没有一个单一的答案,也没有一个真实且唯一的听写。在众多变量中,它取决于:
- 数据集的大小;
- 唯一键的数量与数据映射集中重复键的数量;
- defaultdict 底层工厂的速度;
- OrderDict 的速度与稍后的订购步骤;
- Python 版本。
我不愿一概而论,但这里有一些概括:
- 这种说法
This technique is simpler and faster than an equivalent technique using dict.setdefault()
是完全错误的。这取决于数据;
setdefault
使用小数据集更快更简单;
defaultdict
对于具有更多同质键集的更大数据集(即,添加元素后字典有多短)更快;
setdefault
具有更多异构密钥集的优势;
- 这些结果对于 Python 3 和 Python 2 是不同的;
OrderedDict
在所有情况下都较慢,除了依赖于顺序的算法并且顺序不容易重构或排序;
dict
对于大多数操作,Python 3 通常更快;
- Python 3.6 的 dict 现在按插入顺序排序(降低 的有用性
OrderedDict
)。
唯一的事实:这取决于!这三种技术都很有用。
这是一些要显示的时序代码:
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)