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[Windows] Python 3.13 New REPL prompt swallows characters with print(..., end=" ") #128067

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[Windows] Python 3.13 New REPL prompt swallows characters with print(..., end=" ")#128067
Labels
3.13only security fixes3.14bugs and security fixes3.15bugs and security fixesOS-windowsstdlibStandard Library Python modules in the Lib/ directorytopic-replRelated to the interactive shelltype-bugAn unexpected behavior, bug, or error

Description

@cessor
cessor
opened on Dec 18, 2024

Bug report

Bug description:

I encountered the following unexpected behavior when working with the new REPL on Windows 10's cmd just now:

Actual Behavior

>>> for i in range(0, 14, 2):
...     print(i, end=" ")
...
>>> 4 6 8 10 12
  • When I use print and pass an argument to the end-parameter, the resulting line is missing 4 characters.
  • The four chars of the REPL prompt >>> appear to write over the line, as the cursor blinks on the first visible position (indicator sits under the number 4)
  • Typing overwrites the shown chars.

Expected Behavior

>>> for i in range(0, 14, 2):
...     print(i, end=" ")
...
0 2 4 6 8 10 12 >>> _
  • REPL on Python 3.12 works as expected.
  • Results are printed in one line, prompt appears at the end, cursor follows at end of line.

Note:

This works as expected:

>>> for i in range(0, 14, 2):
...     print(i)
...
0
2
4
6
8
10
12

CPython versions tested on:

3.13

Operating systems tested on:

Windows

Linked PRs

Activity

  1. cessor
    added
    type-bugAn unexpected behavior, bug, or error
    on Dec 18, 2024
  2. ZeroIntensity
    added
    topic-replRelated to the interactive shell
    on Dec 18, 2024
  3. picnixz
    added
    stdlibStandard Library Python modules in the Lib/ directory
    3.13only security fixes
    3.14bugs and security fixes
    on Dec 20, 2024

  4. devdanzin commented on Dec 23, 2024

    @devdanzin
    devdanzin
    on Dec 23, 2024
    Member

    Confirmed on main:

    >>> print(10, end=' ')
>>>

    It's possible to see the 10 output appear then be overwritten by the prompt. Any string ending without a newline seems to suffer from this:

    >>> print("a\n", 10, end=' ')
a

  5. devdanzin commented on Dec 24, 2024

    @devdanzin
    devdanzin
    on Dec 24, 2024
    Member

    It's possible to capture output in a StringIO in code.InteractiveInterpreter.runcode and add a \n character if the output doesn't end in a newline. It should change the behavior to (in both Windows and Linux):

    >>> for i in range(0, 14, 2):
...     print(i, end=" ")
...
0 2 4 6 8 10 12 
>>> _

    That is, the prompt would always appear on a new line.

    If this is considered a good solution, I can submit a PR.

    Edit: with a PoC change, the issue is resolved and test_pyrepl, test_repl and test_code pass.

  6. devdanzin commented on Dec 24, 2024

    @devdanzin
    devdanzin
    on Dec 24, 2024
    Member

    The proposed fix changes two behaviors that could be considered bugs in how output is handled.

    Without the change, in Windows, printing a very long line ending with a space would result in its last wrapped line being overwritten by the prompt and further input:

    >>> print("abcdef" * 30, end=' ')
abcdefabcdefabcdefabcdefabcdefabcdefabcdefabcdefabcdefabcdefabcdefabcdefabcdefabcdefabcdefabcdefabcdefabcdefabcdefabcdef
>>> efabcdefabcdefabcdefabcdefabcdefabcdefabcdefabcdefabcdef

    While on Linux, without the change, the prompt appears right at the end of the long line:

    >>> print("abcdef" * 30, end=' ')
abcdefabcdefabcdefabcdefabcdefabcdefabcdefabcdefabcdefabcdefabcdefabcdefabcdefabcdefabcdefabcdefabcdefabcdef
abcdefabcdefabcdefabcdefabcdefabcdefabcdefabcdefabcdefabcdefabcdefabcdef >>>

    With the change, the prompt always appears on a new line.

  7. gaogaotiantian
    marked print omiting output when using the parameter end=" " #130239 as a duplicate of this issue on Feb 17, 2025
  8. picnixz
    marked print(..., end='') is corrupting output in Python 3.13.1 REPL (PowerShell) #130766 as a duplicate of this issue on Mar 2, 2025
  9. ZeroIntensity
    added
    OS-windows
    on Mar 2, 2025
  10. picnixz
    marked REPL: Prompt obscures chars for print with end="," #136553 as a duplicate of this issue on Jul 12, 2025
  11. picnixz
    added
    3.15bugs and security fixes
    on Jul 12, 2025
  12. skirpichev
    marked [print()] did not print all the the numbers expected #138689 as a duplicate of this issue on Sep 9, 2025

  13. JanEricNitschke commented on Sep 9, 2025

    @JanEricNitschke
    JanEricNitschke
    on Sep 9, 2025
    Contributor

    @devdanzin Are you still having a look at this or can i try to give it a go?

  14. 23 remaining items

  15. ambv
    added a commit that references this issue on Jan 2, 2026

    gh-128067: Fix pyrepl overriding printed output without newlines (#13…

    8a2deea
  16. miss-islington
    added 2 commits that reference this issue on Jan 2, 2026

    pythongh-128067: Fix pyrepl overriding printed output without newlines (

    2a7dcf0

    pythongh-128067: Fix pyrepl overriding printed output without newlines (

    1686927
  17. ambv
    added 6 commits that reference this issue on Jan 2, 2026

    pythongh-128067: In test_pyrepl, discover escape sequences from termi…

    4c41f8b

    gh-128067: In test_pyrepl, discover escape sequences from terminfo in…

    08a17ed

    pythongh-128067: In test_pyrepl, discover escape sequences from termi…

    b5b2e64

    pythongh-128067: In test_pyrepl, discover escape sequences from termi…

    7ac58e5

    [3.14] gh-128067: Fix pyrepl overriding printed output without newlin…

    a141b84

    [3.13] gh-128067: Fix pyrepl overriding printed output without newlin…

    c09d6a4

  18. ambv commented on Jan 2, 2026

    @ambv
    ambv
    on Jan 2, 2026
    Contributor

    Fixed! Thanks for your contribution.

  19. ambv
    closed this as completedon Jan 2, 2026

  20. amirihekmatallah-code commented on Jan 28, 2026

    @amirihekmatallah-code
    amirihekmatallah-code
    on Jan 28, 2026 · Hidden as spam

  21. amirihekmatallah-code commented on Jan 28, 2026

    @amirihekmatallah-code
    amirihekmatallah-code
    on Jan 28, 2026 · Hidden as spam
  22. thunder-coding
    added 2 commits that reference this issue on Feb 15, 2026

    pythongh-128067: Fix pyrepl overriding printed output without newlines (

    b2f1ca2

    pythongh-128067: In test_pyrepl, discover escape sequences from termi…

    7c0d13e
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