Skip to content

Curses terminal resize problems when Python is in interactive mode #46927

Closed
Closed
Curses terminal resize problems when Python is in interactive mode#46927
Labels
3.16new features, bugs and security fixesdocsDocumentation in the Doc dirextension-modulesC modules in the Modules dirtype-bugAn unexpected behavior, bug, or error

Description

@pbazant
pbazant
mannequin
opened on Apr 23, 2008
BPO 2675
Nosy @bitdancer, @vadmium

Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.

Show more details

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2008-04-23.15:41:39.440>
labels = ['extension-modules', 'type-bug', 'docs']
title = 'Curses terminal resize problems when Python is in interactive mode'
updated_at = <Date 2016-05-15.01:05:47.735>
user = 'https://bugs.python.org/pbazant'

bugs.python.org fields:

activity = <Date 2016-05-15.01:05:47.735>
actor = 'martin.panter'
assignee = 'docs@python'
closed = False
closed_date = None
closer = None
components = ['Documentation', 'Extension Modules']
creation = <Date 2008-04-23.15:41:39.440>
creator = 'pbazant'
dependencies = []
files = []
hgrepos = []
issue_num = 2675
keywords = []
message_count = 6.0
messages = ['65700', '109298', '109325', '161020', '161021', '265564']
nosy_count = 5.0
nosy_names = ['pbazant', 'r.david.murray', 'docs@python', 'poq', 'martin.panter']
pr_nums = []
priority = 'normal'
resolution = None
stage = 'needs patch'
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue2675'
versions = ['Python 2.7', 'Python 3.5', 'Python 3.6']

Linked PRs

Activity

  1. pbazant commented on Apr 23, 2008

    @pbazant
    pbazantmannequin
    on Apr 23, 2008
    MannequinAuthor

    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

  2. pbazant
    added
    type-bugAn unexpected behavior, bug, or error
    on Apr 23, 2008

  3. BreamoreBoy commented on Jul 5, 2010

    @BreamoreBoy
    BreamoreBoymannequin
    on Jul 5, 2010
    Mannequin

    Could somebody witht a nix box try this out, I'd do it myself if I had such a beast.

  4. bitdancer commented on Jul 5, 2010

    @bitdancer
    bitdancer
    on Jul 5, 2010
    Member

    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?

  5. bitdancer
    added
    extension-modulesC modules in the Modules dir
    on Jul 5, 2010

  6. poq commented on May 17, 2012

    @poq
    poqmannequin
    on May 17, 2012
    Mannequin

    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.

  7. poq commented on May 17, 2012

    @poq
    poqmannequin
    on May 17, 2012
    Mannequin

    bpo-3948 is almost certainly a duplicate.

  8. vadmium commented on May 15, 2016

    @vadmium
    vadmium
    on May 15, 2016
    Member

    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.

  9. vadmium
    added
    docsDocumentation in the Doc dir
    on May 15, 2016
  10. vadmium
    assigned
    docspython
    on May 15, 2016
  11. ezio-melotti
    transferred this issue fromon Apr 10, 2022
  12. serhiy-storchaka
    added this to Curses issueson May 20, 2022
  13. erlend-aasland
    moved this to Bugs in Curses issueson May 20, 2022

  14. mbriand commented on May 4, 2025

    @mbriand
    mbriand
    on May 4, 2025
    Contributor

    I've been tracking a similar issue for a few days now: importing readline module is adding LINES and COLUMNS variables to the environment. Values are set to the terminal size at time of module import and never updated.

    Reproduce

    So with this reproduce code:

    #!/usr/bin/env python3

import readline
import subprocess
import time
import pprint

time.sleep(3)
pprint.pprint(subprocess.check_output(["env"]).decode())

    If I run it, we can see the both environment variables are set:

    ./test.py | grep "'LINES=\|COLUMNS="
 'LINES=64\n'
 'COLUMNS=225\n')

    The problem is, if we do resize the terminal or change font size during the sleep() call, LINES and COLUMNS values are not updated correctly.

    You can confirm this is indeed linked to the readline module import, if I remove it, these two variables are not set.

    Analysis

    I've been tracking where these variables are set, and this actually comes from the readline C library. This behaviour can be changed from the cpython source code using rl_change_environment: https://git.savannah.gnu.org/cgit/readline.git/tree/doc/readline.info#n3201

    I made the following modification in cpython, and the variables are not set anymore.

    diff --git i/Modules/readline.c w/Modules/readline.c
index 35655c70a461..1bd558830900 100644
--- i/Modules/readline.c
+++ w/Modules/readline.c
@@ -1247,6 +1247,7 @@ flex_complete(const char *text, int start, int end)
 static int
 setup_readline(readlinestate *mod_state)
 {
+       rl_change_environment = 0;
 #ifdef SAVE_LOCALE
     char *saved_locale = strdup(setlocale(LC_CTYPE, NULL));
     if (!saved_locale) {

    Fix ?

    Now I would be happy to open a PR with this change, but I have some doubts: I'm not sure there would not be any unexpected side effect with this.

  15. mbriand
    added 3 commits that reference this issue on May 7, 2025

    pythongh-46927: Prevent readline from overriding environment

    fb4bbf9

    pythongh-46927: Prevent readline from overriding environment

    6f1df9a

    pythongh-46927: Prevent readline from overriding environment

    bc76d98
  16. mbriand
    added 2 commits that reference this issue on Jul 3, 2026

    pythongh-46927: Prevent readline from overriding environment

    02d53ae

    pythongh-46927: Detect rl_change_environment with a configure probe

    e3bce0d
  17. serhiy-storchaka
    added a commit that references this issue on Jul 8, 2026

    gh-46927: Prevent readline from overriding environment variables (GH-…

    c9687b4

  18. mbriand commented on Jul 10, 2026

    @mbriand
    mbriand
    on Jul 10, 2026
    Contributor

    I tested again the original reproducer script, on today main branch. I do confirm it now shows correct values, both while executed stand-alone of from an interactive session (using exec(open("/home/mathieu/tmp/test.py").read())).

    So I believe we can close this PR, as fixed by the above PR.

  19. serhiy-storchaka
    closed this as completedon Jul 10, 2026
  20. github-project-automation
    moved this from Bugs to Done in Curses issueson Jul 10, 2026
  21. serhiy-storchaka
    added
    3.16new features, bugs and security fixes
    on Jul 10, 2026
Sign up for free to join this conversation on GitHub. Already have an account? Sign in to comment

Metadata

Metadata

Assignees

No one assigned

    Labels

    3.16new features, bugs and security fixesdocsDocumentation in the Doc dirextension-modulesC modules in the Modules dirtype-bugAn unexpected behavior, bug, or error

    Projects

    Milestone

    No milestone

    Relationships

    None yet

    Development

    No branches or pull requests

    Issue actions