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list() allocated its result with PyType_GenericAlloc(), bypassing the freelist that PyList_New() and the [] display use. For an exact list, allocate with PyList_New(0) instead. The object handed to list.__init__() is in the same state either way: empty, with no item array, and tracked by the GC. Subclasses still use PyType_GenericAlloc(). pyperf, PGO+LTO, base 19270d3, two interleaved rounds per side (a third round, disturbed by machine load, was dropped): | Benchmark | main | branch | |----------------|:-------:|:---------------------:| | list() | 21.2 ns | 15.3 ns: 1.38x faster | | list(d) | 36.1 ns | 27.4 ns: 1.32x faster | | list(t) | 30.3 ns | 23.9 ns: 1.27x faster | | list(l) | 32.0 ns | 25.7 ns: 1.24x faster | | list(r) | 43.6 ns | 36.0 ns: 1.21x faster | | x + 1 | 10.4 ns | 9.70 ns: 1.07x faster | | len(l) | 9.87 ns | 9.53 ns: 1.03x faster | | l[0] | 8.24 ns | 8.00 ns: 1.03x faster | | L(t) | 48.2 ns | 47.4 ns: 1.02x faster | | tuple(l) | 15.4 ns | 16.0 ns: 1.04x slower | | Geometric mean | (ref) | 1.13x faster | Not significant: []. x + 1, len(l), l[0], L(t) and tuple(l) don't run the changed code; their movement is layout noise, within the up-to-1.07x spread between two runs of the same main build. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
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cmaloney
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Oct 29, 2026
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| PyObject *list = PyType_GenericAlloc(_PyType_CAST(type), 0); | ||
| /* use PyList_New() to make exact lists from the freelist. */ | ||
| PyObject *list = (type == (PyObject *)&PyList_Type |
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This should always be true because tp_vectorcall is not inherited but with the handwritten vector call, it isn't asserted in debug builds
corona10
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Oct 29, 2026
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| Speed up :class:`list` construction by reusing list objects from the free | ||
| list, as ``[]`` already does. |
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Does not need to be known to end user.
methane
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Oct 29, 2026
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list() allocated its result with PyType_GenericAlloc(), bypassing the freelist that PyList_New() and the [] display use. For an exact list, allocate with PyList_New(0) instead. The object handed to list.init() is in the same state either way: empty, with no item array, and tracked by the GC. Subclasses still use PyType_GenericAlloc().
pyperf, PGO+LTO, base 19270d3 :
Not significant: []. x + 1, len(l), l[0], L(t) and tuple(l) don't run the changed code; their movement is layout noise, within the up-to-1.07x spread between two runs of the same main build.
list()to use the existing freelist #160858