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_pyrepl: Mangled input when sys.ps1 is wider than terminal width #159658

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_pyrepl: Mangled input when sys.ps1 is wider than terminal width#159658
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stdlibStandard Library Python modules in the Lib/ directorytopic-replRelated to the interactive shelltype-bugAn unexpected behavior, bug, or error

Description

@nsyhykui
nsyhykui
opened on Oct 18, 2026

Bug report

Bug description

In 3.16 (main), if the prompt (sys.ps1) is wider than the terminal width, typing characters results in a mangled line and an unexpected backslash.

Reproducer

  1. Open a narrow terminal:
xterm -geometry 20x10
  1. Start Python 3.16 in it, then set a long prompt:
import sys
sys.ps1 = "a" * 64 + " "
  1. Type Hello.

Actual behavior

The prompt wraps to the next physical line, but _pyrepl seems to miscalculate the layout. It inserts a stray \ and splits the first character from the rest:

aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa H    \
ello

CPython versions tested on:

3.16

Operating systems tested on:

Linux

Linked PRs

Activity

  1. nsyhykui
    added
    type-bugAn unexpected behavior, bug, or error
    on Oct 18, 2026

  2. nsyhykui commented on Oct 18, 2026

    @nsyhykui
    nsyhykui
    on Oct 18, 2026
    Author

    I found a similar issue on Python 3.14, but it looks much worse. like this:

    >>> import sys
>>> sys.ps1 = "a" * 128 + " "
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaHelloaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa \
  3. aisk
    added
    topic-replRelated to the interactive shell
    on Oct 18, 2026
  4. StanFromIreland
    added
    stdlibStandard Library Python modules in the Lib/ directory
    on Oct 18, 2026
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