Raymond Hettinger <[email protected]> added the comment:
For the record, here's what is going on. The method_cache() code uses a
slightly different invocation for the first call than for subsequent calls. In
particular, the wrapper() uses **kwargs with an empty dict whereas the first
call didn't use keyword arguments at all. The C version of the lru_cache() is
treating that first call as distinct from the second call, resulting in a cache
miss for the both the first and second invocation but not in subsequent
invocations.
The pure python lru_cache() has a memory saving fast path taken when kwds is an
empty dict. The C version is out-of-sync with because it runs the that path
only when kwds==NULL and it doesn't check for the case where kwds is an empty
dict. Here's a minimal reproducer:
@lru_cache()
def f(x):
pass
f(0)
f(0, **{})
assert f.cache_info().hits == 1
Here's a possible fix:
diff --git a/Modules/_functoolsmodule.c b/Modules/_functoolsmodule.c
index 3f1c01651d..f118119479 100644
--- a/Modules/_functoolsmodule.c
+++ b/Modules/_functoolsmodule.c
@@ -751,7 +751,7 @@ lru_cache_make_key(PyObject *args, PyObject *kwds, int
typed)
Py_ssize_t key_size, pos, key_pos, kwds_size;
/* short path, key will match args anyway, which is a tuple */
- if (!typed && !kwds) {
+ if (!typed && (!kwds || PyDict_GET_SIZE(kwds) == 0)) {
if (PyTuple_GET_SIZE(args) == 1) {
key = PyTuple_GET_ITEM(args, 0);
if (PyUnicode_CheckExact(key) || PyLong_CheckExact(key)) {
----------
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Python tracker <[email protected]>
<https://bugs.python.org/issue36650>
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