PHPz2017-04-17 14:32:50
Because keys() needs to form a list, building a list is very expensive for a large dict. What lihsing said is right, and there is another way to write it, iterkeys(), which has little speed difference.
See test code
import timeit
DICT_SIZE = 100*10000
testDict = dict()
for i in range(DICT_SIZE):
testDict[i] = i # 构建大小为100W的字典
assert len(testDict) == DICT_SIZE
def test1():
for _ in testDict.keys():
pass
def test2():
for _ in testDict:
pass
def test3():
for _ in testDict.iterkeys():
pass
# 分别测试2K次
print timeit.timeit("test1()", setup="from __main__ import test1", number=2000)
print timeit.timeit("test2()", setup="from __main__ import test2", number=2000)
print timeit.timeit("test3()", setup="from __main__ import test3", number=2000)
Output
54.1994677764
30.2660675759
31.3075812315
is the result of windows + python 2.7