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rhettinger merged 1 commit into from
Sep 5, 2017
Merged

Add comment to explain the implications of not sorting keywords #3331

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Add comment to explain the implications of not sorting keywords 
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).
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@rhettinger
rhettinger authored Sep 4, 2017
commit 6f2ce06e71eb1b26660261957778546cdefd988d
4 changes: 4 additions & 0 deletions Lib/functools.py
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Expand Up @@ -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
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