REPL exits with traceback when str(sys.ps1) raises

Bug report

Bug description:

Subject says it all. This is on the latest HEAD of main.

~/cpython$ g pull
Already up to date.
~/cpython$ make
Checked 112 modules (34 built-in, 78 shared, 0 n/a on macosx-14.6-x86_64, 0 disabled, 0 missing, 0 failed on import)
./python.exe -E ./Tools/build/generate-build-details.py `cat pybuilddir.txt`/build-details.json
~/cpython$ ./python.exe 
Python 3.14.0a5+ (heads/main:ab11c097052, Feb 28 2025, 08:56:47) [Clang 16.0.0 (clang-1600.0.26.6)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import sys
>>> class C:
...     def __str__(self): 1/0
...     
>>> sys.ps1 = C()
Traceback (most recent call last):
  File "<frozen runpy>", line 198, in _run_module_as_main
  File "<frozen runpy>", line 88, in _run_code
  File "/Users/guido/cpython/Lib/_pyrepl/__main__.py", line 6, in <module>
    __pyrepl_interactive_console()
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^
  File "/Users/guido/cpython/Lib/_pyrepl/main.py", line 59, in interactive_console
    run_multiline_interactive_console(console)
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^
  File "/Users/guido/cpython/Lib/_pyrepl/simple_interact.py", line 143, in run_multiline_interactive_console
    statement = multiline_input(more_lines, ps1, ps2)
  File "/Users/guido/cpython/Lib/_pyrepl/readline.py", line 389, in multiline_input
    return reader.readline()
           ~~~~~~~~~~~~~~~^^
  File "/Users/guido/cpython/Lib/_pyrepl/reader.py", line 796, in readline
    self.prepare()
    ~~~~~~~~~~~~^^
  File "/Users/guido/cpython/Lib/_pyrepl/historical_reader.py", line 306, in prepare
    super().prepare()
    ~~~~~~~~~~~~~~~^^
  File "/Users/guido/cpython/Lib/_pyrepl/reader.py", line 643, in prepare
    self.calc_screen()
    ~~~~~~~~~~~~~~~~^^
  File "/Users/guido/cpython/Lib/_pyrepl/completing_reader.py", line 261, in calc_screen
    screen = super().calc_screen()
  File "/Users/guido/cpython/Lib/_pyrepl/reader.py", line 371, in calc_screen
    prompt = self.get_prompt(ln, ll >= pos >= 0)
  File "/Users/guido/cpython/Lib/_pyrepl/historical_reader.py", line 326, in get_prompt
    return super().get_prompt(lineno, cursor_on_line)
           ~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/guido/cpython/Lib/_pyrepl/reader.py", line 552, in get_prompt
    prompt = f"{ANSIColors.BOLD_MAGENTA}{prompt}{ANSIColors.RESET}"
                                        ^^^^^^^^
  File "<python-input-1>", line 2, in __str__
    def __str__(self): 1/0
                       ~^~
ZeroDivisionError: division by zero
~/cpython$ 

Probably @pablogsal should look into this.

CPython versions tested on:

CPython main branch

Operating systems tested on:

macOS

Linked PRs

• gh-131110

Reproduce the failure with the interactive command shown in the report, then start in Lib/_pyrepl/reader.py and follow get_prompt through the related _pyrepl files named in the traceback. Check the linked PR gh-131110 before starting; done means an exception from str(sys.ps1) no longer exits the REPL with a traceback.