> On Jun 21, 2016, at 10:18 AM, Guido van Rossum <gu...@python.org> wrote:
>
> Judging from Inada's message there seems to be some confusion about how well
> the compact dict preserves order (personally I think if it doesn't guarantee
> order after deletions it's pretty useless).
Inada should follow PyPy's implementation of the compact dict which does
preserve order after deletions (see below).
My original proof-of-concept code didn't have that feature; instead, it was
aimed at saving space and speeding-up iteration. The key ordering was just a
by-product. Additional logic was needed to preserve order for interleaved
insertions and deletions.
Raymond
---(PyPy test of order
preservation)-------------------------------------------------------------
'Demonstrate PyPy preserves order across repeated insertions and deletions'
from random import randrange
import string
s = list(string.letters)
d = dict.fromkeys(s)
n = len(s)
for _ in range(10000):
i = randrange(n)
c = s.pop(i); s.append(c)
d.pop(c); d[c] = None
assert d.keys() == s
---(PyPy session showing order
preservation)--------------------------------------------------
$ pypy
Python 2.7.10 (c09c19272c990a0611b17569a0085ad1ab00c8ff, Jun 13 2016, 03:59:08)
[PyPy 5.3.0 with GCC 4.2.1 Compatible Apple LLVM 7.3.0 (clang-703.0.31)] on
darwin
Type "help", "copyright", "credits" or "license" for more information.
>>>> d = dict(raymond='red', rachel='blue', matthew='yellow')
>>>> del d['rachel']
>>>> d['cindy'] = 'green'
>>>> d['rachel'] = 'azure'
>>>> d
{'raymond': 'red', 'matthew': 'yellow', 'cindy': 'green', 'rachel': 'azure'}
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