What is a good way to test if a Key exists in Python Dictionary

Question

I wanted to test if a key exists in a dictionary before updating the value for the key. I wrote the following code:

if 'key1' in dict.keys():
  print "blah"
else:
  print "boo"

I think this is not the best way to accomplish this task. Is there a better way to test for a key in the dictionary?

Answer 1

You don't have to call keys:

if 'key1' in dict:
  print "blah"
else:
  print "boo"

That will be much faster as it uses the dictionary's hashing as opposed to doing a linear search, which calling keys would do.

Answer 2

You can shorten this:

if 'key1' in dict:
    ...

However, this is at best a cosmetic improvement. Why do you believe this is not the best way?

Answer 3

I would recommend using the setdefault method instead. It sounds like it will do everything you want.

>>> d = {'foo':'bar'}
>>> q = d.setdefault('foo','baz') #Do not override the existing key
>>> print q #The value takes what was originally in the dictionary
bar
>>> print d
{'foo': 'bar'}
>>> r = d.setdefault('baz',18) #baz was never in the dictionary
>>> print r #Now r has the value supplied above
18
>>> print d #The dictionary's been updated
{'foo': 'bar', 'baz': 18}

Answer 4

in is the intended way to test for the existence of a key in a dict.

d = dict()

for i in xrange(100):
    key = i % 10
    if key in d:
        d[key] += 1
    else:
        d[key] = 1

If you wanted a default, you can always use dict.get():

d = dict()

for i in xrange(100):
    key = i % 10
    d[key] = d.get(key, 0) + 1

... and if you wanted to always ensure a default value for any key you can use defaultdict from the collections module, like so:

from collections import defaultdict

d = defaultdict(lambda: 0)

for i in xrange(100):
    d[i % 10] += 1

... but in general, the in keyword is the best way to do it.

Answer 5

There are two different ways to do it:

1. key in dict
2. dict.has_key(key)

You might also want to check out the Python Quick Reference.

EDIT:

As noted, "has_key" is deprecated in Python 3.0+.