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bpo-30040: new empty dict uses key-sharing dict (GH-1080) 

Sizeof new empty dict becomes 72 bytes from 240 bytes (amd64).
It is same size to empty dict created by dict.clear().
This commit is contained in:
Inada Naoki 2019-03-12 17:25:44 +09:00 committed by GitHub
parent fc06a192fd
commit f2a186712b
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4 changed files with 13 additions and 8 deletions
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6
Lib/test/test_descr.py
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@ -5346,16 +5346,16 @@ class SharedKeyTests(unittest.TestCase):
a, b = A(), B()
self.assertEqual(sys.getsizeof(vars(a)), sys.getsizeof(vars(b)))
self.assertLess(sys.getsizeof(vars(a)), sys.getsizeof({}))
self.assertLess(sys.getsizeof(vars(a)), sys.getsizeof({"a":1}))
# Initial hash table can contain at most 5 elements.
# Set 6 attributes to cause internal resizing.
a.x, a.y, a.z, a.w, a.v, a.u = range(6)
self.assertNotEqual(sys.getsizeof(vars(a)), sys.getsizeof(vars(b)))
a2 = A()
self.assertEqual(sys.getsizeof(vars(a)), sys.getsizeof(vars(a2)))
self.assertLess(sys.getsizeof(vars(a)), sys.getsizeof({}))
self.assertLess(sys.getsizeof(vars(a)), sys.getsizeof({"a":1}))
b.u, b.v, b.w, b.t, b.s, b.r = range(6)
self.assertLess(sys.getsizeof(vars(b)), sys.getsizeof({}))
self.assertLess(sys.getsizeof(vars(b)), sys.getsizeof({"a":1}))
class DebugHelperMeta(type):

4
Lib/test/test_sys.py
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@ -982,8 +982,10 @@ class SizeofTest(unittest.TestCase):
check(int.__add__, size('3P2P'))
# method-wrapper (descriptor object)
check({}.__iter__, size('2P'))
# empty dict
check({}, size('nQ2P'))
# dict
check({}, size('nQ2P') + calcsize('2nP2n') + 8 + (8*2//3)*calcsize('n2P'))
check({"a": 1}, size('nQ2P') + calcsize('2nP2n') + 8 + (8*2//3)*calcsize('n2P'))
longdict = {1:1, 2:2, 3:3, 4:4, 5:5, 6:6, 7:7, 8:8}
check(longdict, size('nQ2P') + calcsize('2nP2n') + 16 + (16*2//3)*calcsize('n2P'))
# dictionary-keyview

2
Misc/NEWS.d/next/Core and Builtins/2019-03-11-22-30-56.bpo-30040.W9z8X7.rst Normal file
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@ -0,0 +1,2 @@
New empty dict uses fewer memory for now. It used more memory than empty
dict created by ``dict.clear()``. Patch by Inada Naoki.

9
Objects/dictobject.c
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@ -691,10 +691,8 @@ clone_combined_dict(PyDictObject *orig)
PyObject *
PyDict_New(void)
{
PyDictKeysObject *keys = new_keys_object(PyDict_MINSIZE);
if (keys == NULL)
return NULL;
return new_dict(keys, NULL);
dictkeys_incref(Py_EMPTY_KEYS);
return new_dict(Py_EMPTY_KEYS, empty_values);
}
/* Search index of hash table from offset of entry table */
@ -1276,6 +1274,9 @@ _PyDict_NewPresized(Py_ssize_t minused)
Py_ssize_t newsize;
PyDictKeysObject *new_keys;
if (minused == 0) {
return PyDict_New();
}
/* There are no strict guarantee that returned dict can contain minused
* items without resize. So we create medium size dict instead of very
* large dict or MemoryError.