From 550370957cb0e40bfc497174c95fee47d01de995 Mon Sep 17 00:00:00 2001 From: Raymond Hettinger Date: Mon, 4 Sep 2017 17:47:53 -0700 Subject: [PATCH] Add comment to explain the implications of not sorting keywords (#3331) In Python 3.6, sorted() was removed from _make_key() for the lru_cache and instead rely on guaranteed keyword argument order preservation. This makes keyword argument handling faster but it also causes multiple callers with a different keyword argument order to be cached as separate items. Depending on your point of view, this is either a performance regression (increased number of cache misses) or a performance enhancement (faster computation of keys). --- Lib/functools.py | 4 ++++ 1 file changed, 4 insertions(+) diff --git a/Lib/functools.py b/Lib/functools.py index 89f2cf4f5f7d..0873f207154b 100644 --- a/Lib/functools.py +++ b/Lib/functools.py @@ -432,6 +432,10 @@ def _make_key(args, kwds, typed, saves space and improves lookup speed. """ + # All of code below relies on kwds preserving the order input by the user. + # Formerly, we sorted() the kwds before looping. The new way is *much* + # faster; however, it means that f(x=1, y=2) will now be treated as a + # distinct call from f(y=2, x=1) which will be cached separately. key = args if kwds: key += kwd_mark