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gh-139476: Optimize range[:] slice #139556

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gh-139476: Optimize range[:] slice #139556

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StanFromIreland Oct 4, 2025
466e293
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StanFromIreland Oct 4, 2025
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5 changes: 4 additions & 1 deletion Lib/test/test_range.py
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Original file line number Diff line number Diff line change
Expand Up @@ -584,11 +584,14 @@ def check(start, stop, step=None):
check(0, -1)
check(-1, -3, -1)

for test in (range(5), range(0), range(1, 10, 2), range(10, 0, -1)):
for test in (range(5), range(0), range(1, 10, 2), range(10, 0, -1), range(-1, 200, 1)):
with self.subTest(test=test):
self.assertIs(test, test[:])
self.assertIs(test, test[:len(test):])
self.assertIs(test, test[0:len(test):1])

if len(test) > 1:
self.assertIsNot(test, test[1:len(test):2])
self.assertIsNot(test, test[1:])
self.assertIsNot(test, test[:-1])

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4 changes: 2 additions & 2 deletions Misc/NEWS.d/next/Core_and_Builtins/2025-10-04-14-06-40.gh-issue-139476.02iIfn.rst
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@@ -1,2 +1,2 @@
:func:`range` slicing with ``[:]`` no longer creates a copy, it now returns
the same object, consistent with :func:`copy.copy`.
:func:`range` slicing with ``[:]`` and ``[:len(r):]`` no longer creates a copy,
it now returns the same object, consistent with :func:`copy.copy`.
11 changes: 7 additions & 4 deletions Objects/rangeobject.c
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Original file line number Diff line number Diff line change
Expand Up @@ -409,14 +409,17 @@ compute_slice(rangeobject *r, PyObject *_slice)
PyObject *substart = NULL, *substop = NULL, *substep = NULL;
int error;

if (slice->start == Py_None && slice->stop == Py_None && slice->step == Py_None) {
return Py_NewRef(r);
}

error = _PySlice_GetLongIndices(slice, r->length, &start, &stop, &step);
if (error == -1)
return NULL;

if (start == _PyLong_GetZero()
&& step == _PyLong_GetOne()
&& (slice->stop == Py_None || PyObject_RichCompareBool(stop, r->length, Py_EQ) == 1))
{
return Py_NewRef(r);
}
Comment on lines +416 to +421
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@picnixz picnixz Oct 5, 2025 •
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PyObject_RichCompareBool can raise an error. Handling it would then make the code more complex. So I'm rather against that change as it's not so trivial.

Also, start == _PyLong_GetZero() is not correct because it's not guaranteed that 0 will always be immortal (though it should in general and I see that there is code that is doing this for 1). I actually thought that we had the indices as C integers and not Python ones, but it appears that it's not the case.


substep = PyNumber_Multiply(r->step, step);
if (substep == NULL) goto fail;
Py_CLEAR(step);
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