How to print between last and second-to-last lines in console?

Question

Using Python 3.4, is there a platform-agnostic method of printing between the last and second-to-last lines? What I'm trying to accomplish is basically a progress bar always on the last line. The 2nd line and up are just debug logs. For example:

[DEBUG] Some info...
[DEBUG] More info...
Download Progress: [####      ] 40% (3 of 20)

I run some downloads in a loop. Each iteration of the loop I run this to print the progress bar:

def _print_progress_bar(self):
    amt_done = self._count / self._max
    print('\rDownload Progress: [{:20s}] {:.1f}% ({})'.format(
        '#' * int(amt_done * 20),
        amt_done * 100,
        self._padded_counter(' of ')), end='')

Between calls to this method to render the progress bar, I want to insert diagnostic messages above it (I do not want print statements after this to overwrite the progress bar, basically).

How can I do this?

score 0 · Answer 1 · answered May 21 '15 at 02:06

You can get this effect by using the carriage return, '\r'. On Windows, this is the character that gets combined with a linefeed ('\n') to create a new line. On Unix systems, a bare linefeed automatically returns the carriage, but '\r' is still a valid character. For example, print('hi\rhello') will print 'hi', go back to the beginning of the line, and print 'hello', so only 'hello' ends up actually displayed. You can use end='' to suppress print()'s new line. The following function demonstrates what to do - I recommend running it to see what happens for yourself, as the output is, of course, dynamic.

>>> import time, sys
>>> def doit():
...     print('part 1')
...     print(0, end='')
...     sys.stdout.flush()
...     time.sleep(1)
...     print('\rpart 2')
...     print(1)
...
>>> doit()
part 1
part 2
1

score 0 · Answer 2 · answered May 21 '15 at 02:06

0

The best way I could find was to use terminal escape codes like this:

for i in range(10):
    time.sleep(0.1)
    print('%s\n%s\r\033[F' % (i,i), end="")

This will print a number in one line, then in another line, then move to beginning of the line and one line up to print again. This way you can freely edit lines above you.

This solution however has the potential of not working on Windows.

answered May 21 '15 at 02:06

Andrzej Pronobis

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I tried that on my Windows system and got this result (in handy `repr` form): `'0\n←[F1\n←[F2\n←[F3\n←[F4\n←[F5\n←[F6\n←[F7\n←[F8\n←[F9\n←[F'`. Can you try my answer on your Unix machine? – TigerhawkT3 May 21 '15 at 02:24
Yes, you will get those pretty arrows in windows. In Linux, and most ANSI terminals it will work nicely. – Andrzej Pronobis May 21 '15 at 02:40
It might be possible to get your windows terminal emulator to understand ANSI codes. http://stackoverflow.com/questions/16755142/how-to-make-win32-console-recognize-ansi-vt100-escape-sequences – Håken Lid Jan 23 '16 at 08:02

score 0 · Answer 3 · answered Jan 23 '16 at 07:37

You need to modify your DEBUG function to erase the line before printing. It is the only platform agnostic way I know.

Platform specific way on Windows is to use WinAPI to scroll the upper part of the Window and write exactly to the line above the last. Maybe a library like https://pypi.python.org/pypi/asciimatics/1.5.0.post1 can help you.

How to print between last and second-to-last lines in console?

3 Answers3