[Python-Dev] Python 3.6 dict becomes compact and gets a private version; and keywords become ordered (original) (raw)

Victor Stinner victor.stinner at gmail.com
Thu Sep 15 03:20:17 EDT 2016

2016-09-15 8:31 GMT+02:00 Serhiy Storchaka <storchaka at gmail.com>:

Note that this is made at the expense of the 20% slowing down an iteration.

$ ./python -m timeit -s "d = dict.fromkeys(range(10**6))" -- "list(d)" Python 3.5: 66.1 msec per loop Python 3.6: 82.5 msec per loop Fortunately the cost of the lookup (the most critical operation for dicts) seems left the same. But this can be an argument against using this technique in sets.

My small benchmarks on dict memory usage and dict lookup:

http://bugs.python.org/issue27350#msg275581

It seems like the memory usage is between 20% and 25% smaller. Great job!

Memory usage, Python 3.5 => Python 3.6 on Linux x86_64:

./python -c 'import sys; print(sys.getsizeof({str(i):i for i in range(10)}))'

Note: the size is the the size of the container itself, not of keys nor values.

http://bugs.python.org/issue27350#msg275587

As I expected, a dictionary lookup is a little bit slower (3%) between Python 3.5 and Python 3.6:

$ ./python -m perf timeit -s 'd={str(i):i for i in range(100)}' 'd["10"]; d["20"]; d["30"]; d["40"]; d["50"]; d["10"]; d["20"]; d["30"]; d["40"]; d["50"]' --rigorous

Median +- std dev: [lookup35] 309 ns +- 10 ns -> [lookup36] 320 ns +- 8 ns: 1.03x slower

Victor

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