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bpo-33234 Improve list() pre-sizing for inputs with known lengths (no __length_hint__) #9846

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Merged
pablogsal merged 10 commits into from
Oct 28, 2018
Merged

bpo-33234 Improve list() pre-sizing for inputs with known lengths (no __length_hint__) #9846

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7602879
Improve list() pre-sizing for inputs with known lengths
pablogsal Apr 17, 2018
39a8757
Preallocate only when __len__ is available to avoid loss of performan…
pablogsal Oct 13, 2018
ab450b3
Fix NEWS entry and correct style
pablogsal Oct 15, 2018
717ca67
Use _PyObject_HasLen to check if the object has lenght to avoid overhead
pablogsal Oct 23, 2018
b678442
Simplify implementation and don't capture all expections on error
pablogsal Oct 23, 2018
b40bd62
Add assert and fix formatting
pablogsal Oct 25, 2018
3837486
Remove uneeded test modification
pablogsal Oct 25, 2018
b6ff7ce
Don't preallocate if list is modified when calling PyObject_Size
pablogsal Oct 26, 2018
e7f8c46
Improve readability in the error handling branch
pablogsal Oct 26, 2018
0a6c8ba
Capture only TypeError as suggested by Serhiy
pablogsal Oct 28, 2018
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Fix NEWS entry and correct style
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@pablogsal
pablogsal committed Oct 15, 2018
commit ab450b3d5a453f36a4988d83b391a79bfbfa5a61
4 changes: 2 additions & 2 deletions Misc/NEWS.d/next/Core and Builtins/2018-04-17-01-24-51.bpo-33234.l9IDtp.rst
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@@ -1,2 +1,2 @@
The list constructor will pre-size and not over-allocate when the input size
is known or can be reasonably estimated.
The list constructor will pre-size and not over-allocate when
the input lenght is known.
2 changes: 1 addition & 1 deletion Objects/listobject.c
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Expand Up @@ -2676,7 +2676,7 @@ list___init___impl(PyListObject *self, PyObject *iterable)
}
if (iterable != NULL) {
Py_ssize_t iter_len = PyObject_Length(iterable);
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@vstinner vstinner Oct 23, 2018

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To reduce or avoid the overhead of the worst case, you can avoid this call if _PyObject_HasLen() is false.

This change doesn't require to implement a new _PyObject_ProbeLength().

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@pablogsal pablogsal Oct 23, 2018

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Updated run of first experiment using _PyObject_HasLen():

Mean +- std dev: [old] 795 ns +- 25 ns -> [new] 776 ns +- 25 ns: 1.02x faster (-6%)

if (iter_len == -1){
if (iter_len == -1) {
PyErr_Clear();
}
if (iter_len > 0 && list_preallocate_exact(self, iter_len)) {
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