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bpo-36650: Fix handling of empty keyword args in C version of lru_cache. #12881

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Merged
rhettinger merged 4 commits into from
Apr 20, 2019
Merged

bpo-36650: Fix handling of empty keyword args in C version of lru_cache. #12881

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3c83439
bpo-36650: Fix handling of empty keyword args in C version of lru_cache.
rhettinger Apr 20, 2019
d08a9a6
Fix reST markup
rhettinger Apr 20, 2019
a36d107
Move up definition of kwds_size so that it can be reused
rhettinger Apr 20, 2019
6bfc053
Apply suggestions from PR reviewers
rhettinger Apr 20, 2019
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14 changes: 14 additions & 0 deletions Lib/test/test_functools.py
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Original file line number Diff line number Diff line change
Expand Up @@ -1271,6 +1271,20 @@ def f(x):
self.assertEqual(f(20), '.20.')
self.assertEqual(f.cache_info().currsize, 10)

def test_lru_bug_36650(self):
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# C version of lru_cache was treating a call with an empty **kwargs
# dictionary as being distinct from a call with no keywords at all.
# This did not result in an incorrect answer, but it did trigger
# an unexpected cache miss.

@self.module.lru_cache()
def f(x):
pass

f(0)
f(0, **{})
self.assertEqual(f.cache_info().hits, 1)

def test_lru_hash_only_once(self):
# To protect against weird reentrancy bugs and to improve
# efficiency when faced with slow __hash__ methods, the
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4 changes: 4 additions & 0 deletions Misc/NEWS.d/next/Library/2019-04-19-15-29-55.bpo-36650._EVdrz.rst
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@@ -0,0 +1,4 @@
The C version of functools.lru_cache() was treating calls with an empty
``**kwargs`` dictionary as being distinct from calls with no keywords at all.
This did not result in an incorrect answer, but it did trigger an unexpected
cache miss.
7 changes: 3 additions & 4 deletions Modules/_functoolsmodule.c
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Expand Up @@ -750,8 +750,10 @@ lru_cache_make_key(PyObject *args, PyObject *kwds, int typed)
PyObject *key, *keyword, *value;
Py_ssize_t key_size, pos, key_pos, kwds_size;

kwds_size = kwds ? PyDict_GET_SIZE(kwds) : 0;
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/* short path, key will match args anyway, which is a tuple */
if (!typed && !kwds) {
if (!typed && !kwds_size) {
if (PyTuple_GET_SIZE(args) == 1) {
key = PyTuple_GET_ITEM(args, 0);
if (PyUnicode_CheckExact(key) || PyLong_CheckExact(key)) {
Expand All @@ -765,9 +767,6 @@ lru_cache_make_key(PyObject *args, PyObject *kwds, int typed)
return args;
}

kwds_size = kwds ? PyDict_GET_SIZE(kwds) : 0;
assert(kwds_size >= 0);

key_size = PyTuple_GET_SIZE(args);
if (kwds_size)
key_size += kwds_size * 2 + 1;
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