fix(stl_bind): correct __delitem__ for negative-step slices and re-enable contiguous erase fast path#6088
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…able contiguous erase fast path The slice __delitem__ binding advanced the erase index by step - 1 for all steps. That correction is only valid for positive steps, where erasing shifts later elements down by one. For negative steps the visited indices are strictly decreasing and erasing never shifts them, so the extra -1 deleted the wrong elements (e.g. del v[::-2] on [0,1,2,3] yielded [1,2] instead of [0,2]) and del v[::-1] walked off the front of the vector (v.begin() - 1, observed SIGBUS). Switch to the signed slice::compute overload so negative steps stay signed, advance by step for negative steps and step - 1 for positive ones, and drop the && false that had disabled the O(n) contiguous fast path since 2016. Assisted-by: ClaudeCode:claude-fable-5
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Skylion007
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Jun 12, 2026
| ssize_t start = 0, stop = 0, step = 0, slicelength = 0; | ||
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| if (!slice.compute(v.size(), &start, &stop, &step, &slicelength)) { | ||
| if (!slice.compute((ssize_t) v.size(), &start, &stop, &step, &slicelength)) { |
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Could we use Cpp style static cast?
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Done — switched to static_cast<ssize_t>(v.size()) in e5c1c6d.
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Ahh, it doesn't ask me sometimes. Answer is yes. :)
| assert len(v_int2) == 0 | ||
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| def test_vector_delitem_slice(): |
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Why not invert the for loop and just pytest parameterize?
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Done — the test is now parametrized over the slice cases with the size loop inside, in e5c1c6d.
Use static_cast instead of a C-style cast for the slice.compute() size argument, and convert the __delitem__ slice test to pytest.mark.parametrize over the slice cases. Assisted-by: ClaudeCode:claude-fable-5
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Bug
bind_vector's__delitem__(slice)instl_bind.hadvanced the eraseindex by
step - 1for every slice, but that correction is only validfor positive steps (where erasing an element shifts the later
elements down by one). For negative steps the visited indices are
strictly decreasing, so erasing never shifts them and the
- 1iswrong:
del v[::-2]on[0, 1, 2, 3]produced[1, 2]instead of thecorrect
[0, 2](silent data corruption).del v[::-1]ended up callingv.erase(v.begin() - 1), anout-of-bounds access (observed SIGBUS).
Separately, the contiguous fast path was guarded by
step == 1 && false— a debugging artifact from 2016 (25c03ce) — so a contiguous delete
like
del v[1:1000]did 999 individualerasecalls instead of onerange erase.
Fix
slice::computeoverload so negative steps stay signedinstead of arriving as wrapped
size_tvalues.stepfor negative steps andstep - 1for positivesteps.
&& false, restoring the O(n) contiguous range erase forstep == 1.Semantics now match CPython
del l[slice]exactly for forward,strided, negative, empty, and full slices.
Tests
Added
test_vector_delitem_sliceintests/test_stl_binders.py,looping over forward / strided / negative-step / empty / full slices for
vectors of several lengths and comparing against the equivalent Python
listdel.Verification
Built a scratch opaque
VectorIntmodule against this branch's headersand compared
del v[...]to Pythonlistsemantics across all the slicecases above (and more) for lengths 0–11 — all match, no out-of-bounds
access.
Part of #6084