Python 字典查找性能，get vs in

Question

这不是过早的优化。我的用例在内部循环的最内部对 dict 的权限进行了双重检查，一直在运行。此外，它在智力上令人厌烦（见结果）。

这些方法中哪个更快？

mydict = { 'hello': 'yes', 'goodbye': 'no' }
key = 'hello'

# (A)
if key in mydict:
    a = mydict[key]
    do_things(a)
else:
    handle_an_error()

# vs (B)
a = mydict.get(key,None)
if a is not None:
    do_things(a)
else:
    handle_an_error()

编辑：这些速度相同。常识告诉我 (B) 应该明显更快，因为它只是 1 次 dict 查找与 2 次，但结果不同。我在挠头。

基准测试的结果平均超过 12 次运行，其中 1/2 是命中，另一半是未命中：

doing in
switching to get
total time for IN:  0.532250006994
total time for GET:  0.480916659037
times found: 12000000
times not found: 12000000

当一个类似的运行（*10 多个循环）没有找到密钥时，

doing in
switching to get
total time for IN:  2.35899998744
total time for GET:  4.13858334223

为什么！？

（正确）代码

import time
smalldict = {}
for i in range(10):
    smalldict[str(i*4)] = str(i*18)

smalldict["8"] = "hello"

bigdict = {}
for i in range(10000):
    bigdict[str(i*100)] = str(i*4123)
bigdict["hello"] = "yes!"

timetotal = 0
totalin = 0
totalget = 0
key = "hello"
found= 0
notfound = 0

ddo = bigdict # change to smalldict for small dict gets
print 'doing in'


for r in range(12):
    start = time.time()
    a = r % 2
    for i in range(1000000):
        if a == 0:
            if str(key) in ddo:
                found = found + 1
                foo = ddo[str(key)]
            else:
                notfound = notfound + 1
                foo = "nooo"
        else:
            if 'yo' in ddo:
                found = found + 1
                foo = ddo['yo']
            else:
                notfound = notfound + 1
                foo = "nooo"
    timetotal = timetotal + (time.time() - start)

totalin = timetotal / 12.0 

print 'switching to get'
timetotal = 0
for r in range(12):
    start = time.time()
    a = r % 2
    for i in range(1000000):
        if a == 0:
            foo = ddo.get(key,None)
            if foo is not None:
                found = found + 1
            else:
                notfound = notfound + 1
                foo = "nooo"
        else:
            foo = ddo.get('yo',None)
            if foo is not None:
                found = found + 1
                notfound = notfound + 1
            else:
                notfound = notfound + 1
                foo = "oooo"
    timetotal = timetotal + (time.time() - start)

totalget = timetotal / 12

print "total time for IN: ", totalin
print 'total time for GET: ', totalget
print 'times found:', found
print 'times not found:', notfound

（原）代码导入时间 smalldict = {} for i in range(10): smalldict[str(i*4)] = str(i*18)

smalldict["8"] = "hello"

bigdict = {}
for i in range(10000):
    bigdict[str(i*100)] = str(i*4123)
bigdict["8000"] = "hello"

timetotal = 0
totalin = 0
totalget = 0
key = "hello"
found= 0
notfound = 0

ddo = bigdict # change to smalldict for small dict gets
print 'doing in'
for r in range(12):
    start = time.time()
    a = r % 2
    for i in range(10000000):
        if a == 0:
            if key in ddo:
                foo = ddo[key]
            else:
                foo = "nooo"
        else:
            if 'yo' in ddo:
                foo = ddo['yo']
            else:
                foo = "nooo"
    timetotal = timetotal + (time.time() - start)

totalin = timetotal / 12.0 

print 'switching to get'
timetotal = 0
for r in range(12):
    start = time.time()
    a = r % 2
    for i in range(10000000):
        if a == 0:
            foo = ddo.get(key,None)
            if foo is not None:
                # yaaay
                pass
            else:
                foo = "nooo"
        else:
            foo = ddo.get('yo',None)
            if foo is not None:
                #yaaay
                pass
            else:
                foo = "oooo"
    timetotal = timetotal + (time.time() - start)

totalget = timetotal / 12

print "total time for IN: ", totalin
print 'total time for GET: ', totalget

Answer 1

我们可以做一些更好的计时：

import timeit

d = dict.fromkeys(range(10000))

def d_get_has(d):
    return d.get(1)

def d_get_not_has(d):
    return d.get(-1)

def d_in_has(d):
    if 1 in d:
        return d[1]

def d_in_not_has(d):
    if -1 in d:
        return d[-1]


print timeit.timeit('d_get_has(d)', 'from __main__ import d, d_get_has')
print timeit.timeit('d_get_not_has(d)', 'from __main__ import d, d_get_not_has')
print timeit.timeit('d_in_has(d)', 'from __main__ import d, d_in_has')
print timeit.timeit('d_in_not_has(d)', 'from __main__ import d, d_in_not_has')

在我的电脑上，“in”变体比.get变体更快。这可能是因为.getdict 上的属性查找和属性查找可能与 dict 上的成员资格测试一样昂贵。请注意，使用in和项目查找dict[x]可以直接在字节码中完成，因此可以绕过正常的方法查找...

还值得指出的是，如果我只使用 pypy :-)，我会得到一个巨大的优化：

$ python ~/sandbox/test.py
0.169840812683
0.1732609272
0.122044086456
0.0991759300232

$ pypy ~/sandbox/test.py
0.00974893569946
0.00752687454224
0.00812077522278
0.00686597824097