NameError has different actions when be raised in IDLE

Bug report

Bug description:

In IDLE, I raised three different exceptions. The output is:

>>>raise Exception("123\n456")
Traceback (most recent call last):
  File "<pyshell#0>", line 1, in <module>
    raise Exception("123\n456")
Exception: 123
456
>>>raise NameError("123\n456")
Traceback (most recent call last):
  File "<pyshell#1>", line 1, in <module>
    raise NameError("123\n456")
456
>>>raise OSError("123\n456")
Traceback (most recent call last):
  File "<pyshell#2>", line 1, in <module>
    raise OSError("123\n456")
OSError: 123
456

Only NameError cannot show the Exception Type if the text has two or more lines.
In shell the action is same with other exception.

CPython versions tested on:

3.13

Operating systems tested on:

Windows

Linked PRs

3.12.10:

latest build:

3.12 won't get updates as it now only accepts security-only patches but we'll try to fix this in 3.13 and later

I confirmed that this is IDLE and exception specific. Also, it does not happen if IDLE is started with the -n (no subprocess) option. So it is specific to the exception display in normal mode, which I am sure uses a method in run.py instead of the one in pyshell.py.

The same error occurs when the raise statement in run from an editor file instead of entered interactively.

The run.py function print_exception calls nested function print_exc, which prints the lines returned by get_message_lines(typ, exc, tb). The latter special cases AttributeError and NameError to indirectly get the type hint, if available, that is not directly available to Python code. It does this by calling sys.excepthook with stderr temporarily redirected to an io.StringIO. For example: input (NameError, "name 'aab' is not defined.") might result in output "NameError name 'aab' is not defined. Did you mean: 'aaa'?". The returned message is always a single logical line for NameErrors raised by Python itself. The function extracts and return it.

This bug would only affect a situation where user code catches a NameError, adds second line to the message, and re-raises a NameError with the 2 (or more) line message. A fix should check the message for embedded \ns and if present, extract more lines from the output.

After the change in the PR.

@Locked-chess-official

@Locked-chess-official @shuimu5418 The little images you two posted above took several minutes to appear. If you right-click in the Shell sidebar, you can copy a selection with prompts. Then paste between triple backtick (```) lines.

gh-135511 Fixed truncated exception message for Name- and AttributeEr…

[3.13] gh-135511: Fixed truncated exception message for Name- and Att…

[3.14] gh-135511: Fixed truncated exception message for Name- and Att…

[3.15] gh-135511 Fixed truncated exception message for Name- and Attr…

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`NameError` has different actions when be raised in IDLE #135511

Bug report

Bug description:

CPython versions tested on:

Operating systems tested on:

Linked PRs

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NameError has different actions when be raised in IDLE #135511

Description

Bug report

Bug description:

CPython versions tested on:

Operating systems tested on:

Linked PRs

Activity

Locked-chess-official commented on Jun 14, 2025

shuimu5418 commented on Jun 14, 2025

picnixz commented on Jun 14, 2025

terryjreedy commented on Jun 14, 2025

terryjreedy commented on Jun 14, 2025

terryjreedy commented on Jun 14, 2025

Locked-chess-official commented on Jun 15, 2025

terryjreedy commented on Sep 26, 2026

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`NameError` has different actions when be raised in IDLE #135511