has_key or in

'has_key()' or 'in'?

up vote 393 down vote favorite 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 share improve this question edited Aug 24 '09 at 18:20 SilentGhost 112k 30 208 231 asked  Aug 24 '09 at 16:30 igorgue 4,564 9 25 40
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11 Answers

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up vote 546 down vote accepted

in is definitely more pythonic. In fact has_key() was removed in Python 3.x. share improve this answer edited Apr 30 '13 at 15:56 bluish 8,326 11 66 109 answered  Aug 24 '09 at 16:33 tonfa 11.8k 1 24 33

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 show 1 more comment

up vote 179 down 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!-) share improve this answer answered  Aug 24 '09 at 18:12 Alex Martelli 440k 75 813 1119

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 add a comment

up vote 43 down vote	According to python docs: has_key() is deprecated in favor of key in d. share improve this answer edited Apr 11 '13 at 6:27 jamylak 51.1k 9 116 151 answered  Aug 24 '09 at 16:33 Nadia Alramli 53k 12 128 139
	thanks a lot, it really helped! –  Aaron Socurites  Aug 19 '15 at 7:17 add a comment

up vote 28 down 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).  share improve this answer edited Jan 18 '12 at 16:52 Mike Samuel 69.7k 16 127 172 answered  Aug 24 '09 at 22:11 John Machin 48.3k 3 59 123

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 add a comment

up vote 13 down 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) share improve this answer answered  Jan 18 '12 at 16:45 schlenk 4,659 11 19

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up vote 8 down 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. share improve this answer