Check if a given key already exists in a dictionary
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?
Accepted Answer
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.
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.
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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).
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?
For additional info on speed execution of the accepted answer's proposed methods (10m loops):
'key' in mydict
elapsed time 1.07 secmydict.get('key')
elapsed time 1.84 secmydefaultdict['key']
elapsed time 1.07 sec
Therefore using in
or defaultdict
are recommended against get
.
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}
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)