Does python logging flush every log?

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).