gh-138860: Lazy import rlcompleter in pdb to avoid deadlock in subprocess

Lib/pdb.py

@@ -100,7 +100,6 @@
 import _pyrepl.utils
 
 from contextlib import ExitStack, closing, contextmanager
-from rlcompleter import Completer
 from types import CodeType
 from warnings import deprecated
 
@@ -364,6 +363,15 @@ def __init__(self, completekey='tab', stdin=None, stdout=None, skip=None,
             readline.set_completer_delims(' \t\n`@#%^&*()=+[{]}\\|;:\'",<>?')
         except ImportError:
             pass
+
+        # GH-138860
+        # We need to lazy-import rlcompleter to avoid deadlock
+        # We cannot import it during self.complete* methods because importing
+        # rlcompleter for the first time will overwrite readline's completer
+        # So we import it here and save the Completer class
+        from rlcompleter import Completer
+        self.RlCompleter = Completer
+
         self.allow_kbdint = False
         self.nosigint = nosigint
         # Consider these characters as part of the command so when the users type
@@ -1186,10 +1194,9 @@ def completedefault(self, text, line, begidx, endidx):
             conv_vars = self.curframe.f_globals.get('__pdb_convenience_variables', {})
             return [f"${name}" for name in conv_vars if name.startswith(text[1:])]
 
-        # Use rlcompleter to do the completion
         state = 0
         matches = []
-        completer = Completer(self.curframe.f_globals | self.curframe.f_locals)
+        completer = self.RlCompleter(self.curframe.f_globals | self.curframe.f_locals)
         while (match := completer.complete(text, state)) is not None:
             matches.append(match)
             state += 1
@@ -1204,8 +1211,8 @@ def _enable_rlcompleter(self, ns):
             return
 
         try:
+            completer = self.RlCompleter(ns)
             old_completer = readline.get_completer()
-            completer = Completer(ns)
             readline.set_completer(completer.complete)
             yield
         finally:

Lib/test/test_pdb.py

@@ -4688,6 +4688,28 @@ def foo():
         stdout, _ = self._run_script(script, commands)
         self.assertIn("42", stdout)
 
+    def test_readline_not_imported(self):
+        """GH-138860
+        Directly or indirectly importing readline might deadlock a subprocess
+        if it's launched with process_group=0 or preexec_fn=setpgrp
+
+        It's also a pattern that readline is never imported with just import pdb.
+
+        This test is to ensure that readline is not imported for import pdb.
+        It's possible that we have a good reason to do that in the future.
+        """
+
+        script = textwrap.dedent("""
+            import sys
+            import pdb
+            if "readline" in sys.modules:
+                print("readline imported")
+        """)
+        commands = ""
+        stdout, stderr = self._run_script(script, commands)
+        self.assertNotIn("readline imported", stdout)
+        self.assertEqual(stderr, "")
+
 
 @support.force_colorized_test_class
 class PdbTestColorize(unittest.TestCase):

Misc/NEWS.d/next/Library/2025-09-20-17-50-31.gh-issue-138860.Y9JXap.rst

@@ -0,0 +1 @@
+Lazy import :mod:`rlcompleter` in :mod:`pdb` to avoid deadlock in subprocess.