gh-154467: Fix empty (Pdb) prompt when attaching to a process with pdb -p (#154469)

_PdbServer inherits _cmdloop, which wraps cmdloop() in
_maybe_use_pyrepl_as_stdin(). That context manager blanks self.prompt to ''
so that a local pyrepl draws the prompt itself. The remote server, however,
never reads from a local pyrepl -- it transmits self.prompt to the client
over the socket -- so the blanking made it send an empty prompt whenever the
target process had a pyrepl-capable terminal (pyrepl_input is set).
Override _maybe_use_pyrepl_as_stdin() in _PdbServer to a no-op, keeping the
real prompt. Add an integration test that attaches to a target running under
a pty, so PyREPL is genuinely enabled inside it, and asserts the transmitted
prompt is "(Pdb) ".
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: ambv

Lib/pdb.py

@@ -3158,6 +3158,15 @@ def postloop(self):
 ifself.quitting:
 self.detach()
 
+@contextmanager
+def_maybe_use_pyrepl_as_stdin(self):
+# The server reads every command from the client over the socket, never
+# from a local pyrepl. The base implementation swaps in pyrepl as stdin
+# and blanks `self.prompt` to '' (pyrepl would draw the prompt itself),
+# which here would transmit an empty prompt to the client whenever the
+# target process happens to be pyrepl-capable. Keep the real prompt.
+yield
+
 defdetach(self):
 # Detach the debugger and close the socket without raising BdbQuit
 self.quitting=False

Lib/test/test_remote_pdb.py

@@ -8,6 +8,7 @@
 importsubprocess
 importsys
 importtextwrap
+importthreading
 importunittest
 importunittest.mock
 fromcontextlibimportclosing, contextmanager, redirect_stdout, redirect_stderr, ExitStack
@@ -18,6 +19,11 @@
 importpdb
 frompdbimport_PdbServer, _PdbClient
 
+try:
+importpty
+exceptImportError:
+pty=None
+
 
 ifnotsys.is_remote_debug_enabled():
 raiseunittest.SkipTest('remote debugging is disabled')
@@ -1090,6 +1096,45 @@ def _connect_and_get_client_file(self):
 
 returnprocess, client_file
 
+def_connect_and_get_client_file_via_pty(self):
+"""Like _connect_and_get_client_file, but run the target under a pty.
+
+ With a real terminal on stdin/stdout, PyREPL is available *inside the
+ target process*, which is the condition that exercises pdb's PyREPL
+ input handling in the remote server.
+ """
+controller, worker=pty.openpty()
+self.addCleanup(os.close, controller)
+env=dict(os.environ, TERM="xterm-256color")
+env.pop("PYTHON_BASIC_REPL", None) # don't opt out of PyREPL
+process=subprocess.Popen(
+ [sys.executable, self.script_path],
+stdin=worker, stdout=worker, stderr=worker, env=env, close_fds=True,
+ )
+os.close(worker) # only the child keeps the worker end open
+
+# Continuously drain the terminal so the child never blocks on a write.
+drainer=threading.Thread(
+target=self._drain_until_eof, args=(controller,), daemon=True
+ )
+drainer.start()
+self.addCleanup(drainer.join, SHORT_TIMEOUT)
+
+client_sock, _=self.server_sock.accept()
+client_file=client_sock.makefile('rwb')
+self.addCleanup(client_file.close)
+self.addCleanup(client_sock.close)
+
+returnprocess, client_file
+
+@staticmethod
+def_drain_until_eof(fd):
+try:
+whileos.read(fd, 1024):
+pass
+exceptOSError:
+pass# controller closed, or the pty went away with the child
+
 def_read_until_prompt(self, client_file):
 """Helper to read messages until a prompt is received."""
 messages= []
@@ -1292,6 +1337,32 @@ def test_handle_eof(self):
 self.assertEqual(process.returncode, 0)
 self.assertEqual(stderr, "")
 
+@unittest.skipUnless(pty, "requires pty")
+deftest_prompt_with_interactive_terminal(self):
+"""The server must send "(Pdb) " even when the target owns a terminal.
+
+ The remote server transmits its prompt string to the client, which
+ displays it. When the target process has an interactive terminal,
+ PyREPL is enabled inside it; the base pdb machinery then blanks
+ ``self.prompt`` (a local PyREPL would draw the prompt itself). The
+ remote server has no local PyREPL -- it reads input from the socket --
+ so it must keep the real prompt rather than transmit an empty one.
+ Regression test for gh-154467, where attaching with ``pdb -p`` to a
+ process running at an interactive prompt showed a blank prompt.
+ """
+self._create_script()
+process, client_file=self._connect_and_get_client_file_via_pty()
+
+withkill_on_error(process):
+messages=self._read_until_prompt(client_file)
+# The message that ended the read is the prompt request.
+self.assertEqual(messages[-1], {"prompt": "(Pdb) ", "state": "pdb"})
+
+# Let the target run to completion so nothing is left attached.
+self._send_command(client_file, "c")
+process.wait(timeout=SHORT_TIMEOUT)
+self.assertEqual(process.returncode, 0)
+
 deftest_protocol_version(self):
 """Test that incompatible protocol versions are properly detected."""
 # Create a script using an incompatible protocol version

Misc/NEWS.d/next/Library/2026-07-22-12-59-26.gh-issue-154467.AA9w1D.rst

@@ -0,0 +1,3 @@
+Fixed :mod:`pdb` remote attaching (``python -m pdb -p PID``) sending an
+empty prompt to the client instead of ``(Pdb)`` when the target process has
+an interactive terminal.