(fh)
The second approach wouldn't require the user knowing that your library uses the logging module.

In your application, e.g. in the if __name__ == '__main__' clause, configure the root logger with the handlers you want, using e.g.
logging.basicConfig(level=logging.DEBUG,
filename='/some/file/location/log.txt',
filemode='w',
format='%(asctime)s %(message)s')
and then all logging from your application, your libraries and third-party libraries should write log messages to the file. The sources of logged events (application or libraries) don't need to know or care where the events they log end up - that's taken care of by the configuration. If you need more involved configuration than basicConfig() provides, you can use logging's dictConfig() API to configure logging.

Python logging using modules

I have multiple modules, which are called by a principal script. Each one does log messages using logging Python builtin package.
How can I log an session ID, set during the execution of the main script, across all modules, without needing to push this variable in each module?
I have set up a python configuration file, called config, with:
import logging
logging.basicConfig(
level=logging.DEBUG,
format="%(filename)s:%(lineno)s|%(funcName)3s()|%(asctime)s|%(levelname)s|%(message)s",
handlers=[
logging.FileHandler("debug.log"),
logging.StreamHandler()
]
)
Other modules are using this pre configured logging object, via import, so I am using this:
from config import logging
But I need to log an ID from my current session, my log should look like:
module_name.py:25|function_name()|2020-04-27 18:28:26,518|INFO|Session_ID=abc123|some_message_here
I have tried to put this variable in the config file, set it, and then use it in a function named "log_info" and "log_debug" in this file, but my output log does not trace python script name and function name any more.
Does anyone knows how to handle this situation?

I don't know if this is the 100% best solution, or for sure if it'll work in your situation, but I used it for something similar (verbosity printing levels that would persist across several scripts).
Create a Python file, named, say, sessionid. Inside that, define a top-level setting named id. To set your id, import sessionid and sessionid.id = 'some_id'. Then have your config file import sessionid as well and use sessionid.id as needed.
It took me a bit to figure out that I had to access it like that; changing it using from sessionid import id; id = 'some_id' only persists within the script that does so.
It would make sense to put the id variable in your config file, but only if you don't need to set it in the same script that also needs to from config import logging.

Log with Python logging in Robot Framework

I use robot framework 3.0 under Python 2.7.8. Robot framework's documentation (http://robotframework.org/robotframework/latest/RobotFrameworkUserGuide.html#programmatic-logging-apis) states that
In addition to the new public logging API, Robot Framework offers a built-in support to Python's standard logging module. This works so that all messages that are received by the root logger of the module are automatically propagated to Robot Framework's log file.
I made a short library file to test this:
from logging import debug, error, info, warn
def try_logging():
info("This is merely a humble info message.")
debug("Most users never saw me.")
warn("I warn you about something.")
error("Something bad happened.")
My test case is:
*** Test Cases ***
Logtest
Try logging
When I run it it is a PASSED case, but nothing logged into the HTML log. The test execution log has the suit and the case and the keyword as it should but when I expand them nothing is logged but the "Start / End / Elapsed" line.
How could I forward the Python logger messages to Robot? As you can see the so called automatic propagation is not working automatically. My goal is to write a library that can be run with or without Robot Fw.
Ty for your help in advance.

After hours of code digging I managed to find the answer. I think it is worth sharing as it may be help you if you have some similar issue.
In my case I had some unused libraries imported. One of them was a class that was instantiated when Robot Framework imported the library file. This object had some logger settings that messed up the defaults, that is why I got no result in the robot log.
Without it I got the expected results and automatic propagation worked fine.

Python - Define custom unittest.TextTestRunner

In python unittest, can how do i define a custom unittest.TextTestRunner class? I need to use the Python logger module and ensure that all logs go to the log file created by the Python logger module. Especially when exceptions and assert errors are thrown up they need to go to this log file. I need to be able to user the logger.info() and logger.warning() functions and so on.
Can anyone give me some sample code or link to sample code or steps to do so.

Incompatibility between import-time logger naming with logging configuration

I am setting up my Python logging in main.py via reading in a file and using the fileConfig option. I want to be able to switch between testing and live logging configurations, so I want to read in a separate config file first and extract the logging config file path from there.
The problem here is that other files that I import from main.py grab their own logger via log = getLogger(__name__), and this happens at import time. These links then get broken when the new configuration is loaded in, and these modules end up without the logging working the way I expect.
I can't easily delay the importing of these modules without a lot of refactoring, so is there any other way of being able to keep this method of setting up loggers by module name while still loading in the log configuration later?

I'm not sure from your question exactly how things are breaking, but here's how I see it. The various modules which do log = logging.getLogger(__name__) will have valid names for their loggers (logger name = package name), unless you were to somehow actually move the modules to some other package location.
At import time, the logging configuration may or may not have been set, and there shouldn't be any actual logging calls made as a side-effect of the import (if there are, the messages may have nowhere to go).
Loading a new configuration using fileConfig typically just sets handlers, formatters and levels on loggers.
When you subsequently call code in the imported modules, they log via their loggers, which have handlers attached by your previous configuration call - so they will output according to the configuration.
You should be aware that on older versions of Python (<= 2.5), calls to fileConfig would unavoidably disable existing loggers which weren't named in the configuration - in more recent versions of Python (>= 2.6), this is configurable using a disable_existing_loggers=False keyword argument passed to fileConfig. You may want to check this, as it sometimes leads to unexpected behaviour (the default for that parameter is True, for compatibility with behaviour under the older Python versions).
If you post more details about what seems broken, I might be able to provide a better diagnosis of what's going on.