我应该在Python dicts上使用'has_key()'或'in'吗？

Question

我想知道做什么更好:

d = {'a': 1, 'b': 2}
'a' in d
True

要么:

d = {'a': 1, 'b': 2}
d.has_key('a')
True

Answer 1

in 肯定是更加pythonic.

实际上has_key()在Python 3.x中被删除了.

Answer 2

in 赢得胜利,不仅仅是优雅(而不是被弃用;-)而且还有表现,例如:

$ python -mtimeit -s'd=dict.fromkeys(range(99))' '12 in d'
10000000 loops, best of 3: 0.0983 usec per loop
$ python -mtimeit -s'd=dict.fromkeys(range(99))' 'd.has_key(12)'
1000000 loops, best of 3: 0.21 usec per loop

虽然以下观察并非总是如此,但您会注意到,通常,在Python中,更快的解决方案是更优雅和Pythonic; 这就是为什么-mtimeitSO有用 - 它不只是在这里和那里节省一百纳秒! - )

Answer 3

根据python 文档:

has_key()不赞成使用 key in d.

Answer 4

使用dict.has_key()if(且仅当)您的代码需要由早于2.3的Python版本(key in dict引入时)运行.

Answer 5

有一个例子in实际上会杀死你的表现.

如果你使用in一个O(1)集装箱只实现__getitem__和has_key()而不是__contains__你会变成一个O(1)搜索到O(N),搜索(如in回落到通过线性搜索__getitem__).

修复显然是微不足道的:

def __contains__(self, x):
    return self.has_key(x)

Answer 6

has_key是一个字典方法,但in可以在任何集合上工作,即使__contains__缺少,in也会使用任何其他方法迭代集合来查找.

Answer 7

不推荐使用dict.has_key()的解决方案,使用'in' - sublime文本编辑器3

这里我举了一个名为'age'的字典的例子 -

ages = {}

# Add a couple of names to the dictionary
ages['Sue'] = 23

ages['Peter'] = 19

ages['Andrew'] = 78

ages['Karren'] = 45

# use of 'in' in if condition instead of function_name.has_key(key-name).
if 'Sue' in ages:

    print "Sue is in the dictionary. She is", ages['Sue'], "years old"

else:

    print "Sue is not in the dictionary"

Answer 8

亚当·帕金的评论扩展了Alex Martelli的表现测试......

$ python3.5 -mtimeit -s'd=dict.fromkeys(range( 99))' 'd.has_key(12)'
Traceback (most recent call last):
  File "/usr/local/Cellar/python3/3.5.2_3/Frameworks/Python.framework/Versions/3.5/lib/python3.5/timeit.py", line 301, in main
    x = t.timeit(number)
  File "/usr/local/Cellar/python3/3.5.2_3/Frameworks/Python.framework/Versions/3.5/lib/python3.5/timeit.py", line 178, in timeit
    timing = self.inner(it, self.timer)
  File "<timeit-src>", line 6, in inner
    d.has_key(12)
AttributeError: 'dict' object has no attribute 'has_key'

$ python2.7 -mtimeit -s'd=dict.fromkeys(range(  99))' 'd.has_key(12)'
10000000 loops, best of 3: 0.0872 usec per loop

$ python2.7 -mtimeit -s'd=dict.fromkeys(range(1999))' 'd.has_key(12)'
10000000 loops, best of 3: 0.0858 usec per loop

$ python3.5 -mtimeit -s'd=dict.fromkeys(range(  99))' '12 in d'
10000000 loops, best of 3: 0.031 usec per loop

$ python3.5 -mtimeit -s'd=dict.fromkeys(range(1999))' '12 in d'
10000000 loops, best of 3: 0.033 usec per loop

$ python3.5 -mtimeit -s'd=dict.fromkeys(range(  99))' '12 in d.keys()'
10000000 loops, best of 3: 0.115 usec per loop

$ python3.5 -mtimeit -s'd=dict.fromkeys(range(1999))' '12 in d.keys()'
10000000 loops, best of 3: 0.117 usec per loop