'has_key()' or 'in'?

Question

Hi,

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

Answer 1

My $0.02: the more Pythonic answer would be to use in.

Answer 2

in is definitely more pythonic, in fact has_key() was removed in python 3.x: http://docs.python.org/3.1/whatsnew/3.0.html#builtins

Answer 3

According to python docs:

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

Answer 4

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!-)

Answer 5

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.

Answer 6

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).