gh-130698: Add safe methods to get prompts for new REPL

Lib/_pyrepl/reader.py

@@ -29,6 +29,7 @@
 
 
 from . import commands, console, input
+from .utils import DEFAULT_PS1, DEFAULT_PS2, DEFAULT_PS3, DEFAULT_PS4
 from .utils import wlen, unbracket, disp_str
 from .trace import trace
 
@@ -474,22 +475,40 @@ def get_arg(self, default: int = 1) -> int:
             return default
         return self.arg
 
+    @staticmethod
+    def __get_prompt_str(prompt: object, default_prompt: str) -> str:
+        """
+        Convert prompt object to string.
+
+        If str(prompt) raises BaseException, MemoryError or SystemError then stop
+        the REPL. For other exceptions return default_prompt.
+        """
+        try:
+            return str(prompt)
+        except (MemoryError, SystemError):
+            raise
+        except Exception:
+            return default_prompt
+
     def get_prompt(self, lineno: int, cursor_on_line: bool) -> str:
         """Return what should be in the left-hand margin for line
         'lineno'."""
         if self.arg is not None and cursor_on_line:
-            prompt = f"(arg: {self.arg}) "
+            prompt = DEFAULT_PS1
+            arg = self.__get_prompt_str(self.arg, "")
+            if arg:
+                prompt = f"(arg: {self.arg}) "
         elif self.paste_mode and not self.in_bracketed_paste:
             prompt = "(paste) "
         elif "\n" in self.buffer:
             if lineno == 0:
-                prompt = self.ps2
+                prompt = self.__get_prompt_str(self.ps2, DEFAULT_PS2)
             elif self.ps4 and lineno == self.buffer.count("\n"):
-                prompt = self.ps4
+                prompt = self.__get_prompt_str(self.ps4, DEFAULT_PS4)
             else:
-                prompt = self.ps3
+                prompt = self.__get_prompt_str(self.ps3, DEFAULT_PS3)
         else:
-            prompt = self.ps1
+            prompt = self.__get_prompt_str(self.ps1, DEFAULT_PS1)
 
         if self.can_colorize:
             prompt = f"{ANSIColors.BOLD_MAGENTA}{prompt}{ANSIColors.RESET}"

Lib/_pyrepl/utils.py

@@ -10,6 +10,12 @@
 ZERO_WIDTH_TRANS = str.maketrans({"\x01": "", "\x02": ""})
 
 
+DEFAULT_PS1 = ">>> "
+DEFAULT_PS2 = ">>> "
+DEFAULT_PS3 = "... "
+DEFAULT_PS4 = "... "
+
+
 @functools.cache
 def str_width(c: str) -> int:
     if ord(c) < 128:

Lib/test/test_pyrepl/test_reader.py

@@ -9,6 +9,24 @@
 from .support import prepare_reader, prepare_console
 from _pyrepl.console import Event
 from _pyrepl.reader import Reader
+from _pyrepl.utils import DEFAULT_PS1, DEFAULT_PS2, DEFAULT_PS3, DEFAULT_PS4
+
+
+def prepare_reader_with_prompt(
+    console, ps1=DEFAULT_PS1, ps2=DEFAULT_PS2, ps3=DEFAULT_PS3, ps4=DEFAULT_PS4):
+    reader = prepare_reader(
+        console,
+        can_colorize=False,
+        paste_mode=False,
+        ps1=ps1,
+        ps2=ps2,
+        ps3=ps3,
+        ps4=ps4
+    )
+
+    # we should use original get_prompt from reader to get exceptions
+    del reader.get_prompt
+    return reader
 
 
 class TestReader(ScreenEqualMixin, TestCase):
@@ -298,6 +316,89 @@ def test_prompt_length(self):
         self.assertEqual(prompt, "\033[0;32m樂>\033[0m> ")
         self.assertEqual(l, 5)
 
+    def test_prompt_ps1_raise_exception(self):
+        # Handles exceptions from ps1 prompt
+        class Prompt:
+            def __str__(self): 1/0
+
+        _prepare_reader = functools.partial(
+            prepare_reader_with_prompt,
+            ps1=Prompt(),
+        )
+
+        reader, _ = handle_all_events(
+            events=code_to_events("a=1"),
+            prepare_reader=_prepare_reader
+        )
+
+        prompt = reader.get_prompt(0, False)
+        self.assertEqual(prompt, DEFAULT_PS1)
+
+    def test_prompt_ps2_ps3_ps4_raise_exception(self):
+        # Handles exceptions from ps2, ps3 and ps4 prompts
+        class Prompt:
+            def __str__(self): 1/0
+
+        _prepare_reader = functools.partial(
+            prepare_reader_with_prompt,
+            ps1=Prompt(),
+            ps2=Prompt(),
+            ps3=Prompt(),
+            ps4=Prompt(),
+        )
+
+        reader, _ = handle_all_events(
+            events=code_to_events("if cond:\nfunc()\nfunc()"),
+            prepare_reader=_prepare_reader
+        )
+
+        prompt = reader.get_prompt(0, False)
+        self.assertEqual(prompt, DEFAULT_PS2)
+
+        prompt = reader.get_prompt(1, False)
+        self.assertEqual(prompt, DEFAULT_PS3)
+
+        prompt = reader.get_prompt(2, False)
+        self.assertEqual(prompt, DEFAULT_PS4)
+
+    def test_prompt_arg_raise_exception(self):
+        # Handles exceptions from arg prompt
+        class Prompt:
+            def __str__(self): 1/0
+            def __rmul__(self, b): return b
+
+        reader, _ = handle_all_events(
+            events=code_to_events("if some_condition:\nsome_function()"),
+            prepare_reader=prepare_reader_with_prompt,
+        )
+
+        reader.arg = Prompt()
+        prompt = reader.get_prompt(0, True)
+        self.assertEqual(prompt, DEFAULT_PS1)
+
+    def test_prompt_raise_exception(self):
+        # Tests unrecoverable exceptions from prompts
+        cases = [
+            (MemoryError, "No memory for prompt"),
+            (SystemError, "System error for prompt"),
+        ]
+        for cls, msg in cases:
+            with self.subTest(msg):
+
+                class Prompt:
+                    def __str__(self): raise cls(msg)
+
+                _prepare_reader = functools.partial(
+                    prepare_reader_with_prompt,
+                    ps1=Prompt(),
+                )
+
+                with self.assertRaisesRegex(cls, msg):
+                    handle_events_narrow_console(
+                        events=code_to_events("a=1"),
+                        prepare_reader=_prepare_reader,
+                    )
+
     def test_completions_updated_on_key_press(self):
         namespace = {"itertools": itertools}
         code = "itertools."

Misc/NEWS.d/next/Library/2025-03-13-00-39-54.gh-issue-130698.o2aU3e.rst

@@ -0,0 +1 @@
+Avoid exiting the new REPL when prompt object raises an exception.