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
