New dict implementation in Python 3.6 Python 3.6 release party Inada Naoki (@methane)

稲田　直哉 (Inada Naoki) @methane KLab Inc. (We're hiring) CPython core developer C, Go, Network, MySQL clients

Table of contents Open addressing hash table New implementation Key sharing dict (PEP 412) Future idea

Open addressing hash table

Key hash value 0 1 2 3 4 5 6 7 d["foo"] = "spam" hash("foo") = 42 42 % 8 = 2

Key hash value 0 1 2 3 4 5 6 7 d["foo"] = "spam" hash("foo") = 42 42 % 8 = 2 "foo" 42 "spam"

Key hash value 0 1 2 3 4 5 6 7 d["bar"] = "ham" hash("bar") = 52 52 % 8 = 4 "foo" 42 "spam" "bar" 52 "ham"

Key hash value 0 1 2 3 4 5 6 7 d["baz"] = "egg" hash("baz") = 58 58 % 8 = 2 (conflict!) "foo" 42 "spam" "bar" 52 "ham"

Key hash value 0 1 2 3 4 5 6 7 "probing" "linear probing" uses next entry. (CPython uses "5i + 1" | (hash >> 5) probing, but use simpler way in this example) "foo" 42 "spam" "bar" 52 "ham" "baz" 58 "egg"

Key hash value 0 1 2 3 4 5 6 7 del d["foo"] hash("foo") = 42 42 % 8 = 2 "foo" 42 "spam" "bar" 52 "ham" "baz" 58 "egg"

Key hash value 0 1 2 3 4 5 6 7 del d["foo"] hash("foo") = 42 42 % 8 = 2 "bar" 52 "ham" "baz" 58 "egg"

Key hash value 0 1 2 3 4 5 6 7 x = d["baz"] hash("baz") = 58 58 % 8 = 2 (!!?) "bar" 52 "ham" "baz" 58 "egg"

Key hash value 0 1 2 3 4 5 6 7 del d["foo"] remains DUMMY key "bar" 52 "ham" "baz" 58 "egg" DMMY

Key hash value 0 1 2 3 4 5 6 7 x = d["baz"] hash("baz") = 58 58 % 8 = 2 (conflict with dummy, then linear probing) "bar" 52 "ham" "baz" 58 "egg" DMMY

Problems in classical open addressing hash table ● Large memory usage ○ at least 1/3 of entries are empty ■ otherwise, "probing" can be too slow ○ one entry uses 3 words. (24bytes in 64bit machine) ○ 8 * 8 * 3 = 192 bytes for minimum dict

New dict implementation

Compact dict Original idea is from Raymond Hettinger. PyPy implements it with some customize. https://morepypy.blogspot.jp/2015/01/faster-more-memory-eff icient-and-more.html CPython 3.6 has almost same as PyPy

Key hash value 0 1 2 3 4 5 6 7 d["foo"] = "spam" d["bar"] = "ham" "bar" 52 "ham" "foo" 42 "spam" 0 1 index

Key hash value 0 1 2 3 4 5 6 7 d["foo"] = "spam" d["bar"] = "ham" d["baz"] = "egg" del d["foo"] "bar" 52 "ham" "baz" 58 "egg" DMMY 2 1 index

Pros and cons ● Less memory usage ○ index can be 1 byte for size < 255 ○ 3 * 8 * 5 + 8 = 128bytes (was 192bytes) ● Faster iteration ● Keep insertion order ● (cons) One more lookup stage

PEP 412: Key sharing dict

PEP 412: Key sharing dict Introduced in Python 3.4 Instances of same class can share keys object

class A: def __init__(self, a, b): self.foo = a self.bar = b a = A("spam", "ham") b = A("bacon", "egg")

Key Class value 0 1 2 3 4 5 6 7 "bar" 52 "foo" 42 0 1 index "ham" "spam" values "egg" "bacon" values instance instance

Problem ● Two instances can have different insertion order ○ drop key sharing dict? ■ key sharing dict can save more memory. ● But __slots__ can be used for such cases! ■ performance improvements in some microbench ● Is it matter for real case? __slots__? ■ Needs consensus ● it's more difficult than implementation

Keep key sharing dict support ● Only exactly same order can be permitted ○ "skipped" keys are prohibited ○ deletion is also prohibited ● Otherwise, stop "key sharing" ○ `self.x = None` is faster than `del self.x`

Future ideas

Ideas that will be tried later... ● specialized dict for namespace ○ all keys are interned string ○ only pointer comparison ○ no "hash" in entry -> more compact ● OrderedKey based on new dict ○ no more doubly linked list ○ `od.move_to_end(k, last=False)` is difficult, but it's possible ● functools.lru_cache ○ no more doubly linked list ○ Using `od.move_to_end(key)`

We're moving to Github! New contributors are welcome!