没有一个单一的答案和唯一的字典是正确的。在许多变量中,这取决于:
- 数据集的大小;
- 数据映射集中唯一键的数量与重复键的数量;
- 默认字典底层工厂的速度;
- OrderDict 的速度与某些后续排序步骤;
- Python 版本。
我不想做出概括,但以下是一些普遍性:
- 声明“这种技术比使用 dict.setdefault() 相当技术更简单和更快”是错误的。它取决于数据;
- 对于小数据集,setdefault 更快、更简单;
- 对于具有更同质键集(即添加元素后 dict 有多短)的大型数据集,defaultdict 更快;
- 对于更异构键集,setdefault 有优势;
- Python 3 和 Python 2 的结果不同;
- OrderedDict 在除依赖于顺序且难以重构或排序的算法之外的所有情况下都较慢;
- Python 3 对于大多数 dict 操作通常更快;
- Python 3.6 的 dict 现在按插入顺序排序(降低了 OrderedDict 的实用性)。
唯一的真理是:它取决于! 这三种技术都很有用。
这里有一些计时代码以供参考:
from __future__ import print_function
from collections import defaultdict
from collections import OrderedDict
try:
t=unichr(100)
except NameError:
unichr=chr
def f1(li):
'''defaultdict'''
d = defaultdict(list)
for k, v in li:
d[k].append(v)
return d.items()
def f2(li):
'''setdefault'''
d={}
for k, v in li:
d.setdefault(k, []).append(v)
return d.items()
def f3(li):
'''OrderedDict'''
d=OrderedDict()
for k, v in li:
d.setdefault(k, []).append(v)
return d.items()
if __name__ == '__main__':
import timeit
import sys
print(sys.version)
few=[('yellow', 1), ('blue', 2), ('yellow', 3), ('blue', 4), ('red', 1)]
fmt='{:>12}: {:10.2f} micro sec/call ({:,} elements, {:,} keys)'
for tag, m, n in [('small',5,10000), ('medium',20,1000), ('bigger',1000,100), ('large',5000,10)]:
for f in [f1,f2,f3]:
s = few*m
res=timeit.timeit("{}(s)".format(f.__name__), setup="from __main__ import {}, s".format(f.__name__), number=n)
st=fmt.format(f.__doc__, res/n*1000000, len(s), len(f(s)))
print(st)
s = [(unichr(i%0x10000),i) for i in range(1,len(s)+1)]
res=timeit.timeit("{}(s)".format(f.__name__), setup="from __main__ import {}, s".format(f.__name__), number=n)
st=fmt.format(f.__doc__, res/n*1000000, len(s), len(f(s)))
print(st)
print()
Python 2.7 结果:
2.7.5 (default, Aug 25 2013, 00:04:04)
[GCC 4.2.1 Compatible Apple LLVM 5.0 (clang-500.0.68)]
defaultdict: 10.20 micro sec/call (25 elements, 3 keys)
defaultdict: 21.08 micro sec/call (25 elements, 25 keys)
setdefault: 13.41 micro sec/call (25 elements, 3 keys)
setdefault: 18.24 micro sec/call (25 elements, 25 keys)
OrderedDict: 49.47 micro sec/call (25 elements, 3 keys)
OrderedDict: 102.16 micro sec/call (25 elements, 25 keys)
defaultdict: 28.28 micro sec/call (100 elements, 3 keys)
defaultdict: 79.78 micro sec/call (100 elements, 100 keys)
setdefault: 45.68 micro sec/call (100 elements, 3 keys)
setdefault: 68.66 micro sec/call (100 elements, 100 keys)
OrderedDict: 117.78 micro sec/call (100 elements, 3 keys)
OrderedDict: 343.17 micro sec/call (100 elements, 100 keys)
defaultdict: 1123.60 micro sec/call (5,000 elements, 3 keys)
defaultdict: 4250.44 micro sec/call (5,000 elements, 5,000 keys)
setdefault: 2089.86 micro sec/call (5,000 elements, 3 keys)
setdefault: 3803.03 micro sec/call (5,000 elements, 5,000 keys)
OrderedDict: 4399.16 micro sec/call (5,000 elements, 3 keys)
OrderedDict: 16279.14 micro sec/call (5,000 elements, 5,000 keys)
defaultdict: 5609.39 micro sec/call (25,000 elements, 3 keys)
defaultdict: 25351.60 micro sec/call (25,000 elements, 25,000 keys)
setdefault: 10267.00 micro sec/call (25,000 elements, 3 keys)
setdefault: 24091.51 micro sec/call (25,000 elements, 25,000 keys)
OrderedDict: 22091.98 micro sec/call (25,000 elements, 3 keys)
OrderedDict: 94028.00 micro sec/call (25,000 elements, 25,000 keys)
Python 3.3 结果:
3.3.2 (default, May 21 2013, 11:50:47)
[GCC 4.2.1 Compatible Apple Clang 4.1 ((tags/Apple/clang-421.11.66))]
defaultdict: 8.58 micro sec/call (25 elements, 3 keys)
defaultdict: 21.18 micro sec/call (25 elements, 25 keys)
setdefault: 10.42 micro sec/call (25 elements, 3 keys)
setdefault: 14.58 micro sec/call (25 elements, 25 keys)
OrderedDict: 45.43 micro sec/call (25 elements, 3 keys)
OrderedDict: 92.69 micro sec/call (25 elements, 25 keys)
defaultdict: 20.47 micro sec/call (100 elements, 3 keys)
defaultdict: 77.48 micro sec/call (100 elements, 100 keys)
setdefault: 34.22 micro sec/call (100 elements, 3 keys)
setdefault: 54.86 micro sec/call (100 elements, 100 keys)
OrderedDict: 107.37 micro sec/call (100 elements, 3 keys)
OrderedDict: 318.98 micro sec/call (100 elements, 100 keys)
defaultdict: 714.70 micro sec/call (5,000 elements, 3 keys)
defaultdict: 3892.92 micro sec/call (5,000 elements, 5,000 keys)
setdefault: 1502.91 micro sec/call (5,000 elements, 3 keys)
setdefault: 2888.08 micro sec/call (5,000 elements, 5,000 keys)
OrderedDict: 3912.95 micro sec/call (5,000 elements, 3 keys)
OrderedDict: 14863.02 micro sec/call (5,000 elements, 5,000 keys)
defaultdict: 3649.02 micro sec/call (25,000 elements, 3 keys)
defaultdict: 22313.17 micro sec/call (25,000 elements, 25,000 keys)
setdefault: 7447.28 micro sec/call (25,000 elements, 3 keys)
setdefault: 18426.88 micro sec/call (25,000 elements, 25,000 keys)
OrderedDict: 19202.17 micro sec/call (25,000 elements, 3 keys)
OrderedDict: 85946.45 micro sec/call (25,000 elements, 25,000 keys)
defaultdict
并不一定比普通的dict
更慢。那里的计时方式有缺陷,因为计时包括了创建对象的时间。除此之外,还有不同类型的性能,其中易于维护就是其中之一。你没有指定任何衡量性能的标准。只需使用 适合工作的正确工具。 - Martijn Pieters{}
)比defaultdict(...)
工厂调用(全局查找,堆栈推送,调用)快得多,这会导致在小数据集上结果失衡。认为defaultdict
更慢是错误的。 - Martijn Pietersdefaultdict
比调用dict.setdefault
更加方便,但为什么OrderedDict
在可能的情况下被认为是比dict
更优选的呢?我认为我从来没有关心过字典中键的插入顺序,这个特性比仅仅提供新键的默认值更加具体。 - chepner