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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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9 changes: 9 additions & 0 deletions Lib/test/test_list.py
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Original file line number Diff line number Diff line change
@@ -1,5 +1,6 @@
import sys
from test import list_tests
from test.support import cpython_only
import pickle
import unittest

Expand Down Expand Up @@ -157,5 +158,13 @@ class L(list): pass
with self.assertRaises(TypeError):
(3,) + L([1,2])

@cpython_only
def test_preallocation(self):
iterable = [0] * 10
iter_size = sys.getsizeof(iterable)

self.assertEqual(iter_size, sys.getsizeof(list([0] * 10)))
self.assertEqual(iter_size, sys.getsizeof(list(range(10))))

if __name__ == "__main__":
unittest.main()
2 changes: 2 additions & 0 deletions Misc/NEWS.d/next/Core and Builtins/2018-04-17-01-24-51.bpo-33234.l9IDtp.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1,2 @@
The list constructor will pre-size and not over-allocate when
the input lenght is known.
41 changes: 41 additions & 0 deletions Objects/listobject.c
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Original file line number Diff line number Diff line change
Expand Up @@ -76,6 +76,33 @@ list_resize(PyListObject *self, Py_ssize_t newsize)
return 0;
}

static int
list_preallocate_exact(PyListObject *self, Py_ssize_t size)
{
assert(self->ob_item == NULL);

PyObject **items;
size_t allocated;

allocated = (size_t)size;
if (allocated > (size_t)PY_SSIZE_T_MAX / sizeof(PyObject *)) {
PyErr_NoMemory();
return -1;
}

if (size == 0) {
allocated = 0;
}
items = (PyObject **)PyMem_New(PyObject*, allocated);
if (items == NULL) {
PyErr_NoMemory();
return -1;
}
self->ob_item = items;
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self->allocated = allocated;
return 0;
}

/* Debug statistic to compare allocations with reuse through the free list */
#undef SHOW_ALLOC_COUNT
#ifdef SHOW_ALLOC_COUNT
Expand Down Expand Up @@ -2649,6 +2676,20 @@ list___init___impl(PyListObject *self, PyObject *iterable)
(void)_list_clear(self);
}
if (iterable != NULL) {
if (_PyObject_HasLen(iterable) && self->ob_item == NULL) {
Py_ssize_t iter_len = PyObject_Size(iterable);
if (iter_len == -1) {
if (PyErr_ExceptionMatches(PyExc_Exception)) {

@serhiy-storchaka serhiy-storchaka Oct 26, 2018

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What tests will fail if not silence any exceptions at all?

@pablogsal pablogsal Oct 26, 2018 •
edited
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At least the following:

test_generators failed
**********************************************************************
File "/home/pablogsal/github/cpython/Lib/test/test_generators.py", line ?, in test.test_generators.__test__.weakref
Failed example:
    list(p)
Exception raised:
    Traceback (most recent call last):
      File "/home/pablogsal/github/cpython/Lib/doctest.py", line 1329, in __run
        compileflags, 1), test.globs)
      File "<doctest test.test_generators.__test__.weakref[9]>", line 1, in <module>
        list(p)
    TypeError: object of type 'generator' has no len()
**********************************************************************
1 items had failures:
   1 of  10 in test.test_generators.__test__.weakref
***Test Failed*** 1 failures.


test_genexps failed
**********************************************************************
File "/home/pablogsal/github/cpython/Lib/test/test_genexps.py", line ?, in test.test_genexps.__test__.doctests
Failed example:
    list(p)
Exception raised:
    Traceback (most recent call last):
      File "/home/pablogsal/github/cpython/Lib/doctest.py", line 1329, in __run
        compileflags, 1), test.globs)
      File "<doctest test.test_genexps.__test__.doctests[75]>", line 1, in <module>
        list(p)
    TypeError: object of type 'generator' has no len()
**********************************************************************
1 items had failures:
   1 of  76 in test.test_genexps.__test__.doctests
***Test Failed*** 1 failures.

@serhiy-storchaka serhiy-storchaka Oct 27, 2018

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How can _PyObject_HasLen() return true for generators?

@serhiy-storchaka serhiy-storchaka Oct 27, 2018

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Ah, it is not true generator, it is a weakref. What if silence just TypeError?

@pablogsal pablogsal Oct 27, 2018

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I am a bit afraid if someone has similar tests/usage like these two cases and then they will receive new errors as raising all exceptions would be technically a backwards incompatible change. But if you think we should raise (maybe ignoring TypeError) I am happy to change the implementation.

@serhiy-storchaka serhiy-storchaka Oct 28, 2018

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See PyObject_LengthHint().

@pablogsal pablogsal Oct 28, 2018

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Done in 0a6c8ba

PyErr_Clear();
}
else {

@vstinner vstinner Oct 26, 2018

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nitpick: for readability, I suggest to invert the if condition and exchange the true/false blocks, and move PyErr_Clear() after the if:

if (!not exception) { return -1; }
PyErr_Clear();

return -1;
}
}
if (iter_len > 0 && list_preallocate_exact(self, iter_len)) {
return -1;
}
}
PyObject *rv = list_extend(self, iterable);
if (rv == NULL)
return -1;
Expand Down