From 05d9e9e8329e9813e942692f5afa5f75b1a99eda Mon Sep 17 00:00:00 2001
From: Eduardo Villalpando Mello <eduardovil@microsoft.com>
Date: Tue, 26 May 2026 10:14:15 -0700
Subject: [PATCH 1/2] gh-150462: Fix cursor position when using `\\b`

Fixes length miscalculation when dealing with \b character that caused the cursor to be placed in an incorrect position
---
 Lib/_pyrepl/utils.py                | 20 ++++++++++++++------
 Lib/test/test_pyrepl/test_reader.py |  6 ++++++
 Lib/test/test_pyrepl/test_utils.py  |  7 +++++++
 3 files changed, 27 insertions(+), 6 deletions(-)

diff --git a/Lib/_pyrepl/utils.py b/Lib/_pyrepl/utils.py
index b50426c31ead53a..da881a6051a5e66 100644
--- a/Lib/_pyrepl/utils.py
+++ b/Lib/_pyrepl/utils.py
@@ -93,13 +93,21 @@ def str_width(c: str) -> int:
 
 
 def wlen(s: str) -> int:
-    if len(s) == 1 and s != "\x1a":
+    if len(s) == 1 and s!= "\x1a":
         return str_width(s)
-    length = sum(str_width(i) for i in s)
-    # remove lengths of any escape sequences
-    sequence = ANSI_ESCAPE_SEQUENCE.findall(s)
-    ctrl_z_cnt = s.count("\x1a")
-    return length - sum(len(i) for i in sequence) + ctrl_z_cnt
+    length = 0
+    # Strip ANSI escapes first
+    cleaned = ANSI_ESCAPE_SEQUENCE.sub("", s)
+    for c in cleaned:
+        if c == "\b":
+            # Backspace decreases cursor position, and cannot move before column 0.
+            length = max(length - 1, 0)
+        elif c == "\x1a":
+            # Control-Z is treated as width 2 to account for the fact that it will be rendered as ^Z
+            length += 2
+        else:
+            length += str_width(c)
+    return length
 
 
 def unbracket(s: str, including_content: bool = False) -> str:
diff --git a/Lib/test/test_pyrepl/test_reader.py b/Lib/test/test_pyrepl/test_reader.py
index 0b32ead357c2c0d..01043ebcbd0c717 100644
--- a/Lib/test/test_pyrepl/test_reader.py
+++ b/Lib/test/test_pyrepl/test_reader.py
@@ -359,6 +359,12 @@ def test_prompt_length(self):
         self.assertEqual(prompt, "\033[0;32m樂>\033[0m> ")
         self.assertEqual(l, 5)
 
+        # Handles backspace in prompt
+        ps1 = "Question? _\b"
+        prompt, l = Reader.process_prompt(ps1)
+        self.assertEqual(prompt, ps1)
+        self.assertEqual(l, 10)
+
     def test_prepare_with_zero_width_does_not_crash(self):
         console = prepare_console([], width=0)
         reader = ReadlineAlikeReader(console=console, config=ReadlineConfig())
diff --git a/Lib/test/test_pyrepl/test_utils.py b/Lib/test/test_pyrepl/test_utils.py
index 3c55b6bdaeee9e8..637905c5e74ecc3 100644
--- a/Lib/test/test_pyrepl/test_utils.py
+++ b/Lib/test/test_pyrepl/test_utils.py
@@ -47,6 +47,13 @@ def test_wlen(self):
         self.assertEqual(wlen('e\N{COMBINING ACUTE ACCENT}'), 1)
         self.assertEqual(wlen('a\N{ZERO WIDTH JOINER}b'), 2)
 
+        # Backspace decreases cursor position, and cannot move before column 0.
+        self.assertEqual(wlen('Question? _\b'), 10)
+        self.assertEqual(wlen('ab\b'), 1)
+        self.assertEqual(wlen('ab\b\b'), 0)
+        self.assertEqual(wlen('\babc'), 3)
+        self.assertEqual(wlen('樂\b'), 1)
+
     def test_prev_next_window(self):
         def gen_normal():
             yield 1

From 6de4d52071fe9df5dd739d6acbea5b18432394d4 Mon Sep 17 00:00:00 2001
From: Eduardo Villalpando Mello <eduardovil@microsoft.com>
Date: Tue, 26 May 2026 11:48:37 -0700
Subject: [PATCH 2/2] Rename variable

---
 Lib/_pyrepl/utils.py | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/Lib/_pyrepl/utils.py b/Lib/_pyrepl/utils.py
index da881a6051a5e66..05bcae932cc8e9a 100644
--- a/Lib/_pyrepl/utils.py
+++ b/Lib/_pyrepl/utils.py
@@ -97,8 +97,8 @@ def wlen(s: str) -> int:
         return str_width(s)
     length = 0
     # Strip ANSI escapes first
-    cleaned = ANSI_ESCAPE_SEQUENCE.sub("", s)
-    for c in cleaned:
+    plain = ANSI_ESCAPE_SEQUENCE.sub("", s)
+    for c in plain:
         if c == "\b":
             # Backspace decreases cursor position, and cannot move before column 0.
             length = max(length - 1, 0)