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Reject format-string field index above Py_ssize_t::MAX #7708

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Merged
youknowone merged 1 commit into from
Apr 28, 2026
Merged

Reject format-string field index above Py_ssize_t::MAX #7708

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1 change: 0 additions & 1 deletion Lib/test/test_str.py
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Original file line number Diff line number Diff line change
Expand Up @@ -1469,7 +1469,6 @@ def test_format_huge_width(self):
with self.assertRaises(ValueError):
result = format(2.34, format_string)

@unittest.expectedFailure # TODO: RUSTPYTHON; IndexError: tuple index out of range
def test_format_huge_item_number(self):
format_string = "{{{}:.6f}}".format(sys.maxsize + 1)
with self.assertRaises(ValueError):
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13 changes: 13 additions & 0 deletions crates/common/src/format.rs
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Expand Up @@ -1102,6 +1102,7 @@ pub enum FormatParseError {
EmptyAttribute,
MissingRightBracket,
InvalidCharacterAfterRightBracket,
TooManyDecimalDigits,
}

impl FromStr for FormatSpec {
Expand Down Expand Up @@ -1185,7 +1186,19 @@ impl FieldName {
let field_type = if first.is_empty() {
FieldType::Auto
} else if let Some(index) = parse_usize(&first) {
// Match CPython's get_integer: digits-only segment must fit in
// Py_ssize_t. Above that, raise ValueError at parse time.
if index > isize::MAX as usize {
return Err(FormatParseError::TooManyDecimalDigits);
}
FieldType::Index(index)
} else if first
.as_str()
.ok()
.is_some_and(|s| s.bytes().all(|b| b.is_ascii_digit()))
{
// All-digit segment whose value overflows usize itself.
return Err(FormatParseError::TooManyDecimalDigits);
} else {
FieldType::Keyword(first)
};
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3 changes: 3 additions & 0 deletions crates/vm/src/format.rs
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Expand Up @@ -114,6 +114,9 @@ impl ToPyException for FormatParseError {
fn to_pyexception(&self, vm: &VirtualMachine) -> PyBaseExceptionRef {
match self {
Self::UnmatchedBracket => vm.new_value_error("expected '}' before end of string"),
Self::TooManyDecimalDigits => {
vm.new_value_error("Too many decimal digits in format string")
}
_ => vm.new_value_error("Unexpected error parsing format string"),
}
}
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