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).
diff --git a/Lib/functools.py b/Lib/functools.py
index 89f2cf4..0873f20 100644
--- a/Lib/functools.py
+++ b/Lib/functools.py
@@ -432,6 +432,10 @@
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