python.logging: Why does my non-basicConfig setting not work?

I want to log to a single log file from main and all sub modules.

The log messages send from a main file, where I define the logger, work as expected. But the ones send from a call to an imported function are missing.

It is working if I use logging.basicConfig as in Example 1 below. But the second example which allows for more custom settings does not work.

Any ideas why?

# in the submodule I have this code
import logging
logger = logging.getLogger(__name__)

EXAMPLE 1 - Working

Here I create two handlers and just pass them to basicConfig:

# definition of root looger in main module

formatter = logging.Formatter(fmt="%(asctime)s %(name)s.%(levelname)s: %(message)s", datefmt="%Y.%m.%d %H:%M:%S")   

handler = logging.FileHandler('logger.log')
handler.setFormatter(formatter)
handler.setLevel(logging.DEBUG)

handler2 = logging.StreamHandler(stream=None)
handler2.setFormatter(formatter)
handler2.setLevel(logging.DEBUG)

logging.basicConfig(handlers=[handler, handler2], level=logging.DEBUG)
logger = logging.getLogger(__name__)

EXAMPLE 2 - Not working

Here I create two handlers and addHandler() them to the root logger:

# definition of root looger in main module

logger = logging.getLogger(__name__)
logger.setLevel(logging.DEBUG)

handler = logging.FileHandler('logger.log')
handler.setFormatter(formatter)
handler.setLevel(logging.DEBUG)
#handler.setLevel(logging.ERROR)
logger.addHandler(handler)

handler = logging.StreamHandler(stream=None)
handler.setFormatter(formatter)
handler.setLevel(logging.DEBUG)
logger.addHandler(handler)