Why does logging.setLevel() has no effect here with Python?

Question

I try to understand how the logging module really works. The following code doesn't react like I expected.

#!/usr/bin/env python3
import logging

l = logging.getLogger()
l.setLevel(logging.DEBUG)

print('enabled for DEBUG: {}'.format(l.isEnabledFor(logging.DEBUG)))

l.debug('debug')
l.info('info')
l.warning('warning')
l.error('error')
l.critical('critical')

It just print out this to the console.

warning
error
critical

But why? Shouldn't there be info and debug, too? Why not?

The question is not how to fix this. I know about handlers and things like that. I just try to understand how this code work and why it doesn't react like I expect.

Answer 1

When no handler is set, the lastResort handler is used, and by default the lastResort level is set to WARNING.

This is implemented by this bit of code:

_defaultLastResort = _StderrHandler(WARNING)
lastResort = _defaultLastResort

    def callHandlers(self, record):
        ...
        found = 0
        ...
        if (found == 0):
            if lastResort:
                if record.levelno >= lastResort.level:
                    lastResort.handle(record)

Remember also that both loggers and handlers have levels. A record can be filtered by the logger for having too low a level, and it can also be filtered by a handler for having too low a level. Setting the logger level to DEBUG allows the subsequent logging calls to pass the logger's level filter, but they can still be filtered by the handler's level filter, which is set to lastResort.level, i.e. WARNING, by default.