How to write script output to file and command-line?

You are on the right path but the problem is python buffering the output.

Fortunately there is a way to tell it not to buffer output:

python -u MyLongRunngingScript.py | tee log.txt

The fact that you don't see anything is probably related to the fact that buffering is occurring. So you only get output every 4 Ko of text or so.

instead, try something like this :

class OutputSplitter(object):
    def __init__(self, real_output, *open_files):
        self.__stdout = real_output
        self.__fds = open_files
        self.encoding = real_output.encoding
    def write(self, string):
        self.__stdout.write(string) # don't catch exception on that one.
        self.__stdout.flush()
        for fd in self.__fds:
            try:
                fd.write(string)
                fd.flush()
            except IOError:
                pass # do what you want here. 
    def flush(self):
        pass # already flushed

Then decorate sys.stdout with that class with some code like that :

stdout_saved = sys.stdout
logfile = open("log.txt","a") # check exception on that one.
sys.stdout = OutputSplitter(stdout_saved, logfile)

That way, every output (print included) is flushed to the standard output and to the specified file. Might require tweaking because i haven't tested that implementation.

Of course, expect to see a (small most of the time) performance penalty when printing messages.

Another simple solution could also be

python script.py > output.log