performance - Does python logging flush every log?

Question

When I write a log to file using the standard module logging, will each log be flushed to disk separately? For example, will the following code flush log by 10 times?

logging.basicConfig(level=logging.DEBUG, filename='debug.log')
    for i in xrange(10):
        logging.debug("test")

if so, will it slow down ?

Answer 1

Yes, it does flush the output at every call. You can see this in the source code for the StreamHandler:

def flush(self):
    """
    Flushes the stream.
    """
    self.acquire()
    try:
        if self.stream and hasattr(self.stream, "flush"):
            self.stream.flush()
    finally:
        self.release()

def emit(self, record):
    """
    Emit a record.

    If a formatter is specified, it is used to format the record.
    The record is then written to the stream with a trailing newline.  If
    exception information is present, it is formatted using
    traceback.print_exception and appended to the stream.  If the stream
    has an 'encoding' attribute, it is used to determine how to do the
    output to the stream.
    """
    try:
        msg = self.format(record)
        stream = self.stream
        stream.write(msg)
        stream.write(self.terminator)
        self.flush()   # <---
    except (KeyboardInterrupt, SystemExit): #pragma: no cover
        raise
    except:
        self.handleError(record)

I wouldn't really mind about the performance of logging, at least not before profiling and discovering that it is a bottleneck. Anyway you can always create a Handler subclass that doesn't perform flush at every call to emit(even though you will risk to lose a lot of logs if a bad exception occurs/the interpreter crashes).