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has_key or in

2016-03-11 10:12 405 查看


'has_key()'
or 'in'?

up
vote393down
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51

I wonder what is better to do:
d = {'a': 1, 'b': 2}
'a' in d
True


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


python
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edited Aug
24 '09 at 18:20




SilentGhost
112k30208231

asked Aug 24 '09 at 16:30




igorgue
4,56492540

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11 Answers

activeoldestvotes

up vote546down
voteaccepted
in
is
definitely more pythonic.

In fact 
has_key()
was
removed in Python 3.x.

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edited Apr
30 '13 at 15:56




bluish
8,3261166109

answered Aug 24 '09 at 16:33





tonfa
11.8k12433

2
As an addition, in Python 3, to check for the existence in values, instead of the keys, try >>> 1 in d.values() – riza Aug
24 '09 at 18:12
76
One semi-gotcha to avoid though is to make sure you do: "key in some_dict" rather than "key in some_dict.keys()". Both
are equivalent semantically, but performance-wise the latter is much slower (O(n) vs O(1)). I've seen people do the "in dict.keys()" thinking it's more explicit & therefore better. – Adam
Parkin Nov
9 '11 at 20:55
1
in
works
with 2.6 too right? – Logan Jan
17 '13 at 4:07
@Logan yes it does – Wax
Cage Jul
3 '15 at 13:10
1
@BenjaminSchollnick what's the result? – DerekY Dec
14 '15 at 10:01
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comment
up vote179down
vote
in
wins
hands-down, not just in elegance (and not being deprecated;-) but also in performance, e.g.:
$ 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


While the following observation is not always true, you'll notice that usually, in Python, the faster solution is more elegant and Pythonic; that's
why 
-mtimeit
is
SO helpful -- it's not just about saving a hundred nanoseconds here and there!-)

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answered Aug 24 '09 at 18:12




Alex Martelli
440k758131119

2
thanks a lot, good to know, now I'm changing my code to use 'in' instead of has_key() ;) – igorgue Aug
24 '09 at 18:56
3
Thanks for this, made verifying that "in some_dict" is in fact O(1) much easier (try increasing the 99 to say 1999,
and you'll find the runtime is about the same). – Adam
Parkin Nov
9 '11 at 21:00
has_key
appears
to be O(1) too. – dan-gph Jan
6 '15 at 4:11
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up vote43down
vote
According to python docs:

has_key()
is
deprecated in favor of 
key
in d
.

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edited Apr
11 '13 at 6:27





jamylak
51.1k9116151

answered Aug 24 '09 at 16:33





Nadia Alramli
53k12128139

thanks a lot, it really helped! – Aaron
Socurites Aug
19 '15 at 7:17
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a comment
up vote28down
vote
Use 
dict.has_key()
if
(and only if) your code is required to be runnable by Python versions earlier than 2.3 (when 
key
in dict
was introduced).

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edited Jan
18 '12 at 16:52




Mike Samuel
69.7k16127172

answered Aug 24 '09 at 22:11





John Machin
48.3k359123

6
While one would hope no one is using a Python earlier than 2.3 (released in 2003), I'm quite confident there are still
some holdouts. So this answer is a valuable footnote to all the "use 
in
of
course, duh" answers. – John
Y Aug
3 '11 at 21:07
2
@JohnY This really comes into play with the embedded linux variants. I'm currently stuck using 2.3 on two projects :( – Adam
Lewis Feb
23 '13 at 23:37
1
The WebSphere update in 2013 uses Jython 2.1 as its main scripting language. So this is unfortunately still a useful
thing to note, five years after you noted it. – ArtOfWarfare Sep
24 '14 at 11:49
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a comment
up vote13down
vote
There is one example where 
in
actually
kills your performance.

If you use 
in
on
a O(1) container that only implements 
__getitem__
and 
has_key()
but
not 
__contains__
you
will turn an O(1) search into an O(N) search (as 
in
falls
back to a linear search via 
__getitem__
).

Fix is obviously trivial:
def __contains__(self, x):
return self.has_key(x)


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answered Jan 18 '12 at 16:45





schlenk
4,6591119

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up vote8down
vote
has_key
is
a dictionary method, but 
in
will
work on any collection, and even when 
__contains__
is
missing, 
in
will
use any other method to iterate the collection to find out.

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