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Match CPython's str subscript TypeError message #8319

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3ad8e93
Match CPython's str subscript TypeError message
kim-jaedeok Jul 16, 2026
b5b36a8
Fold str's subscript TypeError into the shared index helper
kim-jaedeok Jul 28, 2026
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Fold str's subscript TypeError into the shared index helper 
The previous commit added `SequenceIndex::try_from_borrowed_object_with_err`
(a closure-taking function) so `str` could raise CPython's
"string indices must be integers, not ..." message. Per review, a whole
helper API plus a thin wrapper was overkill for a single caller.

Instead, inline that one error case into `try_from_borrowed_object`: when
`type_name` is "str", raise the str-specific message; every other type keeps
the shared sequence wording. `str._getitem` goes back to the plain `match`
over `SequenceIndex` with no duplicated index parsing. A bad slice key
(e.g. `'a'[::'x']`) still surfaces the slice-specific error, since only the
non-index/non-slice branch is customized.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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@kim-jaedeok @claude
kim-jaedeok and claude committed Jul 28, 2026
commit b5b36a822665f26a06d3e234c665e2a31f9146f6
8 changes: 1 addition & 7 deletions crates/vm/src/builtins/str.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -660,13 +660,7 @@ impl PyStr {
}

fn _getitem(&self, needle: &PyObject, vm: &VirtualMachine) -> PyResult {
let index = SequenceIndex::try_from_borrowed_object_with_err(vm, needle, || {
vm.new_type_error(format!(
"string indices must be integers, not '{}'",
needle.class()
))
})?;
let item = match index {
let item = match SequenceIndex::try_from_borrowed_object(vm, needle, "str")? {
SequenceIndex::Int(i) => self.getitem_by_index(vm, i)?.to_pyobject(vm),
SequenceIndex::Slice(slice) => self.getitem_by_slice(vm, slice)?.to_pyobject(vm),
};
Expand Down
32 changes: 12 additions & 20 deletions crates/vm/src/sliceable.rs
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Original file line number Diff line number Diff line change
@@ -1,7 +1,7 @@
// export through sliceable module, not slice.
use crate::{
PyObject, PyResult, VirtualMachine,
builtins::{PyBaseExceptionRef, int::PyInt, slice::PySlice},
builtins::{int::PyInt, slice::PySlice},
};
use core::ops::Range;
use malachite_bigint::BigInt;
Expand Down Expand Up @@ -262,24 +262,6 @@ impl SequenceIndex {
vm: &VirtualMachine,
obj: &PyObject,
type_name: &str,
) -> PyResult<Self> {
Self::try_from_borrowed_object_with_err(vm, obj, || {
vm.new_type_error(format!(
"{} indices must be integers or slices or classes that override __index__ operator, not '{}'",
type_name,
obj.class()
))
})
}

/// Like [`Self::try_from_borrowed_object`], but lets the caller supply the
/// `TypeError` raised for a non-index object. Used by types whose CPython
/// message differs from the shared sequence wording (e.g. `str`, whose
/// `unicode_subscript` says "string indices must be integers, not ...").
pub fn try_from_borrowed_object_with_err(
vm: &VirtualMachine,
obj: &PyObject,
err: impl FnOnce() -> PyBaseExceptionRef,
) -> PyResult<Self> {
if let Some(i) = obj.downcast_ref::<PyInt>() {
// TODO: number protocol
Expand All @@ -293,8 +275,18 @@ impl SequenceIndex {
i?.try_to_primitive(vm)
.map_err(|_| vm.new_index_error("cannot fit 'int' into an index-sized integer"))
.map(Self::Int)
} else if type_name == "str" {
// CPython's unicode_subscript raises a distinct message here.
Err(vm.new_type_error(format!(
"string indices must be integers, not '{}'",

@moreal moreal Aug 9, 2026

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It is trivial but there is a limit for the class name

https://github.com/python/cpython/blob/823f0323ee6ec1402088b73bce1a38473cac36dc/Objects/unicodeobject.c#L14645-L14648

obj.class()
)))
} else {
Err(err())
Err(vm.new_type_error(format!(
"{} indices must be integers or slices or classes that override __index__ operator, not '{}'",
type_name,
obj.class()
)))
}
}
}
Expand Down
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