classification
Title: Curses terminal resize problems when Python is in interactive mode
Type: behavior Stage: needs patch
Components: Documentation, Extension Modules Versions: Python 3.6, Python 3.5, Python 2.7
process
Status: open Resolution:
Dependencies: Superseder:
Assigned To: docs@python Nosy List: docs@python, martin.panter, pbazant, poq, r.david.murray
Priority: normal Keywords:

Created on 2008-04-23 15:41 by pbazant, last changed 2016-05-15 01:05 by martin.panter.

Messages (6)
msg65700 - (view) Author: Pavel Bazant (pbazant) Date: 2008-04-23 15:41
When python is in interactive mode, curses does not react to resize 
events properly.

test.py:

import curses

def run():
 stdscr=curses.initscr()
 key=0
 while(key!=ord('q')):
  key=stdscr.getch()
  stdscr.addstr(0,0,str(stdscr.getmaxyx())+' '+str(key))
  stdscr.refresh()
 curses.endwin()

run()

When this is run directly, everything is ok. When it is called via 
execfile() from the interactive prompt, it shows the right screen size 
after the first keypress, but behaves oddly after the resize.

IMHO, the following happens:
For some reason, env. variables LINES and COLUMNS are set but they are 
not reflected in the os.environ structure nor they respond to screen 
size changes. If these variables are set then the ncurses library (see 
man pages) uses their values instead of getting the term size via 
ioctl. 
The ncurses library receives a SIGWINCH and sees that LINES
and COLUMNS are set. However, their values are same as the
screen dimensions before the resize, so it is perplexed why there is
a SIGWINCH if the screen did not change and it just ungetchs an ERR
and ncurses internal structures are not changed appropriately.

From the resizeterm man page:
"If the environment variables LINES or COLUMNS are set, this overrides 
the library's use of the window size obtained from the operating 
system. Thus, even if a SIGWINCH is received, no screen size change may 
be recorded. In that case, no KEY_RESIZE is queued for the next call to 
getch; an ERR will be returned instead."

Executing

import os
os.environ['LINES']="blah"
del os.environ['LINES']
os.environ['COLUMNS']="blah"
del os.environ['COLUMNS']

solves the problem for me.
Perhaps the problem has sth to do with python using readline
in interactive mode???
PB
msg109298 - (view) Author: Mark Lawrence (BreamoreBoy) * Date: 2010-07-05 05:21
Could somebody witht a nix box try this out, I'd do it myself if I had such a beast.
msg109325 - (view) Author: R. David Murray (r.david.murray) * (Python committer) Date: 2010-07-05 15:21
Well, I can run it, but I have no idea what it is telling me or how it demonstrates a bug.  Pavel, is this still a problem in 2.6/2.7?
msg161020 - (view) Author: (poq) Date: 2012-05-17 21:51
Just to confirm: curses SIGWINCH handling is still (2.7/3.2) broken after importing readline.

Readline seems to set the LINES/COLUMNS environment vars, and this confuses curses, even if curses is imported after readline.

Clearing the LINES/COLUMNS vars after import readline fixes the issue:
os.environ['LINES'] = os.environ['COLUMNS'] = ''
or
os.unsetenv('LINES'); os.unsetenv('COLUMNS')
(or other variations).

I spent a couple hours tearing my hair out over this, before I found this report.

It may be possible for Python to work around this readline behavior by saving LINES/COLUMNS and restoring them after initializing readline.

Or maybe this should just be documented somewhere.
msg161021 - (view) Author: (poq) Date: 2012-05-17 22:06
Issue 3948 is almost certainly a duplicate.
msg265564 - (view) Author: Martin Panter (martin.panter) * (Python committer) Date: 2016-05-15 01:05
For the record, this is my interpretation of Pavel’s demo:

If you press a key (other than lowercase “q”) or resize the terminal, it displays the terminal dimensions in the top-left corner. Without Readline, the dimensions are updated when the terminal resizes.

When Readline is used (interactive mode, or simply “import readline”), the reported dimensions are static, which I understand is the bug.

Documentating this quirk may be a good start.
History
Date User Action Args
2016-05-15 01:05:47martin.pantersetassignee: docs@python
components: + Documentation
versions: + Python 2.7, Python 3.5, Python 3.6, - Python 3.3
nosy: + docs@python, martin.panter

messages: + msg265564
stage: test needed -> needs patch
2016-05-15 00:57:41martin.panterlinkissue3948 superseder
2014-12-31 16:20:57akuchlingsetnosy: - akuchling
2014-02-03 19:12:11BreamoreBoysetnosy: - BreamoreBoy
2012-05-17 22:06:23poqsetmessages: + msg161021
2012-05-17 21:51:38poqsetnosy: + poq

messages: + msg161020
versions: + Python 3.3, - Python 2.4
2010-07-05 15:21:49r.david.murraysetnosy: + r.david.murray, akuchling
messages: + msg109325

components: + Extension Modules, - None
stage: test needed
2010-07-05 05:21:46BreamoreBoysetnosy: + BreamoreBoy
messages: + msg109298
2008-04-23 15:41:39pbazantcreate