Check if a given key already exists in a dictionary

Question

Check if a given key already exists in a 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?

Mohan Gulati

2013/05/02

Popular Answer

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.

Jason Baker

2020/01/13

Read less...

You can test for the presence of a key in a dictionary, using the in keyword:

d = {'a': 1, 'b': 2}
'a' in d # <== evaluates to True
'c' in d # <== evaluates to False

A common use for checking the existence of a key in a dictionary before mutating it is to default-initialize the value (e.g. if your values are lists, for example, and you want to ensure that there is an empty list to which you can append when inserting the first value for a key). In cases such as those, you may find the collections.defaultdict() type to be of interest.

In older code, you may also find some uses of has_key(), a deprecated method for checking the existence of keys in dictionaries (just use key_name in dict_name, instead).

Michael Aaron Safyan

2016/06/27

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?

Greg Hewgill

2009/10/21

For additional info on speed execution of the accepted answer's proposed methods (10m loops):

'key' in mydict elapsed time 1.07 sec
mydict.get('key') elapsed time 1.84 sec
mydefaultdict['key'] elapsed time 1.07 sec

Therefore using in or defaultdict are recommended against get.

Wtower

2015/05/29

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}

David Berger

2009/10/21

Dictionary in python has a get('key', default) method. So you can just set a default value in case there is no key.

values = {...}
myValue = values.get('Key', None)

mafonya

2018/03/23

Chris B. · Accepted Answer · 1/22/2020 6:50:10 AM

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

d = {"key1": 10, "key2": 23}

if "key1" in d:
    print("this will execute")

if "nonexistent key" in d:
    print("this will not")

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

d = dict()

for i in range(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 either use dict.setdefault() repeatedly or defaultdict from the collections module, like so:

from collections import defaultdict

d = defaultdict(int)

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

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