Making logging.getLogger() simpler

[Lie Ryan]

I was expecting this to work:

import logging
logger = logging.getLogger(__name__)
logger.warn('this is a warning')

instead it produced the error:

No handlers could be found for logger "__main__"


However, if instead I do:

import logging
logging.warn('creating logger')
logger = logging.getLogger(__name__)
logger.warn('this is a warning')

then it does work.

Is there any reason why getLogger()-created logger shouldn't
automatically create a default handler?

 

[Kev Dwyer]

I was expecting this to work:

import logging
logger = logging.getLogger(__name__)
logger.warn('this is a warning')

instead it produced the error:

No handlers could be found for logger "__main__"


However, if instead I do:

import logging
logging.warn('creating logger')
logger = logging.getLogger(__name__)
logger.warn('this is a warning')

then it does work.

Is there any reason why getLogger()-created logger shouldn't
automatically create a default handler?

Hello Lie,

Calling logging.warn(), or logging.debug() etc. before handlers
have been assigned to the root logger will result in
logging.basicConfig() being called automatically. By default a
StreamHandler will be created on the root logger and your logger
inherits the StreamHandler. So you can avoid the "No handlers..."
warning by calling logging.basicConfig() before your program
does any logging.

I don't know why getLogger() doesn't so something similar when
it's called. Perhaps so that the logger is explicitly
initialised with basic, file or dictConfig?

Cheers,

Kev

 

[Vinay Sajip]

I was expecting this to work:

  importlogging
  logger =logging.getLogger(__name__)
  logger.warn('this is a warning')

instead it produced the error:

  No handlers could be found for logger "__main__"

However, if instead I do:

  importlogging
 logging.warn('creating logger')
  logger =logging.getLogger(__name__)
  logger.warn('this is a warning')

then it does work.

Is there any reason why getLogger()-created logger shouldn't
automatically create a default handler?

There is a good reason why a getLogger()-created logger doesn't add a
default handler. Imagine if it did, and the code you were writing was
a library module that was used in an application being written by
another developer. Imagine if there were several other libraries which
also declared loggers (and therefore several default handlers). Then
the result of running the application would be to get a whole bunch of
unexpected messages from those libraries, including yours. This would
annoy the application developer no end, and rightly so.

For simple uses of logging (where you are writing a utility script,
for example, when you are the application developer) you can call
logging.warning() etc. which will (for your convenience) add a handler
to write to the console for you, if there isn't a handler already.

If you are doing something more involved, you will need to configure
logging to do whatever it is you want, and when you run your program
your log will contain messages from your application as well as third-
party libraries you use, all playing well together and with no
unpleasant surprises.

If you are writing a library, you will typically:

1. Add a NullHandler to your top-level logger.
2. If you want to force your users (application developers) to add
handlers explicitly to their loggers, set your top-level logger's
propagate flag to False.
3. If you want to have a specific verbosity on your loggers, which is
different from the default (WARNING), you need to set that level on
your top-level logger, or on individual loggers for which you want
that specific, non-default verbosity.

I hope that helps,


Vinay Sajip

 

[Vinay Sajip]

If you are writing a library, you will typically:
2. If you want to force your users (application developers) to add
handlers explicitly to their loggers, set your top-level logger's
propagate flag to False.

Sorry, in the above text, where it says "their loggers" it should say
"your loggers"

Regards,

Vinay Sajip