python:使用列表值遍历字典
python:使用列表值遍历字典
提问于 2013-08-17 22:06:03
给定一个列表字典，例如
d = {'1':[11,12], '2':[21,21]}哪一个更有蟒蛇气息或者更可取：
for k in d:
for x in d[k]:
# whatever with k, x或
for k, dk in d.iteritems():
for x in dk:
# whatever with k, x或者还有其他需要考虑的事情吗？
编辑，以防列表可能有用(例如，标准字典不保留顺序)，这可能是合适的，尽管它要慢得多。
d2 = d.items()
for k in d2:
for x in d2[1]:
# whatever with k, x回答 4
Stack Overflow用户
发布于 2013-08-17 22:21:23
这是一个速度测试，为什么不：
import random
numEntries = 1000000
d = dict(zip(range(numEntries), [random.sample(range(0, 100), 2) for x in range(numEntries)]))
def m1(d):
for k in d:
for x in d[k]:
pass
def m2(d):
for k, dk in d.iteritems():
for x in dk:
pass
import cProfile
cProfile.run('m1(d)')
print
cProfile.run('m2(d)')
# Ran 3 trials:
# m1: 0.205, 0.194, 0.193: average 0.197 s
# m2: 0.176, 0.166, 0.173: average 0.172 s
# Method 1 takes 15% more time than method 2cProfile示例输出：
3 function calls in 0.194 seconds
Ordered by: standard name
ncalls tottime percall cumtime percall filename:lineno(function)
1 0.000 0.000 0.194 0.194 <string>:1(<module>)
1 0.194 0.194 0.194 0.194 stackoverflow.py:7(m1)
1 0.000 0.000 0.000 0.000 {method 'disable' of '_lsprof.Profiler' objects}
4 function calls in 0.179 seconds
Ordered by: standard name
ncalls tottime percall cumtime percall filename:lineno(function)
1 0.000 0.000 0.179 0.179 <string>:1(<module>)
1 0.179 0.179 0.179 0.179 stackoverflow.py:12(m2)
1 0.000 0.000 0.000 0.000 {method 'disable' of '_lsprof.Profiler' objects}
1 0.000 0.000 0.000 0.000 {method 'iteritems' of 'dict' objects}Stack Overflow用户
发布于 2014-11-07 08:02:22
我考虑了几种方法：
import itertools
COLORED_THINGS = {'blue': ['sky', 'jeans', 'powerline insert mode'],
'yellow': ['sun', 'banana', 'phone book/monitor stand'],
'red': ['blood', 'tomato', 'test failure']}
def forloops():
""" Nested for loops. """
for color, things in COLORED_THINGS.items():
for thing in things:
pass
def iterator():
""" Use itertools and list comprehension to construct iterator. """
for color, thing in (
itertools.chain.from_iterable(
[itertools.product((k,), v) for k, v in COLORED_THINGS.items()])):
pass
def iterator_gen():
""" Use itertools and generator to construct iterator. """
for color, thing in (
itertools.chain.from_iterable(
(itertools.product((k,), v) for k, v in COLORED_THINGS.items()))):
pass我使用ipython和memory_profiler来测试性能：
>>> %timeit forloops()
1000000 loops, best of 3: 1.31 µs per loop
>>> %timeit iterator()
100000 loops, best of 3: 3.58 µs per loop
>>> %timeit iterator_gen()
100000 loops, best of 3: 3.91 µs per loop
>>> %memit -r 1000 forloops()
peak memory: 35.79 MiB, increment: 0.02 MiB
>>> %memit -r 1000 iterator()
peak memory: 35.79 MiB, increment: 0.00 MiB
>>> %memit -r 1000 iterator_gen()
peak memory: 35.79 MiB, increment: 0.00 MiB正如您所看到的，该方法对峰值内存使用没有明显的影响，但是嵌套for循环在速度方面是无与伦比的(更不用说可读性了)。
Stack Overflow用户
发布于 2013-08-17 22:26:32
下面是列表理解方法。嵌套的.
r = [[i for i in d[x]] for x in d.keys()]
print r
[[11, 12], [21, 21]]页面原文内容由Stack Overflow提供。腾讯云小微IT领域专用引擎提供翻译支持
原文链接：
https://stackoverflow.com/questions/18289678
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