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gh-150462: Fix cursor position when using \b #150488

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gh-150462: Fix cursor position when using \b #150488

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05d9e9e
gh-150462: Fix cursor position when using `\\b`
edvilme May 26, 2026
6de4d52
Rename variable
edvilme May 26, 2026
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20 changes: 14 additions & 6 deletions Lib/_pyrepl/utils.py
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Original file line number Diff line number Diff line change
Expand Up @@ -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
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)
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:
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6 changes: 6 additions & 0 deletions Lib/test/test_pyrepl/test_reader.py
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Expand Up @@ -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())
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7 changes: 7 additions & 0 deletions Lib/test/test_pyrepl/test_utils.py
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Expand Up @@ -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
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