将字典复制到 defaultdict

Question

我想高效地将字典复制到 defaultdict。
这是我的代码：

[In]    from collections import defaultdict  
[In]    new_dict = {'a': 1, 'b': 2, 'c': 3, 'e':4, 'f':5}  
[In]    def_dict = defaultdict(list)  
[In]    timeit for k, v in new_dict.items(): def_dict[k].append(v)  
[Out]    1000000 loops, best of 3: 910 ns per loop

我想改进它的性能。欢迎提出任何建议。

Answer 1

我坚持我的建议，即使用 def_dict[k] = [v] 会比 def_dict[k].append(v) 快，根据我自己的测试发现它快了将近两倍。

下面是我用来确定这一点的完整基准测试代码。我还尝试了其他几种可能性，但 none 更好。

Python 3.5.0 和 Python 2.7.10 都获得了相似的结果。

from __future__ import print_function
import sys
from textwrap import dedent
import timeit

N = 1000000  # number of executions of each "algorithm"
R = 3  # number of Repeations of executions

# common setup for all algorithms (not timed)
setup = dedent("""
    from collections import defaultdict

    new_dict = {'a': 1, 'b': 2, 'c': 3, 'e': 4, 'f': 5}
""")

algorithms = {
    "def_dict[k].append(v)": dedent("""
        def_dict = defaultdict(list)
        for k, v in new_dict.items(): def_dict[k].append(v)
    """),

    "def_dict[k] = [v]": dedent("""
        def_dict = defaultdict(list)
        for k, v in new_dict.items(): def_dict[k] = [v]
    """),

    "defaultdict(list, <gen-xprsn>)": dedent("""
        def_dict = defaultdict(list, ((k, [v]) for k, v in new_dict.items()))
        """),

    "defaultdict(list, <dict-comp>)": dedent("""
        def_dict = defaultdict(list, {k:[v] for k, v in new_dict.items()})
        """),
}

# collecting results of executing and timing each algorithm snippet
timings = [
    (label,
     min(timeit.repeat(algorithms[label], setup=setup, repeat=R, number=N)),
    ) for label in algorithms
]

# display results
print('fastest to slowest execution speeds (Python {}.{}.{})\n'.format(
        *sys.version_info[:3]),
        '  ({:,d} executions, best of {:d} repetitions)\n'.format(N, R))

longest = max(len(timing[0]) for timing in timings)  # length of longest label
ranked = sorted(timings, key=lambda t: t[1]) # ascending sort by execution time
fastest = ranked[0][1]
for timing in ranked:
    print("{:>{width}} : {:9.6f} secs, rel speed {:4.2f}x, {:6.2f}% slower".
            format(timing[0], timing[1], round(timing[1]/fastest, 2),
                   round((timing[1]/fastest - 1) * 100, 2), width=longest))

以下是运行在我的系统上的结果：

fastest to slowest execution speeds (Python 3.5.0)
   (1,000,000 executions, best of 3 repetitions)

             def_dict[k] = [v] :  0.977644 secs, rel speed 1.00x,   0.00% slower
defaultdict(list, <dict-comp>) :  1.330040 secs, rel speed 1.36x,  36.05% slower
defaultdict(list, <gen-xprsn>) :  1.761532 secs, rel speed 1.80x,  80.18% slower
         def_dict[k].append(v) :  1.929081 secs, rel speed 1.97x,  97.32% slower

将字典复制到 defaultdict

Copying a dictionary to defaultdict

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python-3.x