python – Check if a given key already exists in a dictionary

python – Check if a given key already exists in a dictionary

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.

You dont have to call keys:

if key1 in dict:
  print(blah)
else:
  print(boo)

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

python – Check if a given key already exists in a dictionary

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).