python
@@ -785,6 +785,10 @@ pdb
   (if any).
   (Contributed by Tian Gao in :gh:`130493`.)
 
+* ``<tab>`` at the beginning of the line in :mod:`pdb` multi-line input will
+  fill in a 4-space indentation now, instead of inserting a ``\t`` character.
+  (Contributed by Tian Gao in :gh:`130471`.)
+
 
 pickle
 ------

@@ -652,10 +652,10 @@ def displayhook(self, obj):
             self.message(repr(obj))
 
     @contextmanager
-    def _disable_command_completion(self):
+    def _enable_multiline_completion(self):
         completenames = self.completenames
         try:
-            self.completenames = self.completedefault
+            self.completenames = self.complete_multiline_names
             yield
         finally:
             self.completenames = completenames
@@ -753,7 +753,7 @@ def default(self, line):
             buffer = line
             if (code := codeop.compile_command(line + '\n', '<stdin>', 'single')) is None:
                 # Multi-line mode
-                with self._disable_command_completion():
+                with self._enable_multiline_completion():
                     buffer = line
                     continue_prompt = "...   "
                     while (code := codeop.compile_command(buffer, '<stdin>', 'single')) is None:
@@ -996,6 +996,21 @@ def _complete_expression(self, text, line, begidx, endidx):
             # Complete a simple name.
             return [n for n in ns.keys() if n.startswith(text)]
 
+    def _complete_indentation(self, text, line, begidx, endidx):
+        try:
+            import readline
+        except ImportError:
+            return []
+        # Fill in spaces to form a 4-space indent
+        return [' ' * (4 - readline.get_begidx() % 4)]
+
+    def complete_multiline_names(self, text, line, begidx, endidx):
+        # If text is space-only, the user entered <tab> before any text.
+        # That normally means they want to indent the current line.
+        if not text.strip():
+            return self._complete_indentation(text, line, begidx, endidx)
+        return self.completedefault(text, line, begidx, endidx)
+
     def completedefault(self, text, line, begidx, endidx):
         if text.startswith("$"):
             # Complete convenience variables

@@ -4487,6 +4487,32 @@ def test_multiline_completion(self):
 
         self.assertIn(b'42', output)
 
+    def test_multiline_indent_completion(self):
+        script = textwrap.dedent("""
+            import pdb; pdb.Pdb().set_trace()
+        """)
+
+        # \t should always complete a 4-space indent
+        # This piece of code will raise an IndentationError or a SyntaxError
+        # if the completion is not working as expected
+        input = textwrap.dedent("""\
+            def func():
+            \ta = 1
+             \ta += 1
+              \ta += 1
+               \tif a > 0:
+                    a += 1
+            \t\treturn a
+
+            func()
+            c
+        """).encode()
+
+        output = run_pty(script, input)
+
+        self.assertIn(b'4', output)
+        self.assertNotIn(b'Error', output)
+
 
 def load_tests(loader, tests, pattern):
     from test import test_pdb

diff --git a/Misc/NEWS.d/next/Library/2025-02-22-19-44-00.gh-issue-125377.LFTK0H.rst b/Misc/NEWS.d/next/Library/2025-02-22-19-44-00.gh-issue-125377.LFTK0H.rst
@@ -0,0 +1 @@
+``<tab>`` at the beginning of the line in :mod:`pdb` multi-line input will fill in a 4-space indentation now, instead of inserting a ``\t`` character.
Original file line number	Diff line number	Diff line change
		@@ -0,0 +1 @@
		``<tab>`` at the beginning of the line in :mod:`pdb` multi-line input will fill in a 4-space indentation now, instead of inserting a ``\t`` character.
iritkatriel Mar 3, 2025 Copy link Copy Markdown Member Choose a reason for hiding this comment The reason will be displayed to describe this comment to others. Learn more. Choose a reason Spam Abuse Off Topic Outdated Duplicate Resolved Low Quality I'd add this to what's new in 3.14. All reactions gaogaotiantian Mar 4, 2025 Copy link Copy Markdown Member Author Choose a reason for hiding this comment The reason will be displayed to describe this comment to others. Learn more. Choose a reason Spam Abuse Off Topic Outdated Duplicate Resolved Low Quality Done. All reactions